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massive stars, multiple stellar systems, and clusters are born of the gravitational collapse of massive, dense, gaseous clumps, and the way these systems form strongly depends on how the parent clump fragments into cores during collapse. numerical simulations show that magnetic fields may be the key ingredient in regulating fragmentation. here we present alma observations at ~ 0.25'' resolution of the thermal dust continuum emission at ~ 278 ghz towards a turbulent, dense, and massive clump, iras 16061-5048c1, in a very early evolutionary stage. the alma image shows that the clump has fragmented into many cores along a filamentary structure. we find that the number, the total mass, and the spatial distribution of the fragments are consistent with fragmentation dominated by a strong magnetic field. our observations support the theoretical prediction that the magnetic field plays a dominant role in the fragmentation process of massive turbulent clumps. | magnetically regulated fragmentation of a massive, dense, and turbulent clump |
polarized magnetic dipole (m1) emission lines provide important diagnostics for the magnetic field dominating the evolution of the solar corona. this paper advances a multi-line technique using specific combinations of m1 lines to infer the full vector magnetic field for regions of optically thin emission that can be localized along a given line of sight. our analytical formalism is a generalization of the "single-point inversion" approach introduced by plowman. we show that combinations of m1 transitions for which each is either a $j=1\to 0$ transition or has equal landé g-factors for the upper and lower levels contain degenerate spectropolarimetric information that prohibits the application of the single-point inversion technique. this may include the pair of fe xiii lines discussed by plowman. we identify the fe xiii 10747 å and si x 14301 å lines as one alternative combination for implementing this technique. our sensitivity analysis, based on coronal loop properties, suggests that for photon noise levels around 10-4 of the line intensity, which will be achievable with the national science foundation's daniel k. inouye solar telescope, magnetic fields with sufficient strength (∼10 g) and not severely inclined to the line of sight (≲35°) can be recovered with this method. degenerate solutions exist, though we discuss how added constraints may help resolve them or reduce their number. | using multi-line spectropolarimetric observations of forbidden emission lines to measure single-point coronal magnetic fields |
we present scexao/charis 1.1-2.4 μm integral field direct spectroscopy of the young hip 79124 triple system. hip 79124 is a member of the scorpius-centaurus association, consisting of an a0v primary with two low-mass companions at a projected separation of < 1″. thanks to the high quality wavefront corrections provided by scexao, both companions are decisively detected without the employment of any psf-subtraction algorithm to eliminate quasi-static noise. the spectrum of the outer c object is very well matched by upper scorpius m4 ± 0.5 standard spectra, with a teff = 2945 ± 100 k and a mass of ∼350 mjup. hip 79124 b is detected at a separation of only 180 mas in a highly-correlated noise regime, and it falls in the spectral range m6 ± 0.5 with teff = 2840 ± 190 k and ∼100 mjup. previous studies of stellar populations in sco-cen have highlighted a discrepancy in isochronal ages between the lower-mass and higher-mass populations. this could be explained either by an age spread in the region, or by conventional isochronal models failing to reproduce the evolution of low-mass stars. the hip 79124 system should be coeval, and therefore it provides an ideal laboratory to test these scenarios. we place the three components in a color-magnitude diagram and find that the models predict a younger age for the two low-mass companions (∼3 myr) than for the primary star (∼6 myr). these results imply that the omission of magnetic effects in conventional isochronal models inhibit them from reproducing early low-mass stellar evolution, which is further supported by the fact that new models that include such effects provide more consistent ages in the hip 79124 system. | isochronal age-mass discrepancy of young stars: scexao/charis integral field spectroscopy of the hip 79124 triple system |
in this paper, we present steady-state relativistic magnetohydrodynamic simulations that include a mass-load term to study the process of jet deceleration. the mass load mimics the injection of a proton-electron plasma from stellar winds within the host galaxy into initially pair plasma jets, with mean stellar mass-losses ranging from 10-14 to $10^{-9}\, {\mathrm{m}_\odot \, \mathrm{yr}^{-1}}$ . the spatial jet evolution covers $\sim 500\, {\rm pc}$ from jet injection in the grid at 10 pc from the jet nozzle. our simulations use a relativistic gas equation of state and a pressure profile for the ambient medium. we compare these simulations with previous dynamical simulations of relativistic, non-magnetized jets. our results show that toroidal magnetic fields can prevent fast jet expansion and the subsequent embedding of further stars via magnetic tension. in this sense, magnetic fields avoid a runaway deceleration process. furthermore, when the mass load is large enough to increase the jet density and produce fast, differential jet expansion, the conversion of magnetic energy flux into kinetic energy flux (i.e. magnetic acceleration), helps to delay the deceleration process with respect to non-magnetized jets. we conclude that the typical stellar population in elliptical galaxies cannot explain jet deceleration in classical fanaroff-riley type i radio galaxies. however, we observe a significant change in the jet composition, thermodynamical parameters, and energy dissipation along its evolution, even for moderate values of the mass load. | on the deceleration of fanaroff-riley class i jets: mass loading of magnetized jets by stellar winds |
we have studied the filaments extracted from the column density maps of the nearby lupus 1, 3, and 4 molecular clouds, derived from photometric maps observed with the herschel satellite. filaments in the lupus clouds have quite low column densities, with a median value of ∼1.5 × 1021 cm-2 and most have masses per unit length lower than the maximum critical value for radial gravitational collapse. indeed, no evidence of filament contraction has been seen in the gas kinematics. we find that some filaments, that on average are thermally subcritical, contain dense cores that may eventually form stars. this is an indication that in the low column density regime, the critical condition for the formation of stars may be reached only locally and this condition is not a global property of the filament. finally, in lupus we find multiple observational evidences of the key role that the magnetic field plays in forming filaments, and determining their confinement and dynamical evolution. | filaments in the lupus molecular clouds |
we consider the existence of a neutron star magnetic field by the detected cyclotron lines. we collected data on nine sources of high-mass x-ray binaries with supergiant companions as a test case for our model, to demonstrate their distribution and evolution. the wind velocity, spin period and magnetic field strength are studied under different mass loss rates. in our model, correlations between mass-loss rate and wind velocity are found and can be tested in further observations. we examine the parameter space where wind accretion is allowed, avoiding the barrier of rotating magnetic fields, with robust data on the magnetic field of neutron stars. our model shows that most sources (six of nine systems) can be fed by the wind with relatively slow velocity, and this result is consistent with previous predictions. in a few sources, our model cannot fit the standard wind accretion scenario. in these peculiar cases, other scenarios (disk formation, partial roche lobe overflow) should be considered. this would provide information about the evolutionary tracks of various types of binaries, and thus exhibit a clear dichotomy behavior in wind-fed x-ray binary systems. | on the possibility of disk-fed formation in supergiant high-mass x-ray binaries |
the paper presents magnetic field measurements for 15 chemically peculiar (cp) stars of subgroup 1b in the orion ob1 association. we have found that the proportion of stars with strong magnetic fields among these 15 cp stars is almost twice as large as in subgroup 1a. along with this, the age of subgroup 1b is estimated as 2 myr, and the age of subgroup 1a is in the order of 10 myr. the average root-mean-square magnetic field $\left\langle {{{b}e}} \right\rangle $ (all) for stars in subgroup 1b is 2.3 times higher than that for stars in subgroup 1a. the conclusions obtained fall within the concept of the fossil origin of large-scale magnetic fields in b and a stars, but the rate of field weakening with age appears anomalously high. we present our results as an important observational test for calibrating the theory of stellar magnetic field formation and evolution. | magnetic fields of cp stars in the orion ob1 association. iv. stars of subgroup 1b |
particle-accelerating colliding-wind binaries (pacwbs) are multiple systems made of early-type stars able to accelerate particles up to relativistic velocities. the relativistic particles can interact with different fields (magnetic or radiation) in the colliding-wind region and produce non-thermal emission. in many cases, non-thermal synchrotron radiation might be observable and thus constitute an indicator of the existence of a relativistic particle population in these multiple systems. to date, the catalogue of pacwbs includes about 40 objects spread over many stellar types and evolutionary stages, with no clear trend pointing to privileged subclasses of objects likely to accelerate particles. this paper aims at discussing critically some criteria for selecting new candidates among massive binaries. the subsequent search for non-thermal radiation in these objects is expected to lead to new detections of particle accelerators. on the basis of this discussion, some broad ideas for observation strategies are formulated. at this stage of the investigation of pacwbs, there is no clear reason to consider particle acceleration in massive binaries as an anomaly or even as a rare phenomenon. we therefore consider that several pacwbs will be detected in the forthcoming years, essentially using sensitive radio interferometers which are capable of measuring synchrotron emission from colliding-wind binaries. prospects for high-energy detections are also briefly addressed. | an investigation into the fraction of particle accelerators among colliding-wind binaries. towards an extension of the catalogue |
we assess the systematic uncertainties in stellar evolutionary calculations for low- to intermediate-mass, main-sequence stars. we compare published stellar tracks from several different evolution codes with our own tracks computed using the stellar codes stars and mesa. in particular, we focus on tracks of 1 and 3 m⊙ at solar metallicity. we find that the spread in the available 1 m⊙ tracks (computed before the recent solar composition revision) can be covered by tracks between 0.97-1.01 m⊙ computed with the stars code. we assess some possible causes of the origin of this uncertainty, including how the choice of input physics and the solar constraints used to perform the solar calibration affect the tracks. we find that for a 1 m⊙ track, uncertainties of around 10% in the initial hydrogen abundance and initial metallicity produce around a 2% error in mass. for the 3 m⊙ tracks, there is very little difference between the tracks from the various different stellar codes. the main difference comes in the extent of the main sequence, which we believe results from the different choices of the implementation of convective overshooting in the core. uncertainties in the initial abundances lead to a 1-2% error in the mass determination. these uncertainties cover only part of the total error budget, which should also include uncertainties in the input physics (e.g., reaction rates, opacities, convective models) and any missing physics (e.g., radiative levitation, rotation, magnetic fields). uncertainties in stellar surface properties such as luminosity and effective temperature will further reduce the accuracy of any potential mass determinations. | confronting uncertainties in stellar physics. ii. exploring differences in main-sequence stellar evolution tracks |
we summarize the results of a dedicated effort made between 2012 and 2019 to follow the evolution of the cyclotron line in her x-1 through repeated nustar observations. the previously observed nearly 20-year-long decay of the cyclotron line energy has ended in 2012: from then on, the pulse-phase-averaged flux-corrected cyclotron line energy has remained stable and constant at an average value of ecyc = (37.44 ± 0.07) kev (normalized to a flux level of 6.8 rxte/asm-cts s-1). the flux dependence of ecyc discovered in 2007 is now measured with high precision, giving a slope of (0.675 ± 0.075) kev/(asm-cts s-1), corresponding to an increase of 6.5% of ecyc for an increase in flux by a factor of two. we also find that all line parameters as well as the continuum parameters show a correlation with x-ray flux. while a correlation between ecyc and x-ray flux (both positive and negative) is now known for several accreting binaries with various suggestions for the underlying physics, the phenomenon of a long-term decay has so far only been seen in her x-1 and vela x-1, with far less convincing explanations. | cyclotron line energy in hercules x-1: stable after the decay |
context. surface differential rotation (dr) is one major ingredient of the magnetic field generation process in the sun and likely in other stars. the term solar-like differential rotation describes the observation that solar equatorial regions rotate faster than polar ones. the opposite effect of polar regions rotating faster than equatorial ones (termed as antisolar dr) has only been observed in a few stars, although there is evidence from theoretical dynamo models.aims: we present a new method of detecting the sign of dr (i.e., solar-like or antisolar dr) by analyzing long-term high-precision light curves with the lomb-scargle periodogram.methods: we compute the lomb-scargle periodogram and identify a set of significant periods pk, which we associate with active regions located at different latitudes on the stellar surface. if detectable, the first harmonics (p_k') of these periods were identified to compute their peak-height-ratios rk:= h(p'k)/h(pk) . spots rotating at lower latitudes generate less sine-shaped light curves, which requires additional power in the harmonics, and results in larger ratios rk. comparing different ratios rk and the associated periods pk yields information about the spot latitudes, and reveals the sign of dr.results: we tested our method on different sets of synthetic light curves all exhibiting solar-like dr. the number of cases where our method detects antisolar dr is the false-positive rate of our method. depending on the set of light curves, the noise level, the required minimum peak separation, and the presence or absence of spot evolution, our method fails to detect the correct sign in at most 20%. we applied our method to 50 kepler g stars and found 21-34 stars with solar-like dr and 5-10 stars with antisolar dr, depending on the minimum peak separation.conclusions: the method is able to determine the sign of dr in a statistical way with a low false-positive rate. applying our method to real data might suggest that - within the uncertainties - antisolar dr was detected in 5-10 kepler stars. table 3 is available in electronic form at http://www.aanda.org | discriminating solar and antisolar differential rotation in high-precision light curves |
it has been suggested that the absence of macroturbulence in the atmosphere of ngc 1624-2 is due its strong magnetic field (the strongest known for a massive o star) suppressing convection in its outer layers, removing the mechanism thought responsible for the observed macroturbulence in stars with lower field strengths. here, we develop and apply a criterion for a uniform magnetic field to suppress convection in stellar envelopes in which radiation pressure is a significant contributor to hydrostatic balance. we find upper mass limits of ∼55 and ∼30 m⊙ for magnetic suppression to be possible in zero-age main-sequence and terminal-age main-sequence stars, respectively. for evolved stars, magnetic suppression of convection can significantly alter the stars' evolution. for ngc 1624-2, we find that a polar dipole strength of 16.5 ± 5.9 kg is required to suppress convection, in good agreement with the value ∼20 kg measured by spectropolarimetry. | magnetic inhibition of convection in o-star envelopes |
in the context of the high resolution, high signal-to-noise ratio, high sensitivity, spectropolarimetric survey britepol, which complements observations by the brite constellation of nanosatellites for asteroseismology, we are looking for and measuring the magnetic field of all stars brighter than v = 4. in this paper, we present circularly polarized spectra obtained with harpspol at eso in la silla (chile) and espadons at cfht (hawaii) for three hot evolved stars: ι car, hr 3890 and ɛ cma. we detected a magnetic field in all three stars. each star has been observed several times to confirm the magnetic detections and check for variability. the stellar parameters of the three objects were determined and their evolutionary status was ascertained employing evolution models computed with the geneva code. ɛ cma was already known and is confirmed to be magnetic, but our modelling indicates that it is located near the end of the main sequence, i.e. it is still in a core hydrogen burning phase. ι car and hr 3890 are the first discoveries of magnetic hot supergiants located well after the end of the main sequence on the hertzsprung-russell diagram. these stars are probably the descendants of main-sequence magnetic massive stars. their current field strength (a few g) is compatible with magnetic flux conservation during stellar evolution. these results provide observational constraints for the development of future evolutionary models of hot stars including a fossil magnetic field. | discovery of magnetic a supergiants: the descendants of magnetic main-sequence b stars |
the hosking integral, which characterizes magnetic helicity fluctuations in subvolumes, is known to govern the decay of magnetically dominated turbulence. here, we show that, when the evolution of the magnetic field is controlled by the motion of electrons only, as in neutron star crusts, the decay of the magnetic field is still controlled by the hosking integral, but now it has effectively different dimensions than in ordinary magnetohydrodynamic (mhd) turbulence. this causes the correlation length to increase with time $t$ like $t^{4/13}$ instead of $t^{4/9}$ in mhd. the magnetic energy density decreases like $t^{-10/13}$, which is slower than in mhd, where it decays like $t^{-10/9}$. these new analytic results agree with earlier numerical simulations for the non-helical hall cascade. | hosking integral in non-helical hall cascade |
large-scale magnetic fields play a vital role in determining the angular momentum transport and generating jets/outflows in accreting systems, yet their origins remain poorly understood. we focus on radiatively inefficient accretion flows (riafs) around black holes (bhs), and conduct 3d general-relativistic magnetohydrodynamic simulations using the athena++ code. we first reconfirm that the magnetorotational instability driven dynamo in the riaf alone does not spontaneously form a magnetically arrested disk (mad), conducive for strong-jet formation. we next investigate the other possibility, where the large-scale magnetic fields are advected inward from external sources (e.g., the companion star in x-ray binaries and the magnetized ambient medium in active galactic nuclei). although the actual configurations of the external fields could be complex and uncertain, they are likely to be closed. as a first study, we treat them as closed field loops of different sizes, shapes, and field strengths. unlike earlier studies of flux transport, where the magnetic flux is injected into the initial laminar flow, we inject the magnetic field loops into the quasi-stationary turbulent riaf in inflow equilibrium, then follow their evolution. we find that a substantial fraction (~15%-40%) of the flux injected at large radii reaches the bh, with a weak dependence on the loop parameters, except when the loops are injected at high latitudes, away from the midplane. the relatively high efficiency of the flux transport observed in our study hints that a mad might easily be formed relatively close to the bh, provided that a source of the large-scale field exists at larger radii. | magnetic flux transport in radiatively inefficient accretion flows and the pathway toward a magnetically arrested disk |
we develop new angular momentum evolution models for stars with masses of 0.5 to 1.6 m⊙ and from the pre-main-sequence (pms) through the end of their main-sequence (ms) lifetime. the parametric models include magnetic braking based on numerical simulations of magnetized stellar winds, mass-loss-rate prescription, core-envelope decoupling as well as disc locking phenomena. we have also accounted for recent developments in modelling dramatically weakened magnetic braking in stars more evolved than the sun. we fit the free parameters in our model by comparing model predictions to rotational distributions of a number of stellar clusters as well as individual field stars. our model reasonably successfully reproduces the rotational behaviour of stars during the pms phase to the zero-age main-sequence (zams) spin-up, sudden zams spin-down and convergence of the rotation rates afterwards. we find that including core-envelope decoupling improves our models, especially for low-mass stars at younger ages. in addition, by accounting for the almost complete suppression of magnetic braking at slow-spin periods, we provide better fits to observations of stellar rotations compared to previous models. | a semi-empirical model for magnetic braking of solar-type stars |
we analyze nonlinear evolution of torsional alfvén waves in a straight magnetic flux tube filled in with a low-β plasma, and surrounded with a plasma of lower density. such magnetic tubes model, in particular, a segment of a coronal loop or a polar plume. the wavelength is taken comparable to the tube radius. we perform a numerical simulation of the wave propagation using ideal magnetohydrodynamics. we find that a torsional wave nonlinearly induces three kinds of compressive flows: the parallel flow at the alfvén speed, which constitutes a bulk plasma motion along the magnetic field, the tube wave, and also transverse flows in the radial direction, associated with sausage fast magnetoacoustic modes. in addition, the nonlinear torsional wave steepens and its propagation speed increases. the latter effect leads to the progressive distortion of the torsional wave front, i.e., nonlinear phase mixing. because of the intrinsic non-uniformity of the torsional wave amplitude across the tube radius, the nonlinear effects are more pronounced in regions with higher wave amplitudes. they are always absent at the axes of the flux tube. in the case of a linear radial profile of the wave amplitude, the nonlinear effects are localized in an annulus region near the tube boundary. thus, the parallel compressive flows driven by torsional alfvén waves in the solar and stellar coronae, are essentially non-uniform in the perpendicular direction. the presence of additional sinks for the wave energy reduces the efficiency of the nonlinear parallel cascade in torsional alfvén waves. | nonlinear evolution of short-wavelength torsional alfvén waves |
magnetic activity of stars like the sun evolves in time because of spin-down owing to angular momentum removal by a magnetized stellar wind. these magnetic fields are generated by an internal dynamo driven by convection and differential rotation. spin-down therefore converges at an age of about 700 myr for solar-mass stars to values uniquely determined by the stellar mass and age. before that time, however, rotation periods and their evolution depend on the initial rotation period of a star after it has lost its protostellar/protoplanetary disk. this non-unique rotational evolution implies similar non-unique evolutions for stellar winds and for the stellar high-energy output. i present a summary of evolutionary trends for stellar rotation, stellar wind mass loss and stellar high-energy output based on observations and models. | the sun through time |
although galactic outflows play a key role in our understanding of the evolution of galaxies, the exact mechanism by which galactic outflows are driven is still far from being understood and, therefore, our understanding of associated feedback mechanisms that control the evolution of galaxies is still plagued by many enigmas. in this work, we present a simple toy model that can provide insight on how non-axisymmetric instabilities in galaxies (bars, spiral arms, warps) can lead to local exponential magnetic field growth by radial flows beyond the equipartition value by at least two orders of magnitude on a timescale of a few 100 myr. our predictions show that the process can lead to galactic outflows in barred spiral galaxies with a mass-loading factor η ≈ 0.1, in agreement with our numerical simulations. moreover, our outflow mechanism could contribute to an understanding of the large fraction of barred spiral galaxies that show signs of galactic outflows in the chang-es survey. extending our model shows the importance of such processes in high-redshift galaxies by assuming equipartition between magnetic energy and turbulent energy. simple estimates for the star formation rate in our model together with cross correlated masses from the star-forming main sequence at redshifts z ~ 2 allow us to estimate the outflow rate and mass-loading factors by non-axisymmetric instabilities and a subsequent radial inflow dynamo, giving mass-loading factors of η ≈ 0.1 for galaxies in the range of m ⋆ = 109-1012 m ⊙, in good agreement with recent results of sinfoni and kmos 3d. | driving galactic outflows with magnetic fields at low and high redshift |
this paper describes 3d simulations of the formation of collapsing cold clumps via thermal instability inside a larger cloud complex. the initial condition was a diffuse atomic, stationary, thermally unstable, 200 pc diameter spherical cloud in pressure equilibrium with low-density surroundings. this was seeded with 10 per cent density perturbations at the finest initial grid level (0.29 pc) around nh= 1.1 cm-3 and evolved with self-gravity included. no magnetic field was imposed. resimulations at a higher resolution of a region extracted from this simulation (down to 0.039 pc) show that the thermal instability forms sheets, then filaments, and finally clumps. the width of the filaments increases overtime, in one particular case from 0.26 to 0.56 pc. thereafter, clumps with sizes of around 5 pc grow at the intersections of filaments. 21 distinct clumps, with properties similar to those observed in molecular clouds, are found using the fellwalker algorithm to find minima in the gravitational potential. not all of these are gravitationally bound, but the convergent nature of the flow and increasing central density suggest they are likely to form stars. further simulation of the most massive clump shows the gravitational collapse to a density >106 cm-3. these results provide realistic initial conditions that can be used to study feedback in individual clumps, interacting clumps, and the entire molecular cloud complex. | sheets, filaments, and clumps - high-resolution simulations of how the thermal instability can form molecular clouds |
most massive galaxies are thought to contain a supermassive black hole in their centre surrounded by a tenuous gas environment, leading to no significant emission. in these quiescent galaxies, tidal disruption events represent a powerful detection method for the central black hole. following the disruption, the stellar debris evolves into an elongated gas stream, which partly falls back towards the disruption site and accretes on to the black hole producing a luminous flare. using an analytical treatment, we investigate the interaction between the debris stream and the gas environment of quiescent galaxies. although we find dynamical effects to be negligible, we demonstrate that kelvin-helmholtz instability can lead to the dissolution of the stream into the ambient medium before it reaches the black hole, likely dimming the associated flare. this result is robust against the presence of a typical stellar magnetic field and fast cooling within the stream. furthermore, we find this effect to be enhanced for disruptions involving more massive black holes and/or giant stars. consequently, although disruptions of evolved stars have been proposed as a useful probe of black holes with masses ≳ 108 m⊙, we argue that the associated flares are likely less luminous than expected. | bad prospects for the detection of giant stars' tidal disruption: effect of the ambient medium on bound debris |
we report the evolution of the x-ray pulsations of exo 2030+375 during its 2021 outburst using the observations from insight-hxmt. based on the accretion torque model, we study the correlation between the spin frequency derivatives and the luminosity. pulsations can be detected in the energy band of 1-160 kev. the pulse profile evolves significantly with luminosity during the outburst, leading to that the whole outburst can be divided into several parts with different characteristics. the evolution of the pulse profile reveals the transition between the supercritical (fan-beam dominated) and the subcritical accretion (pencil-beam dominated) mode. from the accretion torque model and the critical luminosity model, based on a distance of 7.1 kpc, the inferred magnetic fields are (0.41 - 0.74) × 1012 and (3.48 - 3.96) × 1012 g, respectively, or based on a distance of 3.6 kpc, the estimated magnetic fields are (2.4 - 4.3) × 1013 and (0.98 - 1.11) × 1012 g, respectively. two different sets of magnetic fields both support the presence of multipole magnetic fields of the neutron star. | timing analysis of exo 2030+375 during its 2021 giant outburst observed with insight-hxmt |
seismic and magnetic observations have suggested the presence of a stably stratified layer atop earth's core. such a layer could affect the morphology of the geomagnetic field and the evolution of the core, but the precise impact of this layer depends largely on its internal dynamics. among other physical phenomena, stratified layers host internal gravity waves (igw), which can be excited by adjacent convective motions. internal waves are known to play an important role on the large-scale dynamics of the earth's climate and on the long-term evolution of stars. yet, they have received relatively little attention in the earth's outer core so far and deserve detailed investigations in this context. here, we make a first step in that direction by running numerical simulations of igw in a non-rotating spherical shell in which a stratified layer lies on top of a convective region. we use a nonlinear equation of state to produce self-consistently such a two-layer system. both propagating waves and global modes coexist in the stratified layer. we characterize the spectral properties of these waves and find that energy is distributed across a wide range of frequencies and length scales, that depends on the prandtl number. for the control parameters considered and in the absence of rotational and magnetic effects, the mean kinetic energy in the layer is about 0.1 per cent that of the convective region. igw produce perturbations in the gravity field that may fall within the sensitivity limit of present-day instruments and could potentially be detected in available data. we finally provide a road map for future, more geophysically realistic, studies towards a more thorough understanding of the dynamics and impact of internal waves in a stratified layer atop earth's core. | internal gravity waves in a stratified layer atop a convecting liquid core in a non-rotating spherical shell |
the role of magnetic fields in galaxy evolution is still an unsolved question in astrophysics. we have previously shown that magnetic fields play a crucial role in major mergers between disc galaxies; in hydrodynamic simulations of such mergers, the auriga model produces compact remnants with a distinctive bar and ring morphology. in contrast, in magnetohydrodynamic (mhd) simulations, remnants form radially extended discs with prominent spiral arm structure. in this paper, we analyse a series of cosmological 'zoom-in' simulations of major mergers and identify exactly how magnetic fields are able to alter the outcome of the merger. we find that magnetic fields modify the transport of angular momentum, systematically hastening the merger progress. the impact of this altered transport depends on the orientation of the field, with a predominantly non-azimuthal (azimuthal) orientation increasing the central baryonic concentration (providing support against collapse). both effects act to suppress an otherwise existent bar-instability, which in turn leads to a fundamentally different morphology and manifestation of feedback. we note, in particular, that stellar feedback is substantially less influential in mhd simulations, which allows for the later accretion of higher angular momentum gas and the subsequent rapid radial growth of the remnant disc. a corollary of the increased baryonic concentration in mhd simulations is that black holes are able to grow twice as large, although this turns out to have little impact on the remnant's development. our results show that galaxy evolution cannot be modelled correctly without including magnetic fields. | the impact of magnetic fields on cosmological galaxy mergers - ii. modified angular momentum transport and feedback |
short gamma-ray bursts that are followed by long-duration x-ray plateaus may be powered by the birth, and hydrodynamic evolution, of magnetars from compact binary coalescence events. if the rotation and magnetic axes of the system are not orthogonal to each other, the star will undergo free precession, leading to fluctuations in the luminosity of the source. in some cases, precession-induced modulations in the spin-down power may be discernible in the x-ray flux of the plateau. in this work, 25 x-ray light curves associated with bursts exhibiting a plateau are fitted to luminosity profiles appropriate for precessing, oblique rotators. based on the akaike information criterion, 16 $(64{{\ \rm per\ cent}})$ of the magnetars within the sample display either moderate or strong evidence for precession. additionally, since the precession period of the star is directly tied to its quadrupolar ellipticity, the fits allow for an independent measure of the extent to which the star is deformed by internal stresses. assuming these deformations arise due to a mixed poloidal-toroidal magnetic field, we find that the distribution of magnetic-energy ratios is bimodal, with data points clustering around energetically equal and toroidally dominated partitions. implications of this result for gravitational-wave emission and dynamo activity in newborn magnetars are discussed. | precessing magnetars as central engines in short gamma-ray bursts |
we study the four-dimensional relationships between magnetic activity, rotation, mass, and age for solar-type stars in the age range 5-25 myr. this is the late-pre-main-sequence (l-pms) evolutionary phase when rapid changes in a star's interior may lead to changes in the magnetic dynamo mechanisms. we carefully derive rotational periods and spot sizes for 471 members of several l-pms open clusters using photometric light curves from the zwicky transient facility. magnetic activity was measured in our previous chandra-based study, and additional rotational data were obtained from other work. several results emerge. mass-dependent evolution of rotation through the l-pms phase agrees with astrophysical models of stellar angular momentum changes, although the data suggest a subpopulation of stars with slower initial rotations than commonly assumed. there is a hint of the onset of unsaturated tachoclinal dependency of x-ray activity on rotation, as reported by argiroffi et al., but this result is not confidently confirmed. both x-ray luminosity and starspot area decrease approximately as t -1 for solar-mass stars, suggesting that spot magnetic fields are roughly constant and l-pms stars follow the universal solar-scaling law between the x-ray luminosity and surface magnetic flux. assuming convective dynamos are dominant, theoretical magnetic fluxes fail to reveal the universal law for l-pms stars that enter late henyey tracks. altogether we emerge with a few lines of evidence suggesting that the transition from the turbulent to solar-type dynamo occurs at the later stages of l-pms evolution as stars approach the zero-age main sequence. | magnetic activity-rotation-age-mass relations in late-pre-main-sequence stars |
with forty years since the discovery of the first cyclotron line in her x-1, there have been remarkable advancements in the field related to the study of the physics of accreting neutron stars - cyclotron lines have been a major torchbearer in this regard, from being the only direct estimator of the magnetic field strength, a tracer of accretion geometry and an indicator of the emission beam in these systems. the main flurry of activities have centred around studying the harmonic separations, luminosity dependence, pulse phase dependence and more recently, the shapes of the line and the trend for long-term evolution in the line energy. this article visits the important results related to cyclotron lines since its discovery and reviews their significance. an emphasis is laid on pulse phase resolved spectroscopy and the important clues a joint timing and spectral study in this context can provide, to build a complete picture for the physics of accretion and hence x-ray emission in accreting neutron stars. | cyclotron lines: from magnetic field strength estimators to geometry tracers in neutron stars |
a high-resolution spectropolarimetric survey of all (573) stars brighter than magnitude v= 4 has been undertaken with narval at tbl, espadons at cfht, and harpspol at eso, as a ground-based support to the brite constellation of nano-satellites in the framework of the ground-based observing team (gbot). the goal is to detect magnetic fields in brite targets, as well as to provide one very high-quality, high-resolution spectrum for each star. the survey is nearly completed and already led to the discovery of 42 new magnetic stars and the confirmation of several other magnetic detections, including field discoveries in, e.g., an am star, two δ scuti stars, hot evolved stars, and stars in clusters. follow-up spectropolarimetric observations of approximately a dozen of these magnetic stars have already been performed to characterise their magnetic field configuration and strength in detail. | the brite spectropolarimetric program |
context. runaway stars produce bowshocks that are usually observed at infrared (ir) wavelengths. non-thermal radio emission has been detected so far only from the bowshock of bd+43°3654, whereas the detection of non-thermal radiation from these bowshocks at high energies remains elusive.aims: we aim at characterising in detail the radio, x-ray, and γ-ray emission from stellar bowshocks accounting for the structure of the region of interaction between the stellar wind and its environment.methods: we develop a broadband-radiative, multi-zone model for stellar bowshocks that takes into account the spatial structure of the emitting region and the observational constraints. the model predicts the evolution and the emission of the relativistic particles accelerated and streaming together with the shocked flow.results: we present broadband non-thermal spectral energy distributions for different scenarios, synthetic radio-cm synchrotron maps that reproduce the morphology of bd+43°3654, and updated predictions in x-ray and γ-ray energy ranges. we also compare the results of the multi-zone model applied in this work with those of a refined one-zone model.conclusions: a multi-zone model provides better constraints than a one-zone model on the relevant parameters, namely the magnetic field intensity and the amount of energy deposited in non-thermal particles. however, one-zone models can be improved by carefully characterising the intensity of the ir dust photon field and the escape rate of the plasma from the shocked region. finally, comparing observed radio maps with those obtained from a multi-zone model enables constraints to be obtained on the direction of stellar motion with respect to the observer. | multi-zone non-thermal radiative model for stellar bowshocks |
context. magnetic fields can significantly affect the star formation process. the theory of the magnetically driven collapse in a uniform field predicts that the contraction initially happens along the field lines. when the gravitational pull grows strong enough, the magnetic field lines pinch inwards, giving rise to a characteristic hourglass shape.aims: we investigate the magnetic field structure of a young class 0 object, iras 15398-3359, embedded in the lupus i cloud. previous observations at large scales have suggested that this source evolved in an highly magnetised environment. this object thus appears to be an ideal candidate to study the magnetically driven core collapse in the low-mass regime.methods: we performed polarisation observations of iras 15398-3359 at 214 μm using the sofia telescope, thus tracing the linearly polarised thermal emission of cold dust.results: our data unveil a significant bend of the magnetic field lines from the gravitational pull. the magnetic field appears ordered and aligned with the large-scale b-field of the cloud and with the outflow direction. we estimate a magnetic field strength of b = 78 μg, which is expected to be accurate within a factor of two. the measured mass-to-flux parameter is λ = 0.95, indicating that the core is in a transcritical regime. the stokes maps are also available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/j/a+a/631/a154 | magnetic properties of the protostellar core iras 15398-3359 |
the evolution of the gravitational-wave phase in the signal produced by inspiralling binaries of compact stars is modified by the nonzero deformability of the two stars. hence, the measurement of these corrections has the potential of providing important information on the equation of state of nuclear matter. extensive work has been carried out over the last decade to quantify these corrections, but it has so far been restricted to stars with zero intrinsic magnetic fields. while the corrections introduced by the magnetic tension and magnetic pressure are expected to be subdominant, it is nevertheless useful to determine the precise conditions under which these corrections become important. to address this question, we have carried out a second-order perturbative analysis of the tidal deformability of magnetized compact stars under a variety of magnetic-field strengths and equations of state describing either neutron stars or quark stars. overall, we find that magnetically induced corrections to the tidal deformability will produce changes in the gravitational-wave phase evolution that are unlikely to be detected for a realistic magnetic field i.e., b ∼1010- 1012 g . at the same time, if the magnetic field is unrealistically large, i.e., b ∼1016 g , these corrections would produce a sizeable contribution to the phase evolution, especially for quark stars. in the latter case, and if the neglected higher-order terms will remain negligible also for very high magnetic fields, the induced phase differences would represent a unique tool to measure the properties of the magnetic fields, providing information that is otherwise hard to quantify. | tidal deformability and gravitational-wave phase evolution of magnetized compact-star binaries |
our understanding of large-scale magnetic fields in stellar radiative zones remains fragmented and incomplete. such magnetic fields, which must be produced by some form of dynamo mechanism, are thought to dominate angular-momentum transport, making them crucial to stellar evolution. a major difficulty is the effect of stable stratification, which generally suppresses dynamo action. we explore the effects of stable stratification on mean-field dynamo theory with a particular focus on a non-helical large-scale dynamo (lsd) mechanism known as the magnetic shear-current effect. we find that the mechanism is robust to increasing stable stratification as long as the original requirements for its operation are met: a source of shear and non-helical magnetic fluctuations (e.g. from a small-scale dynamo). both are plausibly sourced in the presence of differential rotation. our idealized direct numerical simulations, supported by mean-field theory, demonstrate the generation of near equipartition large-scale toroidal fields. additionally, a scan over magnetic reynolds number shows no change in the growth or saturation of the lsd, providing good numerical evidence of a dynamo mechanism resilient to catastrophic quenching, which has been an issue for helical dynamos. these properties - the absence of catastrophic quenching and robustness to stable stratification - make the mechanism a plausible candidate for generating in situ large-scale magnetic fields in stellar radiative zones. | on large-scale dynamos with stable stratification and the application to stellar radiative zones |
insufficient numerical resolution of grid-based, direct numerical simulations (dns) hampers the development of instability-driven turbulence at small (unresolved) scales. as an alternative to dns, sub-grid models can potentially reproduce the effects of turbulence at small scales in terms of the resolved scales, and hence can capture physical effects with less computational resources. we present a new sub-grid model, the mhd-instability-induced-turbulence (minit) mean-field model. minit is a physically motivated model based on the evolution of the turbulent (maxwell, reynolds, and faraday) stress tensors and their relation with the turbulent energy densities of the magnetorotational (mri) and parasitic instabilities, modelled with two partial differential evolution equations with stiff source terms. their solution allows obtaining the turbulent stress tensors through the constant coefficients that link them to the energy densities. the model is assessed using data from mri in-box dns and applying a filtering operation to compare the filtered data with that from the model. using the l2-norm as the metric for the comparison, we find less than one order-of-magnitude difference between the two sets of data. no dependence on filter size or length scale of unresolved scales is found, as opposed to results using the gradient model (which we also use to contrast our model) in which the l2-norm of some of the stresses increases with filter size. we conclude that minit can help dns by properly capturing small-scale turbulent stresses which has potential implications on the dynamics of highly magnetized rotating compact objects, such as those formed during binary neutron star mergers. | assessment of a new sub-grid model for magnetohydrodynamical turbulence. i. magnetorotational instability |
we present the analysis of the brightest flare that was recorded in the insight-hmxt data set in a broad energy range (2-200 kev) from the microquasar grs 1915+105 during an unusual low-luminosity state. this flare was detected by insight-hxmt among a series of flares during 2019 june 2 utc 16:37:06-20:11:36 with a 2-200 kev luminosity of 3.4-7.27 × 1038 erg s-1. basing on the broadband spectral analysis, we find that the flare spectrum shows different behaviors during bright and faint epochs. the spectrum of the flare can be fitted with a model dominated by a power-law component. additional components show up in the bright epoch with a hard tail and in the faint epoch with an absorption line of ∼6.78 kev. the reflection component of the latter is consistent with an inner disk radius ∼five times larger than that of the former. these results on the giant flare during the "unusual" low-luminosity state of grs 1915+105 may suggest that the source experiences a possible fast transition from a jet-dominated state to a wind-dominated state. we speculate that the evolving accretion disk and the large-scale magnetic field may play important roles in this peculiar huge flare. | insight-hxmt observations of a possible fast transition from the jet- to wind-dominated state during a huge flare of grs 1915+105 |
deep r-band ccd linear polarimetry collected for fields with lines of sight toward the lupus i molecular cloud is used to investigate the properties of the magnetic field within this molecular cloud. the observed sample contains about 7000 stars, almost 2000 of them with a polarization signal-to-noise ratio larger than 5. these data cover almost the entire main molecular cloud and also sample two diffuse infrared patches in the neighborhood of lupus i. the large-scale pattern of the plane-of-sky projection of the magnetic field is perpendicular to the main axis of lupus i, but parallel to the two diffuse infrared patches. a detailed analysis of our polarization data combined with the herschel/spire 350 μm dust emission map shows that the principal filament of lupus i is constituted by three main clumps that are acted on by magnetic fields that have different large-scale structural properties. these differences may be the reason for the observed distribution of pre- and protostellar objects along the molecular cloud and the cloud’s apparent evolutionary stage. on the other hand, assuming that the magnetic field is composed of large-scale and turbulent components, we find that the latter is rather similar in all three clumps. the estimated plane-of-sky component of the large-scale magnetic field ranges from about 70 to 200 μg in these clumps. the intensity increases toward the galactic plane. the mass-to-magnetic flux ratio is much smaller than unity, implying that lupus i is magnetically supported on large scales. based on observations collected at the observatório do pico dos dias, operated by laboratório nacional de astrofísica (lna/mcti, brazil). | tracing the magnetic field morphology of the lupus i molecular cloud |
the geomagnetic field has been continuously monitored from low-earth orbit (leo) since 1999, complementing ground-based observatory data by providing calibrated scalar and vector measurements with global coverage. the successful three-satellite esa swarm constellation is expected to remain in operation up to at least 2025. further monitoring the field from space with high-precision absolute magnetometry beyond that date is of critical importance for improving our understanding of dynamics of the multiple components of this field, as well as that of the ionospheric environment. here, we will report on the latest status of the nanomagsat project, which aims to deploy and operate a new constellation concept of three identical 16u nanosatellites, using two inclined (approximately 60°) and one polar leo, as well as an innovative payload including an advanced miniaturized absolute scalar and self-calibrated vector magnetometer (mam) combined with a set of precise star trackers (str), a compact high-frequency field magnetometer (hfm, sharing subsystems with the mam), a multi-needle langmuir probe (m-nlp) and dual frequency gnss receivers. the data to be produced will at least include 1 hz absolutely calibrated and oriented magnetic vector field (using the mam and str), 2 khz very low noise magnetic scalar (using the mam) and vector (using the hfm) field, 2 khz local electron density (using the m-nlp) as well as precise timing, location and tec products. in addition to briefly presenting the nanosatellite and constellation concepts, as well as the evolving programmatic status of the mission (which already underwent a consolidation study funded by the esa scout programme), this presentation will illustrate through a number of e2e simulations the ability of nanomagsat to complement and improve on many of the science goals of the swarm mission at a much lower cost, and to bring innovative science capabilities for ionospheric investigations. nanomagsat could form the basis of a permanent collaborative constellation of nanosatellites for low-cost long-term monitoring of the geomagnetic field and ionospheric environment from space. | nanomagsat, a 16u nanosatellite constellation high-precision magnetic project to initiate permanent low-cost monitoring of the earth's magnetic field and ionospheric environment |
context. dwarf novæ (dne) and low mass x-ray binaries (lmxbs) show eruptions that are thought to be due to a thermal-viscous instability in their accretion disk. these eruptions provide constraints on angular momentum transport mechanisms.aims: we explore the idea that angular momentum transport could be controlled by the dynamical evolution of the large-scale magnetic field. we study the impact of different prescriptions for the magnetic field evolution on the dynamics of the disk. this is a first step in confronting the theory of magnetic field transport with observations.methods: we developed a version of the disk instability model that evolves the density, the temperature, and the large-scale vertical magnetic flux simultaneously. we took into account the accretion driven by turbulence or by a magnetized outflow with prescriptions taken, respectively, from shearing box simulations or self-similar solutions of magnetized outflows. to evolve the magnetic flux, we used a toy model with physically motivated prescriptions that depend mainly on the local magnetization β, where β is the ratio of thermal pressure to magnetic pressure.results: we find that allowing magnetic flux to be advected inwards provides the best agreement with dne light curves. this leads to a hybrid configuration with an inner magnetized disk, driven by angular momentum losses to an mhd outflow, sharply transiting to an outer weakly-magnetized turbulent disk where the eruptions are triggered. the dynamical impact is equivalent to truncating a viscous disk so that it does not extend down to the compact object, with the truncation radius dependent on the magnetic flux and evolving as ṁ-2/3.conclusions: models of dne and lmxb light curves typically require the outer, viscous disk to be truncated in order to match the observations. there is no generic explanation for this truncation. we propose that it is a natural outcome of the presence of large-scale magnetic fields in both dne and lmxbs, with the magnetic flux accumulating towards the center to produce a magnetized disk with a fast accretion timescale. | magnetic field transport in compact binaries |
stellar winds govern the angular momentum evolution of solar-like stars throughout their main-sequence lifetime. the efficiency of this process depends on the geometry of the star's magnetic field. there has been a rapid increase recently in the number of stars for which this geometry can be determined through spectropolarimetry. we present a computationally efficient method to determine the 3d geometry of the stellar wind and to estimate the mass-loss rate and angular momentum loss rate based on these observations. using solar magnetograms as examples, we quantify the extent to which the values obtained are affected by the limited spatial resolution of stellar observations. we find that for a typical stellar surface resolution of 20o-30o, predicted wind speeds are within 5 per cent of the value at full resolution. mass-loss rates and angular momentum loss rates are within 5-20 per cent. in contrast, the predicted x-ray emission measures can be underestimated by one-to-two orders of magnitude, and their rotational modulations by 10-20 per cent. | estimating stellar wind parameters from low-resolution magnetograms |
planck galactic cold clumps (pgccs) possibly represent the early stages of star formation. to understand better the properties of pgccs, we studied 16 pgccs in the l1495 cloud with molecular lines and continuum data from herschel, jcmt/scuba-2, and the pmo 13.7 m telescope. thirty dense cores were identified in 16 pgccs from 2d gaussian fitting. the dense cores have dust temperatures of t d = 11-14 k, and h2 column densities of {n}{{{h}}2} = (0.36-2.5) × 1022 cm-2. we found that not all pgccs contain prestellar objects. in general, the dense cores in pgccs are usually at their earliest evolutionary stages. all the dense cores have non-thermal velocity dispersions larger than the thermal velocity dispersions from molecular line data, suggesting that the dense cores may be turbulence-dominated. we have calculated the virial parameter α and found that 14 of the dense cores have α <2, while 16 of the dense cores have α >2. this suggests that some of the dense cores are not bound in the absence of external pressure and magnetic fields. the column density profiles of dense cores were fitted. the sizes of the flat regions and core radii decrease with the evolution of dense cores. co depletion was found to occur in all the dense cores, but is more significant in prestellar core candidates than in protostellar or starless cores. the protostellar cores inside the pgccs are still at a very early evolutionary stage, sharing similar physical and chemical properties with the prestellar core candidates. | the properties of planck galactic cold clumps in the l1495 dark cloud |
context. the solar dynamo consists of a process that converts poloidal magnetic field to toroidal magnetic field followed by a process that creates new poloidal field from the toroidal field.aims: our aim is to observe the poloidal and toroidal fields relevant to the global solar dynamo and to see if their evolution is captured by a babcock-leighton dynamo.methods: we used synoptic maps of the surface radial field from the kpnso/vt and solis observatories, to construct the poloidal field as a function of time and latitude; we also used full disk images from wilcox solar observatory and soho/mdi to infer the longitudinally averaged surface azimuthal field. we show that the latter is consistent with an estimate of the longitudinally averaged surface azimuthal field due to flux emergence and therefore is closely related to the subsurface toroidal field.results: we present maps of the poloidal and toroidal magnetic fields of the global solar dynamo. the longitude-averaged azimuthal field observed at the surface results from flux emergence. at high latitudes this component follows the radial component of the polar fields with a short time lag of between 1-3 years. the lag increases at lower latitudes. the observed evolution of the poloidal and toroidal magnetic fields is described by the (updated) babcock-leighton dynamo model. | observing and modeling the poloidal and toroidal fields of the solar dynamo |
sgr j1745-2900 was detected from its outburst activity in 2013 april and it was the first soft gamma repeater (sgr) detected near the center of the galaxy (sagittarius a*). we use 3.5 yr chandra x-ray light-curve data to constrain some neutron star (ns) geometric parameters. we assume that the flux modulation comes from hot spots on the stellar surface. our model includes the ns mass, radius, a maximum of three spots of any size, temperature and positions, and general relativistic effects. we find that the light curve of sgr j1745-2900 could be described by either two or three hot spots. the ambiguity is due to the small amount of data, but our analysis suggests that one should not disregard the possibility of multi-spots (due to a multipolar magnetic field) in highly magnetized stars. for the case of three hot spots, we find that they should be large and have angular semiapertures ranging from 16° to 67°. the large size found for the spots points to a magnetic field with a nontrivial poloidal and toroidal structure (in accordance with magnetohydrodynamics investigations and neutron star interior composition explorer's (nicer) recent findings for psr j0030+0451) and is consistent with the small characteristic age of the star. finally, we also discuss possible constraints on the mass and radius of sgr j1745-2900 and briefly envisage possible scenarios accounting for the 3.5 yr evolution of sgr j1745-290 hot spots. | evidence for a multipolar magnetic field in sgr j1745-2900 from x-ray light-curve analysis |
short gamma-ray bursts (grbs) are presumably results of binary neutron star mergers, which lead to the formation of a stellar mass black hole, surrounded by remnant matter. the strong magnetic fields help collimate jets of plasma, launched along the axis of the black hole rotation. we study the structure and evolution of the accreting plasma in the short grbs and we model the formation of the base of relativistic, poynting-dominated jets. our numerical models are based on general relativistic mhd, axisymmetric simulations. we discuss the origin of variability in the grb jet emission, the timescales of which are related to the action of the magnetorotational instability in the accreting plasma. we also estimate the value of a maximum achievable lorentz factor in the jets produced by our simulations and reached at large distances, where the gamma-ray emission is produced. | the mri imprint on the short-grb jets |
how the accreted mass settling on the surface of a neutron star affects the topology of the magnetic field and how the secular evolution of the binary system depends on the magnetic field change is still an open issue. we report evidence for a clear drop in the observed magnetic field in the accreting pulsar v0332+53 after undergoing a bright 3-month long x-ray outburst. we determine the field from the position of the fundamental cyclotron line in its x-ray spectrum and relate it to the luminosity. for equal levels of luminosity, in the declining phase we measure a systematically lower value of the cyclotron line energy with respect to the rising phase. this results in a drop of ∼1.7 × 1011 g of the observed field between the onset and the end of the outburst. the settling of the accreted plasma on to the polar cap seems to induce a distortion of the magnetic field lines weakening their intensity along the accretion columns. therefore, the dissipation rate of the magnetic field could be much faster than previously estimated, unless the field is able to restore its original configuration on a time-scale comparable with the outbursts recurrence time. | an unexpected drop in the magnetic field of the x-ray pulsar v0332+53 after the bright outburst occurred in 2015 |
context. the origin of fossil fields in intermediate- and high-mass stars is poorly understood, as is the interplay between binarity and magnetism during stellar evolution. thus we have begun a study of the magnetic properties of a sample of intermediate-mass and massive short-period binary systems as a function of binarity properties.aims: this paper specifically aims to characterise the magnetic field of hd 5550, a double-lined spectroscopic binary system of intermediate mass.methods: we gathered 25 high-resolution spectropolarimetric observations of hd 5550 using the instrument narval. we first fitted the intensity spectra using zeeman/atlas9 lte synthetic spectra to estimate the effective temperatures, microturbulent velocities, and the abundances of some elements of both components, as well as the light ratio of the system. we then applied the multi-line least-square deconvolution (lsd) technique to the intensity and circularly polarised spectra, which provided us with mean lsd i and v line profiles. we fitted the stokes i line profiles to determine the radial and projected rotational velocities of both stars. we then analysed the shape and evolution of the v profiles using the oblique rotator model to characterise the magnetic fields of both stars.results: we confirm the ap nature of the primary, which has previously been reported, and find that the secondary displays spectral characteristics typical of an am star. while a magnetic field is clearly detected in the lines of the primary, no magnetic field is detected in the secondary in any of our observations. if a dipolar field were present at the surface of the am star, its polar strength must be below 40 g. the faint variability observed in the stokes v profiles of the ap star allowed us to propose a rotation period of 6.84-0.39+0.61 d, which is close to the orbital period (~6.82 d), suggesting that the star is synchronised with its orbit. by fitting the variability of the v profiles, we propose that the ap component hosts a dipolar field inclined with the rotation axis at an angle β = 156 ± 17 ° and a polar strength bd = 65 ± 20 g. the field strength is the weakest known for an ap star. based on the binamics large programme (pi: c. neiner, runid: l131n02) obtained at the telescope bernard lyot (usr5026) operated by the observatoire midi-pyrénées, université de toulouse (paul sabatier), centre national de la recherche scientifique of france. | the magnetic field of the double-lined spectroscopic binary system hd 5550 |
context. recently, the colliding-wind region of the binary stellar system hd 93129a was resolved for the first time using very large baseline interferometry. this system, one of the most massive known binaries in our galaxy, presents non-thermal emission in the radio band, which can be used to infer the physical conditions in the system, and make predictions for the high-energy band.aims: we intend to constrain some of the unknown parameters of hd 93129a through modeling the non-thermal emitter. we also aim to analyse the detectability of this source in hard x-rays and γ-rays. finally, we want to predict how the non-thermal emission will evolve in the future, when the stars approach periastron.methods: a broadband radiative model for the wind-collision region (wcr) has been developed taking into account the evolution of the accelerated particles streaming along the shocked region, the emission by different radiative processes, and the attenuation of the emission propagating through the local matter and radiation fields. we reproduce the available radio data, and make predictions of the emission in hard x-rays and γ-rays under different assumptions.results: from the analysis of the radio emission, we find that the binary hd 93129a is more likely to have a low inclination and a high eccentricity, with the more massive star being currently closer to the observer. the minimum energy of the non-thermal electrons seems to be between ~20-100 mev, depending on the intensity of the magnetic field in the wcr. the latter can be in the range ~20-1500 mg.conclusions: our model is able to reproduce the observed radio emission, and predicts that the non-thermal radiation from hd 93129a will increase in the near future. with instruments such as nustar, fermi, and cta, it will be possible to constrain the relativistic particle content of the source, and other parameters such as the magnetic field strength in the wcr which, in turn, can be used to obtain upper-limits of the magnetic field on the surface of the very massive stars, thereby inferring whether magnetic field amplification is taking place in the particle acceleration region. | a model for the non-thermal emission of the very massive colliding-wind binary hd 93129a |
the theoretical prediction that magnetic reconnection spontaneously drives turbulence has been supported by magnetohydrodynamic (mhd) and kinetic simulations. while reconnection with externally driven turbulence is accepted as an effective mechanism for particle acceleration, the acceleration during the reconnection with self-driven turbulence is studied for the first time in this work. by using high-resolution 3d mhd simulations of reconnection with self-generated turbulence, we inject test particles into the reconnection layer to study their acceleration process. we find that the energy gain of the particles takes place when they bounce back and forth between converging turbulent magnetic fields. the particles can be efficiently accelerated in self-driven turbulent reconnection with the energy increase by about 3 orders of magnitude in the range of the box size. the acceleration proceeds when the particle gyroradii become larger than the thickness of the reconnection layer. we find that the acceleration in the direction perpendicular to the local magnetic field dominates over that in the parallel direction. the energy spectrum of accelerated particles is time-dependent with a slope that evolves toward -2.5. our findings can have important implications for particle acceleration in high-energy astrophysical environments. | particle acceleration in self-driven turbulent reconnection |
a glitch of a pulsar is known as a sudden increase in the spin frequency and spin-down rate (frequency time derivative), and it can be caused by a sudden release of the stress built up in the solid crust of the star or pinned vortices in the superfluid interior. psr j2021+4026 is the first pulsar that shows a significant change in the gamma-ray flux and pulse profile at the glitch that occurred around 2011 october 16. we report the results of timing and spectral analysis of psr j2021+4026 using ∼8 yr fermi large area telescope data. we find that the pulsar stayed at a high spin-down rate (∼ 4 % higher than the pre-glitch value) and a low gamma-ray state (∼ 18 % lower) for about 3 yr after the glitch. around 2014 december, the spin-down rate and gamma-ray flux gradually returned to pre-glitch values within a timescale of a few months. the phase-resolved spectra and pulse profiles after the relaxation are also consistent with those before the glitch. the observed long-term evolution of the spin-down rate and the gamma-ray flux indicates that the glitch triggered a mode change in the global magnetosphere. we speculate that the glitch changed the local magnetic field structure around the polar cap and/or the inclination angle of the dipole axis, leading to a change in the electric current circulating in the magnetosphere. | mode change of a gamma-ray pulsar, psr j2021+4026 |
we study the magneto-rotational instability (mri) driven dynamo in a geometrically thin disc ($h/r\ll 1$) using stratified zero net flux (znf) shearing box simulations. we find that mean fields and emfs oscillate with a primary frequency $f_{\rm dyn} = 0.017$ ($\approx 9$ orbital period), but also have higher harmonics at $3f_{\rm dyn}$. correspondingly, the current helicity, has two frequencies $2f_{\rm dyn}$ and $4f_{\rm dyn}$ respectively, which appear to be the beat frequencies of mean fields and emfs as expected from the magnetic helicity density evolution equation. further, we adopt a novel inversion algorithm called the `iterative removal of sources' (iros), to extract the turbulent dynamo coefficients in the mean-field closure using the mean magnetic fields and emfs obtained from the shearing box simulation. we show that an $\alpha-$effect ($\alpha_{yy}$) is predominantly responsible for the creation of the poloidal field from the toroidal field, while shear generates back a toroidal field from the poloidal field; indicating that an $\alpha-\omega$-type dynamo is operative in mri-driven accretion discs. we also find that both strong outflow ($\bar{v}_z$) and turbulent pumping ($\gamma_z$ ) transport mean fields away from the mid-plane. instead of turbulent diffusivity, they are the principal sink terms in the mean magnetic energy evolution equation. we find encouraging evidence that a generative helicity flux is responsible for the effective $\alpha$-effect. finally, we point out potential limitations of horizontal ($x-y$) averaging in defining the `mean' on the extraction of dynamo coefficients and their physical interpretations. | shedding light on the mri driven dynamo in a stratified shearing box |
context. the properties and evolution of magnetic fields surrounding galaxies are observationally largely unconstrained. the detection and study of these magnetic fields is important to understand galaxy evolution since magnetic fields are tracers for dynamical processes in the circumgalactic medium (cgm) and can have a significant impact on the evolution of the cgm.aims: the faraday rotation measure (rm) of the polarized light of background radio sources passing through the magnetized cgm of intervening galaxies can be used as a tracer for the strength and extent of magnetic fields around galaxies.methods: we used rotation measures observed by the mightee-pol (meerkat international ghz tiered extragalactic exploration polarization) survey by meerkat in the xmm-lss and cosmos fields to investigate the rm around foreground star-forming galaxies. we used spectroscopic catalogs of star-forming and blue cloud galaxies to measure the rm of mightee-pol sources as a function of the impact parameter from the intervening galaxy. in addition, we examined the dependence of the rm on redshift. we then repeated this procedure using a deeper galaxy catalog with photometric redshifts.results: for the spectroscopic star-forming sample, we find a redshift-corrected |rm| excess of 5.6 ± 2.3 rad m−2 which corresponds to a 2.5σ significance around galaxies with a median redshift of z = 0.46 for impact parameters below 130 kpc only selecting the intervenor with the smallest impact parameter. making use of a photometric galaxy catalog and taking into account all intervenors with mg < −13.6 mag, the signal disappears. we find no indication for a correlation between redshift and rm, nor do we find a connection between the total number of intervenors to the total |rm|.conclusions: we have presented tentative evidence that the cgm of star-forming galaxies is permeated by coherent magnetic fields within the virial radius. we conclude that mostly bright, star-forming galaxies with impact parameters less than 130 kpc significantly contribute to the rm of the background radio source. | probing magnetic fields in the circumgalactic medium using polarization data from mightee |
when the accreting white dwarf in a magnetic cataclysmic variable star (mcv) has a field strength in excess of 10 mg, it is expected to synchronize its rotational frequency to the binary orbit frequency, particularly at small binary separations, due to the steep radial dependence of the magnetic field. we report the discovery of an mcv (sdss j134441.83+204408.3, hereafter j1344) that defies this expectation by displaying asynchronous rotation (p spin/p orb = 0.893) in spite of a high surface field strength (b = 56 mg) and a short orbital period (114 minutes). previously misidentified as a synchronously rotating mcv, j1344 was observed by transiting exoplanet survey satellite during sector 50, and the resulting power spectrum shows distinct spin and orbital frequencies, along with various sidebands and harmonics. although there are several other asynchronous mcvs at short orbital periods, the presence of cyclotron humps in j1344's sloan digital sky survey spectrum makes it possible to directly measure the field strength in the cyclotron-emitting region, and while a previously study estimated 65 mg based on its identification of two cyclotron humps, we revise this to 56 ± 2 mg based on the detection of a third hump and on our modeling of the cyclotron spectrum. short-period mcvs with field strengths above 10 mg are normally expected to be synchronous, so the highly asynchronous rotation in j1344 presents an interesting challenge for theoretical studies of spin-period evolution. | sdss j134441.83+204408.3: a highly asynchronous short-period magnetic cataclysmic variable with a 56 mg field strength |
plaskett's 'star' appears to be one of a small number of short-period binary systems known to contain a hot, massive, magnetic star. we combine an extensive spectropolarimetric (stokes v) data set with archival photometry and spectropolarimetry to establish the essential characteristics of the magnetic field and magnetosphere of the rapidly rotating, broad-line component of the system. we apply least-squares deconvolution (lsd) to infer the longitudinal magnetic field from each stokes v spectrum. using the time series of longitudinal field measurements, in combination with corot photometry and equivalent width measurements of magnetospheric spectral lines, we infer the rotation period of the magnetic star to be equal to $1.21551^{+0.00028}_{-0.00034}$ d. modelling the stokes v lsd profiles with zeeman-doppler imaging, we produce the first reliable magnetic map of an o-type star. we find a magnetic field that is predominantly dipolar, with an obliquity near 90° and a polar strength of about 850 g. we update the calculations of the theoretical magnetospheric parameters, and in agreement with their predictions we identify clear variability signatures of the h α, h β, and he ii λ4686 lines confirming the presence of a dense centrifugal magnetosphere surrounding the star. finally, we report a lack of detection of radial velocity (rv) variations of the observed stokes v profiles, suggesting that historical reports of the large rv variations of the broad-line star's spectral lines may be spurious. this discovery may motivate a fundamental revision of the historical model of the plaskett's star as a near-equal mass o + o binary system. | the magnetic field and magnetosphere of plaskett's star: a fundamental shift in our understanding of the system |
the first multimessenger observation attributed to a merging neutron star binary provided an enormous amount of observational data. unlocking the full potential of this data requires a better understanding of the merger process and the early postmerger phase, which are crucial for the later evolution that eventually leads to observable counterparts. in this work, we perform standard hydrodynamical numerical simulations of a system compatible with gw170817. we focus on a single equation of state and two mass ratios, while neglecting magnetic fields and neutrino radiation. we then apply newly developed postprocessing and visualization techniques to the results obtained for this basic setting. the focus lies on understanding the three-dimensional structure of the remnant, most notably the fluid flow pattern, and its evolution until collapse. we investigate the evolution of mass and angular momentum distribution up to collapse, as well as the differential rotation along and perpendicular to the equatorial plane. for the cases that we studied, the remnant cannot be adequately modeled as a differentially rotating axisymetric neutron star. further, the dominant aspect leading to collapse is the gravitational wave radiation and not internal redistribution of angular momentum. we relate features of the gravitational wave signal to the evolution of the merger remnant and make the waveforms publicly available. finally, we find that the three-dimensional vorticity field inside the disk is dominated by medium-scale disturbances and not the orbital velocity, with potential consequences for magnetic field amplification effects. | numerical inside view of hypermassive remnant models for gw170817 |
ρ pup is a δ scuti f2 pulsator, known to host a main radial mode as well as non-radial pulsations, with chemical peculiarities typical of evolved am stars. we present a high-precision spectropolarimetric observations of this star, obtained with espadons at the canada-france-hawaii telescope in the frame of the brite spectropolarimetric survey. a magnetic field is clearly detected in ρ pup, with a longitudinal field strength below 1 g. this makes ρ pup the second known magnetic δ scuti discovered, after hd 188774, and a possible cool evolved counterpart of the recently discovered ultraweakly magnetic am family. | discovery of a magnetic field in the δ scuti f2m star ρ pup |
sharp phase transitions allow for the existence of a third family of stable compact stars, twin stars. in this work, we investigate for the first time the role of strong magnetic fields on non-magnetic twin-star sequences and the case in which magnetic fields themselves give rise to a third family of stable stars. we use three sets of equations of state to study such effects from a general point of view. magnetic field effects are introduced in the structure of stars through the solution of the einstein-maxwell equations, assuming a poloidal magnetic field configuration and a metric that allows for the description of deformed stars. we show that strong magnetic fields can destabilize twin-star sequences. on the other hand, magnetic fields can also give rise to twin stars in models that did not predict these sequences. in this sense, magnetic fields can play an important role in the evolution of neutron stars. | can magnetic fields (de)stabilize twin stars? |
the spectral anisotropy of turbulent structures has been measured in the solar wind since 1990, relying on the assumption of axisymmetry about the mean magnetic field, b 0. however, several works indicate that this hypothesis might be partially wrong, thus raising two questions: (i) is it correct to interpret measurements at 1 au (the so-called maltese cross) in term of a sum of slab and two-dimensional (2d) turbulence; and (ii) what information is really contained in the maltese cross? we solve direct numerical simulations of the magnetohydrodynamic equations including the transverse stretching exerted by the solar wind flow and study the genuine 3d anisotropy of turbulence as well as that one resulting from the assumption of axisymmetry about b 0. we show that the evolution of the turbulent spectrum from 0.2 to 1 au depends strongly on its initial anisotropy. an axisymmetric spectrum with respect to b 0 keeps its axisymmetry, i.e., resists stretching perpendicular to radial, while an isotropic spectrum becomes essentially axisymmetric with respect to the radial direction. we conclude that close to the sun, slow-wind turbulence has a spectrum that is axisymmetric around b 0 and the measured 2d component at 1 au describes the real shape of turbulent structures. in contrast, fast-wind turbulence has a more isotropic spectrum at the source and becomes radially symmetric at 1 au. such structure is hidden by the symmetrization applied to the data that instead returns a slab geometry. | beyond the maltese cross: geometry of turbulence between 0.2 and 1 au |
this paper presents detailed consideration of methodologies to calibrate differential light curves for accurate physical starspot modeling. we use the sun and starspot models as a testbed to highlight some factors in this calibration that that have not yet been treated with care. one unambiguously successful procedure for converting a differential light curve into a light deficit curve appears difficult to implement, but methodologies are presented that work in many cases. the years-long time coverage of kepler provides a strong advantage, but unresolved issues concerning the competing and sometimes similar effects of surface differential rotation versus spot number and size evolution can prevent the confident recovery of correct spot covering fractions in certain cases. we also consider whether faculae are detected by kepler and/or must be accounted for. we conclude their effects are such that absolute photometry is not required for spot deficit calibrations. to elucidate their signature, we re-examine correlations between absolute brightness, differential variability, and apparent spot coverage for hundreds of kepler stars with absolute calibrations from montet et al. the results are similar to theirs, but we draw somewhat different conclusions. most of the stars in this active solar-type sample are spot-dominated as expected. partly because of a dearth of longer period stars, the evidence for facular dominance in this sample is both sparse and relatively weak. the facular population exhibits a puzzling lack of dependence on rotation period, which raises questions about the apparent detection of a “facular” signal at short periods. | calibration of differential light curves for physical analysis of starspots |
late-type main-sequence stars exhibit an x-ray to bolometric flux ratio that depends on {tilde{r}o}, the ratio of rotation period to convective turnover time, as {tilde{r}o}^{-ζ } with 2 ≤ ζ ≤ 3 for {tilde{r}o} > 0.13, but saturates with |ζ| < 0.2 for {tilde{r}o} < 0.13. saturated stars are younger than unsaturated stars and show a broader spread of rotation rates and x-ray activity. the unsaturated stars have magnetic fields and rotation speeds that scale roughly with the square root of their age, though possibly flattening for stars older than the sun. the connection between faster rotators, stronger fields, and higher activity has been established observationally, but a theory for the unified time-evolution of x-ray luminosity, rotation, magnetic field and mass loss that captures the above trends has been lacking. here we derive a minimalist holistic framework for the time evolution of these quantities built from combining a parker wind with new ingredients: (1) explicit sourcing of both the thermal energy launching the wind and the x-ray luminosity via dynamo produced magnetic fields; (2) explicit coupling of x-ray activity and mass-loss saturation to dynamo saturation (via magnetic helicity build-up and convection eddy shredding); (3) use of coronal equilibrium to determine how magnetic energy is divided into wind and x-ray contributions. for solar-type stars younger than the sun, we infer conduction to be a subdominant power loss compared to x-rays and wind. for older stars, conduction is more important, possibly quenching the wind and reducing angular momentum loss. we focus on the time evolution for stars younger than the sun, highlighting what is possible for further generalizations. overall, the approach shows promise towards a unified explanation of all of the aforementioned observational trends. | minimalist coupled evolution model for stellar x-ray activity, rotation, mass loss, and magnetic field |
this review has two parts. the first one is devoted to the barbier–chalonge–divan (bcd) spectrophotometric system, also known as the paris spectral classification system. although the bcd system has been applied and is still used for all stellar objects from o to f spectral types, the present account mainly concerns normal and 'active' b-type stars. the second part treats topics related to stellar rotation, considered one of the key phenomena determining the structure and evolution of stars. the first part is eminently observational. in contrast, the second part deals with observational aspects related to stellar rotation but also recalls some supporting or basic theoretical concepts that may help better understand the gains and shortcomings of today's existent interpretation of stellar data. | bcd spectrophotometry and rotation of active b-type stars: theory and observations |
despite their rarity, massive stars dominate the ecology of galaxies via their strong, radiatively-driven winds throughout their lives and as supernovae in their deaths. however, their evolution and subsequent impact on their environment can be significantly affected by the presence of a magnetic field. while recent studies indicate that about 7% of ob stars in the milky way host strong, stable, organised (fossil) magnetic fields at their surfaces, little is known about the fields of very massive stars, nor the magnetic properties of stars outside our galaxy. we aim to continue searching for strong magnetic fields in a diverse set of massive and very massive stars (vms) in the large and small magellanic clouds (lmc/smc), and we evaluate the overall capability of fors2 to usefully search for and detect stellar magnetic fields in extra-galactic environments. we have obtained fors2 spectropolarimetry of a sample of 41 stars, which principally consist of spectral types b, o, of/wn, wnh, and classical wr stars in the lmc and smc. four of our targets are of?p stars; one of them was just recently discovered. each spectrum was analysed to infer the longitudinal magnetic field. no magnetic fields were formally detected in our study, although bayesian statistical considerations suggest that the of?p star smc 159-2 is magnetic with a dipolar field of the order of 2.4-4.4 kg. in addition, our first constraints of magnetic fields in vms provide interesting insights into the formation of the most massive stars in the universe. | a search for strong magnetic fields in massive and very massive stars in the magellanic clouds |
the magnetic field structure, kinematical stability, and evolutionary status of the starless dense core barnard 68 (b68) are revealed based on the near-infrared polarimetric observations of background stars, measuring the dichroically polarized light produced by aligned dust grains in the core. after subtracting unrelated ambient polarization components, the magnetic fields pervading b68 are mapped using 38 stars and axisymmetrically distorted hourglass-like magnetic fields are obtained, although the evidence for the hourglass field is not very strong. on the basis of simple 2d and 3d magnetic field modeling, the magnetic inclination angles on the plane-of-sky and in the line-of-sight direction are determined to be 47° ± 5° and 20° ± 10°, respectively. the total magnetic field strength of b68 is obtained to be 26.1 ± 8.7 μ g. the critical mass of b68, evaluated using both magnetic and thermal/turbulent support, is m_cr = 2.30 ± 0.20 {m}_{⊙}, which is consistent with the observed core mass of m_core=2.1 m_{⊙}, suggesting a nearly critical state. we found a relatively linear relationship between polarization and extinction up to av ∼ 30 mag toward the stars with deepest obscuration. further theoretical and observational studies are required to explain the dust alignment in cold and dense regions in the core. | distortion of magnetic fields in barnard 68 |
blue stragglers and other mass transfer/collision products are likely born with rapid rotation rates due to angular momentum transfer during mass-transfer, merger, or collisional formation. however, less is known about the angular momentum evolution of these stars as they age. here we compare rotation rates and post-formation ages of mass-transfer products to models of angular momentum evolution for normal main-sequence (ms) stars and collisionally formed blue stragglers. in our sample, we include both f- and g-type blue stragglers in the cluster ngc 188 and post-mass-transfer gk ms stars in the field, all binaries with white dwarf (wd) companions. we compare ages derived from wd cooling models to photometric rotation periods and/or spectral v sin i measurements. we demonstrate that these systems have rapid rotation rates soon after formation. they then spin down as they age, much like standard solar-type ms stars do. we discuss the physical implications of this result, which suggests that the spin-down of post-mass transfer stars can be described by standard magnetic-braking prescriptions. this opens up the possibility of using gyrochronology as a method to determine the time since formation of blue straggler stars and other post-mass-transfer binaries. | observations of spin-down in post-mass-transfer stars and the possibility for blue straggler gyrochronology |
context. stars and their winds can contribute to the non-thermal emission in extragalactic jets. because of the complexity of jet-star interactions, the properties of the resulting emission are closely linked to those of the emitting flows.aims: we simulate the interaction between a stellar wind and a relativistic extragalactic jet and use the hydrodynamic results to compute the non-thermal emission under different conditions.methods: we performed relativistic axisymmetric hydrodynamical simulations of a relativistic jet interacting with a supersonic, non-relativistic stellar wind. we computed the corresponding streamlines out of the simulation results and calculated the injection, evolution, and emission of non-thermal particles accelerated in the jet shock, focusing on electrons or e±-pairs. several cases were explored, considering different jet-star interaction locations, magnetic fields, and observer lines of sight. the jet luminosity and star properties were fixed, but the results are easily scalable when these parameters are changed.results: individual jet-star interactions produce synchrotron and inverse compton emission that peaks from x-rays to mev energies (depending on the magnetic field), and at ~100-1000 gev (depending on the stellar type), respectively. the radiation spectrum is hard in the scenarios explored here as a result of non-radiative cooling dominance, as low-energy electrons are efficiently advected even under relatively high magnetic fields. interactions of jets with cold stars lead to an even harder inverse compton spectrum because of the klein-nishina effect in the cross section. doppler boosting has a strong effect on the observer luminosity.conclusions: the emission levels for individual interactions found here are in the line of previous, more approximate, estimates, strengthening the hypothesis that collective jet-star interactions could significantly contribute at high energies under efficient particle acceleration. | coupling hydrodynamics and radiation calculations for star-jet interactions in active galactic nuclei |
mhd turbulence is common in many space physics and astrophysics environments. we first discuss the properties of incompressible mhd turbulence. a well-conductive fluid amplifies initial magnetic fields in a process called small-scale dynamo. below equipartition scale for kinetic and magnetic energies the spectrum is steep (kolmogorov -5/3) and is represented by critically balanced strong mhd turbulence. in this chapter we report the basic reasoning behind universal nonlinear small-scale dynamo and the inertial range of mhd turbulence. we measured the efficiency of the small-scale dynamo ce= 0. 05, kolmogorov constant ck= 4. 2 and anisotropy constant ca= 0. 63 for mhd turbulence in high-resolution direct numerical simulations. we also discuss so-called imbalanced or cross-helical mhd turbulence which is relevant for in many objects, most prominently in the solar wind. we show that properties of incompressible mhd turbulence are similar to the properties of alfvénic part of mhd cascade in compressible turbulence. the other parts of the cascade evolve according to their own dynamics. the slow modes are being cascaded by alfvénic modes, while fast modes create an independent cascade. we show that different ways of decomposing compressible mhd turbulence into alfvén, slow and fast modes provide consistent results and are useful in understanding not only turbulent cascade, but its interaction with fast particles. | mhd turbulence, turbulent dynamo and applications |
a precondition for the radio emission of pulsars is the existence of strong, small-scale magnetic field structures (`magnetic spots') in the polar cap region. their creation can proceed via crustal hall drift out of two qualitatively and quantitatively different initial magnetic field configurations: a field confined completely to the crust and another which penetrates the whole star. the aim of this study is to explore whether these magnetic structures in the crust can deform the star sufficiently to make it an observable source of gravitational waves. we model the evolution of these field configurations, which can develop, within ∼104-105 yr, magnetic spots with local surface field strengths ∼1014 g maintained over ≳106 yr. deformations caused by the magnetic forces are calculated. we show that, under favourable initial conditions, a star undergoing crustal hall drift can have ellipticity ɛ ∼ 10-6, even with sub-magnetar polar field strengths, after ∼105 yr. a pulsar rotating at ∼102 hz with such ɛ is a promising gravitational wave source candidate. since such large deformations can be caused only by a particular magnetic field configuration that penetrates the whole star and whose maximum magnetic energy is concentrated in the outer core region, gravitational wave emission observed from radio pulsars can thus inform us about the internal field structures of young neutron stars. | gravitational radiation from neutron stars deformed by crustal hall drift |
optical, j and k photometric observations of the kv uma black hole x-ray nova in its quiescent state obtained in 2017-2018 are presented. a significant flickering within light curves was not detected, although the average brightness of the system faded by ≈0.1m over 350 d. changes in the average brightness were not accompanied with an increase or decrease in the flickering. from the modelling of five light curves the inclination of the kv uma orbit and the black hole mass were obtained: i = 74 ± 4°, m_bh=(7.06{-}7.24) m_{⊙ }, depending on the mass ratio used. the non-stellar component of the spectrum in the range λ = 6400-22 000 å can be fitted by a power law fλ ∼ λα, α ≈ -1.8. the accretion disc orientation angle changed from one epoch to another. the model with spots on the star was inadequate. evolutionary calculations using the scenario machine code were performed for low-mass x-ray binaries, with a recently discovered anomalously rapid decrease of the orbital period taken into account. we show that the observed decrease can be consistent with the magnetic stellar wind of the optical companion, whose magnetic field was increased during the common-envelope stage. several constraints on evolutionary scenario parameters were developed. | optical, j and k light curves of xte j1118+480 = kv uma: the mass of the black hole and the spectrum of the non-stellar component |
the origin and evolution of the 1/f power law observed in the energy spectrum of solar coronal and solar wind fluctuations at scales of around an hour is not entirely understood. several existing theories aim at explaining it, involving both linear and nonlinear mechanisms. an often overlooked property of the solar corona and solar wind is their highly inhomogeneous nature. in this paper we investigate the linear evolution of pure alfvén and surface alfvén waves propagating through a plasma that is inhomogeneous across the magnetic field. the inhomogeneity is given by density, which we model to be 2d colored noise, with power spectral slopes ranging from -2 to -1. alfvén waves propagate independently on individual magnetic field lines, and eventually get completely out of phase through the process of phase mixing, leading to unrealistic spectra. when the coupling between the inhomogeneous background and the propagating waves is fully accounted for, transverse waves such as surface alfvén waves (also referred to as kink or alfvénic) appear, showing collective wave behavior of neighboring magnetic field lines with different alfvén speeds. we show that the linear cascade of surface alfvén wave energy, induced by phase mixing and resonant absorption, leads to a perpendicular wave energy spectrum, which tends to the perpendicular power spectrum of the background density. based on our model, we propose that a perpendicular density power spectrum of 1/f in the solar corona can induce, through linear processes, the 1/f spectrum of the fluctuations that is observed at the largest scales. | phase mixing and the 1/f spectrum in the solar wind |
a fireball of radiation plasma created near the surface of a neutron star (ns) expands under its own pressure along magnetic field lines, and produces photon emission and relativistic matter outflow. we comprehensively classify the expanding fireball evolution into five cases, and obtain the photospheric luminosity and the kinetic energy of the outflow, taking into account key processes; lateral diffusion of photons escaping from a magnetic flux tube, effects of strong magnetic field, baryon loading from the ns surface, and radiative acceleration via cyclotron resonant scattering, some of which have not been considered in the context of gamma-ray bursts. applying our model to magnetar bursts with fast radio bursts (frbs), in particular the x-ray short bursts from sgr 1935+2154 associated with the galactic frb 20200428a, we show that the burst radiation can accelerate the outflow to high-lorentz factor with sufficient energy to power frbs. | expanding fireball in magnetar bursts and fast radio bursts |
context. the spin evolution of young protostars, surrounded by an accretion disk, still poses problems for observations and theoretical models. in recent studies, the importance of the magnetic star-disk interaction for stellar spin evolution has been elaborated. the accretion disk in these studies, however, is only represented by a simplified model and important features are not considered.aims: a more realistic representation of the accretion disk is indispensable for a better understanding of the star-disk interaction and the stellar spin evolution. the aim of this study is to investigate the influence of a hydrodynamic disk evolution on the stellar rotational period and vice versa during the accretion phase.methods: we combined the implicit hydrodynamic tapir disk code with a stellar spin evolution model. the influence of stellar magnetic fields on the disk dynamics, the radial position of the inner disk radius, as well as the influence of stellar rotation on the disk were calculated self-consistently.results: within a defined parameter space, we can reproduce the majority of fast and slow rotating stars observed in young stellar clusters. additionally, the back reaction of different stellar spin evolutionary tracks on the disk can be analyzed. disks around fast rotating stars are located closer to the star. consequently, the disk midplane temperature in the innermost disk region increases significantly compared to slow rotating stars. we can show the effects of stellar rotation on episodic accretion outbursts. the higher temperatures of disks around fast rotating stars result in more outbursts and a longer outbursting period over the disk lifetime.conclusions: the combination of a long-term hydrodynamic disk and a stellar spin evolution model allows the inclusion of previously unconsidered effects such as the back-reaction of stellar rotation on the long-term disk evolution and the occurrence of accretion outbursts. however, a wider parameter range has to be studied to further investigate these effects. additionally, a possible interaction between our model and a more realistic stellar evolution code (e.g., the mesa code) can improve our understanding of the stellar spin evolution and its effects on the pre-main sequence star. | time-dependent, long-term hydrodynamic simulations of the inner protoplanetary disk. ii. the importance of stellar rotation |
we numerically investigate the dynamics of a supernova fallback accretion confronting with a relativistic wind from a newborn neutron star (ns). the time evolution of the accretion shock in the radial direction is basically characterized by the encounter radius of the flow renc and a dimensionless parameter $\zeta \equiv l/{\dot{m}}_{\mathrm{fb}}{c}^{2}$ , where l is the ns wind luminosity and ${\dot{m}}_{\mathrm{fb}}$ is the fallback mass accretion rate. we find that the critical condition for the fallback matter to reach near the ns surface can be simply described as $\zeta \lt {\zeta }_{\min }\equiv {{gm}}_{* }/{c}^{2}{r}_{\mathrm{enc}}$ or ${r}_{\mathrm{enc}}l/{{gm}}_{* }{\dot{m}}_{\mathrm{fb}}\lt 1$ independent of the wind lorentz factor, where m* is the ns mass. with combining the condition for the fallback matter to bury the surface magnetic field under the ns crust, we discuss the possibility that the trifurcation of nss into rotation-powered pulsars, central compact objects, and magnetars can be induced by supernova fallback. | a necessary condition for supernova fallback invading newborn neutron-star magnetosphere |
the orion nebula is a prime example of a massive star-forming region in our galaxy. observations have shown that gravitational and magnetic energy are comparable in its integral-shaped filament on a scale of ∼1 pc, and that the population of pre-main sequence stars appears dynamically heated compared to the protostars. these results have been attributed to a slingshot mechanism resulting from the oscillation of the filament by stutz & gould. in this paper, we show that radially contracting filaments naturally evolve towards a state where gravitational, magnetic, and rotational energy are comparable. while the contraction of the filament will preferentially amplify the axial component of the magnetic field, the presence of rotation leads to a helical field structure. we show how magnetic tension can give rise to a filament oscillation, and estimate a typical time-scale of 0.7 myr for the motion of the filament to the position of maximum displacement, consistent with the characteristic time-scale of the ejected stars. furthermore, the presence of helical magnetic fields is expected to give rise to magneto-hydrodynamical instabilities. we show here that the presence of a magnetic field significantly enhances the overall instability, which operates on a characteristic scale of about 1 pc. we expect the physics discussed here to be generally relevant in massive star-forming regions, and encourage further investigations in the future. | magnetic tension and instabilities in the orion a integral-shaped filament |
we examine the effects of plastic flow that appear in a neutron-star crust when a magnetic stress exceeds the threshold. the dynamics involved are described using the navier-stokes equation comprising the viscous-flow term, and the velocity fields for the global circulation are determined using quasi-stationary approximation. we simulate the magnetic-field evolution by taking into consideration the hall drift, ohmic dissipation, and fluid motion induced by the lorentz force. the decrease in the magnetic energy is enhanced, as the energy converts to the bulk motion energy and heat. it is found that the bulk velocity induced by the lorentz force has a significant influence in the low-viscosity and strong-magnetic-field regimes. this effect is crucial near magnetar surfaces. | magnetic-field evolution with large-scale velocity circulation in a neutron-star crust |
understanding the origin of stellar masses—the initial mass function (imf)— remains one of the most challenging problems in astrophysics. the imf is a key ingredient for simulations of galaxy formation and evolution, and is used to calibrate star formation relations in extra-galactic observations. modeling the imf directly in hydrodynamical simulations has been attempted in several previous studies, but the most important processes that control the imf remain poorly understood. this is because predicting the imf from direct hydrodynamical simulations involves complex physics such as turbulence, magnetic fields, radiation feedback and mechanical feedback, all of which are difficult to model and the methods used have limitations in terms of accuracy and computational efficiency. moreover, a physical interpretation of the simulated imfs requires a numerically converged solution at high resolution, which has so far not been convincingly demonstrated. here we present a resolution study of star cluster formation aimed at producing a converged imf. we compare a set of magnetohydrodynamical (mhd) adaptive-mesh-refinement simulations with three different implementations of the thermodynamics of the gas: 1) with an isothermal equation of state (eos), 2) with a polytropic eos, and 3) with a simple stellar heating feedback model. we show that in the simulations with an isothermal or polytropic eos, the number of stars and their mass distributions depend on the numerical resolution. by contrast, the simulations that employ the simple radiative feedback module demonstrate convergence in the number of stars formed and in their imfs. | converging on the initial mass function of stars |
the rapidly evolving dust and gas extinction observed towards wd 1145+017 has opened a real-time window on to the mechanisms for destruction-accretion of planetary bodies on to white dwarf stars, and has served to underline the importance of considering the dynamics of dust particles around such objects. here it is argued that the interaction between (charged) dust grains and the stellar magnetic field is an important ingredient in understanding the physical distribution of infrared emitting particles in the vicinity of such white dwarfs. these ideas are used to suggest a possible model for wd 1145+017 in which the unusual transit shapes are caused by opaque clouds of dust trapped in the stellar magnetosphere. the model can account for the observed transit periodicities if the stellar rotation is near 4.5 h, as the clouds of trapped dust are then located near or within the co-rotation radius. the model requires the surface magnetic field to be at least around some tens of kg. in contrast to the eccentric orbits expected for large planetesimals undergoing tidal disintegration, the orbits of magnetospherically-trapped dust clouds are essentially circular, consistent with the observations. | magnetospherically-trapped dust and a possible model for the unusual transits at wd 1145+017 |
eccentric binaries known as heartbeat stars experience strong dynamical tides as the stars pass through periastron, providing a laboratory to study tidal interactions. we measure the rotation periods of 24 heartbeat systems, using the kepler light curves to identify rotation peaks in the fourier transform. where possible, we compare the rotation period to the pseudosynchronization period derived by hut. few of our heartbeat stars are pseudosynchronized with the orbital period. for four systems, we were able to identify two sets of rotation peaks, which we interpret as the rotation from both stars in the binary. most stars in our sample have rotation rates larger than the pseudosynchronization period while a single target rotates much faster than this rate. the majority of the systems have a rotation period that is approximately \tfrac{3}{2} times the pseudosynchronization period, suggesting that other physical mechanisms strongly influence the star’s evolution. | the pseudosynchronization of binary stars undergoing strong tidal interactions |
we discuss the spectral and timing properties of the accreting millisecond x-ray pulsar swift j1756.9-2508 observed by xmm-newton, nicer, and nustar during the x-ray outburst occurred in april 2018. the spectral properties of the source are consistent with a hard state dominated at high energies by a non-thermal power-law component with a cut-off at ∼70 kev. no evidence of iron emission lines or reflection humps has been found. from the coherent timing analysis of the pulse profiles, we derived an updated set of orbital ephemerides. combining the parameters measured from the three outbursts shown by the source in the last ∼11 yr, we investigated the secular evolution of the spin frequency and the orbital period. we estimated a neutron magnetic field of 3.1× 108 < b_{ pc}< 4.5× 108 g and measured an orbital period derivative of -4.1× 10^{-12} < \dot{p}_{ orb}< 7.1× 10^{-12} s s-1. we also studied the energy dependence of the pulse profile by characterizing the behaviour of the pulse fractional amplitude in the energy range 0.3-80 kev. these results are compared with those obtained from the previous outbursts of swift j1756.9-2508 and other previously known accreting millisecond x-ray pulsars. | swift j1756.9-2508: spectral and timing properties of its 2018 outburst |
context. mass-loss rate is one of the most important stellar parameters. mass loss via stellar winds may influence stellar evolution and modifies stellar spectrum. stellar winds of subluminous hot stars, especially subdwarfs, have not been studied thoroughly.aims: we aim to provide mass-loss rates as a function of subdwarf parameters and to apply the formula for individual subdwarfs, to predict the wind terminal velocities, to estimate the influence of the magnetic field and x-ray ionization on the stellar wind, and to study the interaction of subdwarf wind with mass loss from be and cool companions.methods: we used our kinetic equilibrium (nlte) wind models with the radiative force determined from the radiative transfer equation in the comoving frame (cmf) to predict the wind structure of subluminous hot stars. our models solve stationary hydrodynamical equations, that is the equation of continuity, equation of motion, and energy equation and predict basic wind parameters.results: we predicted the wind mass-loss rate as a function of stellar parameters, namely the stellar luminosity, effective temperature, and metallicity. the derived wind parameters (mass-loss rates and terminal velocities) agree with the values derived from the observations. the radiative force is not able to accelerate the homogeneous wind for stars with low effective temperatures and high surface gravities. we discussed the properties of winds of individual subdwarfs. the x-ray irradiation may inhibit the flow in binaries with compact components. in binaries with be components, the winds interact with the disk of the be star.conclusions: stellar winds exist in subluminous stars with low gravities or high effective temperatures. despite their low mass-loss rates, they are detectable in the ultraviolet spectrum and cause x-ray emission. subdwarf stars may lose a significant part of their mass during the evolution. the angular momentum loss in magnetic subdwarfs with wind may explain their low rotational velocities. stellar winds are especially important in binaries, where they may be accreted on a compact or cool companion. | stellar wind models of subluminous hot stars |
context. alma observations at 1.4 mm and ~0."2 (~750 au) angular resolution of the main core in the high-mass star-forming region g31.41+0.31 have revealed a puzzling scenario. on the one hand, the continuum emission looks very homogeneous and the core appears to undergo solid-body rotation, suggesting a monolithic core stabilized by the magnetic field; on the other hand, rotation and infall speed up toward the core center, where two massive embedded free-free continuum sources have been detected, pointing to an unstable core having undergone fragmentation.aims: to establish whether the main core is indeed monolithic or if its homogeneous appearance is due to a combination of large dust opacity and low angular resolution, we carried out millimeter observations at higher angular resolution and different wavelengths.methods: we carried out alma observations at 1.4 mm and 3.5 mm that achieved angular resolutions of ~0."1 (~375 au) and ~0."075 (~280 au), respectively. vla observations at 7 mm and 1.3 cm at even higher angular resolution, ~0."05 (~190 au) and ~0."07 (~260 au), respectively, were also carried out to better study the nature of the free-free continuum sources detected in the core.results: the millimeter continuum emission of the main core has been clearly resolved into at least four sources, a, b, c, and d, within 1″, indicating that the core is not monolithic. the deconvolved radii of the dust emission of the sources, estimated at 3.5 mm, are ~400-500 au; their masses range from ~15 to ~26 m⊙; and their number densities are several 109 cm−3. sources a and b, located closer to the center of the core and separated by ~750 au, are clearly associated with two free-free continuum sources, likely thermal radio jets, and are brightest in the core. the spectral energy distribution of these two sources and their masses and sizes are similar and suggest a common origin. source c has not been detected at centimeter wavelengths, while source d has been clearly detected at 1.3 cm. source d is likely the driving source of an e-w sio outflow previously detected in the region, which suggests that the free-free emission might be coming from a radio jet.conclusions: the observations have confirmed that the main core in g31 is collapsing, that it has undergone fragmentation, and that its homogeneous appearance previously observed at short wavelengths is a consequence of both high dust opacity and insufficient angular resolution. the low level of fragmentation together with the fact that the core is moderately magnetically supercritical, suggests that g31 could have undergone a phase of magnetically regulated evolution characterized by a reduced fragmentation efficiency, eventually leading to the formation of a small number of relatively massive dense cores. the reduced images (fits files) are only available at the cds via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/j/a+a/648/a100 | fragmentation in the massive g31.41+0.31 protocluster |
we present the evolution of the x-ray emission properties of the magnetar 1e 1547.0-5408 since february 2004 over a time period covering three outbursts. we analyzed new and archival observations taken with the swift, nustar, chandra, and xmm-newton x-ray satellites. the source has been observed at a relatively steady soft x-ray flux of ≈10-11 erg cm-2 s-1 (0.3-10 kev) over the last 9 years, which is about an order of magnitude fainter than the flux at the peak of the last outburst in 2009, but a factor of ∼30 larger than the level in 2006. the broad-band spectrum extracted from two recent nustar observations in april 2016 and february 2019 showed a faint hard x-ray emission up to ∼70 kev. its spectrum is adequately described by a flat power law component, and its flux is ∼7 × 10-12 erg cm-2 s-1 (10-70 kev), that is a factor of ∼20 smaller than at the peak of the 2009 outburst. the hard x-ray spectral shape has flattened significantly in time, which is at variance with the overall cooling trend of the soft x-ray component. the pulse profile extracted from these nustar pointings displays variability in shape and amplitude with energy (up to ≈25 kev). our analysis shows that the flux of 1e 1547.0-5408 is not yet decaying to the 2006 level and that the source has been lingering in a stable, high-intensity state for several years. this might suggest that magnetars can hop among distinct persistent states that are probably connected to outburst episodes and that their persistent thermal emission can be almost entirely powered by the dissipation of currents in the corona. | the long-term enhanced brightness of the magnetar 1e 1547.0-5408 |
we study the behaviour of the cyclotron resonant scattering feature (crsf) of the high-mass x-ray binary vela x-1 using the long-term hard x-ray monitoring performed by the burst alert telescope (bat) on board swift. high-statistics, intensity-selected spectra were built along 11 years of bat survey. while the fundamental line is not revealed, the second harmonic of the crsf can be clearly detected in all the spectra, at an energy varying between ∼53 and ∼58 kev, directly correlated with the luminosity. we have further investigated the evolution of the crsf in time, by studying the intensity-selected spectra built along four 33-month time intervals along the survey. for the first time, we find in this source a secular variation in the crsf energy: independent of the source luminosity, the crsf second harmonic energy decreases by ∼0.36 kev yr-1 between the first and the third time intervals, corresponding to an apparent decay of the magnetic field of ∼3 × 1010 g yr-1. the intensity-cyclotron energy pattern is consistent between the third and the last time intervals. a possible interpretation for this decay could be the settling of an accreted mound that produces either a distortion of the poloidal magnetic field on the polar cap or a geometrical displacement of the line forming region. this hypothesis seems supported by the correspondence between the rate of the line shift per unit accreted mass and the mass accreted on the polar cap per unit area in vela x-1 and her x-1, respectively. | the swift-bat monitoring reveals a long-term decay of the cyclotron line energy in vela x-1 |
the presence of ultrahigh excitation (uhe) absorption lines (e.g. o viii) in the optical spectra of several of the hottest white dwarfs poses a decades-long mystery and is something that has never been observed in any other astrophysical object. the occurrence of such features requires a dense environment with temperatures near 106 k, by far exceeding the stellar effective temperature. here we report the discovery of a new hot wind white dwarf, galex j014636.8+323615. astonishingly, we found for the first time rapid changes of the equivalent widths of the uhe features, which are correlated to the rotational period of the star (p = 0.242035 d). we explain this with the presence of a wind-fed circumstellar magnetosphere in which magnetically confined wind shocks heat up the material to the high temperatures required for the creation of the uhe lines. the photometric and spectroscopic variability of galex j014636.8+323615 can then be understood as consequence of the obliquity of the magnetic axis with respect to the rotation axis of the white dwarf. this is the first time a wind-fed circumstellar magnetosphere around an apparently isolated white dwarf has been discovered and finally offers a plausible explanation of the ultrahot wind phenomenon. | unravelling the baffling mystery of the ultrahot wind phenomenon in white dwarfs |
magnetorotational instability (mri) is the most likely mechanism for angular momentum transport in accretion disks. however, despite numerous efforts, the quest for an unambiguous identification of mri in laboratory is still ongoing. for this purpose, a big taylor-couette experiment with liquid sodium is currently under construction within dresdyn project. in preparation for these experiments, we have determined optimal parameters for axisymmetric standard mri (smri) with an axial magnetic field and analyzed its nonlinear evolution. in this sequel paper, we investigated the linear and nonlinear dynamics of non-axisymmetric smri modes in a similar magnetized taylor-couette setup at very small magnetic prandtl numbers $pm \sim 10^{-5}$ of liquid sodium. we showed that the achievable magnetic reynolds $rm\sim 40$ and lundquist $lu\sim10$ numbers in this experiment are large enough for the growth of non-axisymmetric $|m|=1$ smri modes, but the corresponding critical $rm_c$ is about 2-3 times higher than that of axisymmetric smri. then we followed the nonlinear evolution of these modes and analyzed the structure of the saturated state and its scaling properties with $re$. for $re \lesssim 10^4$, the non-axisymmetric smri modes do not saturate and eventually decay due to the modification of the radial profile of the mean azimuthal velocity by the nonlinear saturation of the axisymmetric smri. by contrast, for large $re \gtrsim 10^4$, a rapid growth and saturation of other, nonmagnetic type of non-axisymmetric modes occur, which are radially localized in the turbulent boundary layer near the inner cylinder wall. the saturation amplitude of these non-axisymmetric modes is always a few orders smaller than that of the axisymmetric one. we showed that the scaling relations for the magnetic energy and torque derived for axisymmetric smri in our previous study well carry over to the present 3d case. | non-axisymmetric standard magnetorotational instability in the upcoming dresdyn-mri experiments -- linear and nonlinear dynamics |
the evolutions of a neutron star's rotation and magnetic field (b-field) have remained unsolved puzzles for over half a century. we ascribe the rotational braking torques of pulsar to both components, the standard magnetic dipole radiation (mdr) and particle wind flow (mdr + wind, hereafter named mdrw), which we apply to the crab pulsar (b0531 + 21), the only source with a known age and long-term continuous monitoring by radio telescope. based on the above presumed simple spin-down torques, we obtain the exact analytic solution on the rotation evolution of the crab pulsar, together with the related outcomes as described below: (1) unlike the constant characteristic b-field suggested by the mdr model, this value for the crab pulsar increases by a hundred times in 50 kyr while its real b-field has no change; (2) the rotational braking index evolves from ∼3 to 1 in the long-term, however, it drops from 2.51 to 2.50 in ∼45 years at the present stage, while the particle flow contributes approximately 25% of the total rotational energy loss rate; (3) strikingly, the characteristic age has the maximum limit of ∼10 kyr, meaning that it is not always a good indicator of a real age. furthermore, we discussed the evolutionary path of the crab pulsar from the mdr to the wind domination by comparing with the possible wind braking candidate pulsar psr j1734-3333. | evolution of spin period and magnetic field of the crab pulsar: decay of the braking index by the particle wind flow torque |
in this work, we study the effects of magnetic fields and rotation on the structure and composition of proto-neutron stars. a hadronic chiral su(3) model is applied to cold neutron stars and proto-neutron stars with trapped neutrinos and at fixed entropy per baryon. we obtain general relativistic solutions for neutron and proto-neutron stars endowed with a poloidal magnetic field by solving einstein-maxwell field equations in a self-consistent way. as the neutrino chemical potential decreases in value over time, this alters the chemical equilibrium and the composition inside the star, leading to a change in the structure and in the particle population of these objects. we find that the magnetic field deforms the star and significantly alters the number of trapped neutrinos in the stellar interior, together with strangeness content and temperature in each evolution stage. | internal composition of proto-neutron stars under strong magnetic fields |
we report the first clear detection of the zeeman splitting of a ccs emission line at 45 ghz toward the nearby pre-stellar dense filament, taurus molecular cloud 1 (tmc-1). we observed hc_3n non-zeeman lines simultaneously with the ccs line, and did not detect any significant splitting of the hc_3n lines. thus, we conclude that our detection of ccs zeeman splitting is robust. the derived line-of-sight magnetic field strength is about 117 ± 21 μg, which corresponds to a normalized mass-to-magnetic flux ratio of 2.2 if we adopt an inclination angle of 45°. thus, we conclude that the tmc-1 filament is magnetically supercritical. recent radiative transfer calculations of the ccs and hc_3n lines along the line of sight suggest that the filament is collapsing with a speed of ∼0.6 km s^{-1}, which is comparable to three times the isothermal sound speed. this infall velocity appears to be consistent with the evolution of a gravitationally infalling core. | first clear detection of the ccs zeeman splitting toward the pre-stellar core, taurus molecular cloud 1 |
magnetorotational instability (mri) is considered as the most likely mechanism driving angular momentum transport in astrophysical disks. however, despite many efforts, a direct and conclusive experimental evidence of mri in laboratory is still missing. recently performing one-dimensional linear analysis of the standard version of mri (smri) in a taylor-couette (tc) flow between two rotating coaxial cylinders threaded by an axial magnetic field, we showed that smri can be detected in the upcoming dresdyn-mri experiment based on a cylindrical magnetized tc flow of liquid sodium. in this follow-up study, also related to the dresdyn-mri experiments, we focus on the nonlinear evolution and saturation properties of smri and analyze its scaling behavior with respect to various parameters of the basic tc flow using a pseudospectral code. we conduct a detailed analysis over the extensive ranges of magnetic reynolds number rm ∈[8.5 ,37.1 ] , lundquist number lu ∈[1.5 ,15.5 ] , and reynolds number, re ∈[103,105] . we focus on the small magnetic prandtl number, pm ≪1 , regime down to pm ∼10-4 , aiming ultimately for those very small values typical of liquid sodium used in the experiments. in the saturated state, the magnetic energy of smri and associated normalized torque due to perturbations exerted on the cylinders, which characterizes angular momentum transport, both increase with rm for fixed (lu ,re ) , while for fixed (lu ,rm ) , the magnetic energy decreases and torque increases with increasing re . we also study the scaling of the magnetic energy and torque in the saturated state as a function of re and find a power-law dependence of the form re-0.6 ...-0.5 for the magnetic energy and re0.4 ...0.5 for the torque at all sets of (lu ,rm ) and sufficiently high re ≥4000 . we also explore the dependence on lundquist number and angular velocity of the cylinders. the scaling laws derived here will be instrumental in the subsequent analysis of more realistic finite-length tc flows and comparison of numerical results with those obtained from the dresdyn-mri experiments in order to conclusively and unambiguously identify smri in the laboratory. | nonlinear evolution of magnetorotational instability in a magnetized taylor-couette flow: scaling properties and relation to upcoming dresdyn-mri experiment |
ar scorpii is an intermediate polar binary system composed of a magnetic white dwarf (wd) and an m-type star and shows nonthermal, pulsed, and highly linearly polarized emission. the radio/optical emission modulates with the wd’s spin and shows the double-peak structure in the light curves. in this paper, we discuss a possible scenario for the radiation mechanism of ar scorpii. the magnetic interaction on the surface of the companion star produces an outflow from the companion star, the heating of the companion star surface, and the acceleration of electrons to a relativistic energy. the accelerated electrons, whose typical lorentz factor is ∼50-100, from the companion star move along the magnetic field lines toward the wd surface. the electrons injected with the pitch angle of \sin {θ }p,0> 0.05 are subject to the magnetic mirror effect and are trapped in the closed magnetic field line region. we find that the emission from the first magnetic mirror points mainly contributes to the observed pulsed emission and the formation of the double-peak structure in the light curve. for the inclined rotator, the pulse peak in the calculated light curve shifts the position in the spin phase, and a fourier analysis exhibits a beat frequency feature, which are consistent with the optical/uv observations. the pulse profile also evolves with the orbital phase owing to the effect of the viewing geometry. the model also interprets the global features of the observed spectral energy distribution in radio to x-ray energy bands. we also discuss the curvature radiation and the inverse-compton scattering process in the outer gap accelerator of the wd in ar scorpii and the possibility of the detection by future high-energy missions. | a model for ar scorpii: emission from relativistic electrons trapped by closed magnetic field lines of magnetic white dwarfs |
context. differential rotation has a strong influence on stellar internal dynamics and evolution, notably by triggering hydrodynamical instabilities, by interacting with the magnetic field, and more generally by inducing transport of angular momentum and chemical elements. moreover, it modifies the way waves propagate in stellar interiors and thus the frequency spectrum of these waves, the regions they probe, and the transport they generate.aims: we investigate the impact of a general differential rotation (both in radius and latitude) on the propagation of axisymmetric gravito-inertial waves.methods: we use a small-wavelength approximation to obtain a local dispersion relation for these waves. we then describe the propagation of waves thanks to a ray model that follows a hamiltonian formalism. finally, we numerically probe the properties of these gravito-inertial rays for different regimes of radial and latitudinal differential rotation.results: we derive a local dispersion relation that includes the effect of a general differential rotation. subsequently, considering a polytropic stellar model, we observe that differential rotation allows for a large variety of resonant cavities that can be probed by gravito-inertial waves. we identify that for some regimes of frequency and differential rotation, the properties of gravito-inertial rays are similar to those found in the uniformly rotating case. furthermore, we also find new regimes specific to differential rotation, where the dynamics of rays is chaotic.conclusions: as a consequence, we expect modes to follow the same trend. some parts of oscillation spectra corresponding to regimes similar to those of the uniformly rotating case would exhibit regular patterns, while parts corresponding to the new regimes would be mostly constituted of chaotic modes with a spectrum rather characterised by a generic statistical distribution. | asymptotic theory of gravity modes in rotating stars. ii. impact of general differential rotation |
the properties of supersonic isothermal turbulence influence a variety of astrophysical phenomena, including the structure and evolution of star-forming clouds. this work presents a simple model for the structure of dense regions in turbulence in which the density distribution behind isothermal shocks originates from rough hydrostatic balance between the pressure gradient behind the shock and its deceleration from ram pressure applied by the background fluid. using simulations of supersonic isothermal turbulence and idealized waves moving through a background medium, we show that the structural properties of dense, shocked regions broadly agree with our analytical model. our work provides a new conceptual picture for describing the dense regions, which complements theoretical efforts to understand the bulk statistical properties of turbulence and attempts to model the more complex features of star-forming clouds like magnetic fields, self-gravity, or radiative properties. | dense regions in supersonic isothermal turbulence |
rotation periods from kepler k2 are combined with projected rotation velocities from the wiyn 3.5 m telescope to determine projected radii for fast-rotating, low-mass (0.15 ≤ m/ m_{⊙ } ≤ 0.6) members of the praesepe cluster. a maximum likelihood analysis that accounts for observational uncertainties, binarity, and censored data yields marginal evidence for radius inflation - the average radius of these stars is 6 ± 4 per cent larger at a given luminosity than predicted by commonly used evolutionary models. this overradius is smaller (at 2σ confidence) than was found for similar stars in the younger pleiades using a similar analysis; any decline appears due to changes occurring in higher mass (> 0.25 m_{⊙}) stars. models incorporating magnetic inhibition of convection predict an overradius, but do not reproduce this mass dependence unless superequipartition surface magnetic fields are present at lower masses. models incorporating flux blocking by starspots can explain the mass dependence but there is no evidence that spot coverage diminishes between the pleiades and praesepe samples to accompany the decline in overradius. the fastest rotating stars in both praesepe and the pleiades are significantly smaller than the slowest rotators for which a projected radius can be measured. this may be a selection effect caused by more efficient angular momentum loss in larger stars leading to their progressive exclusion from the analysed samples. our analyses assume random spin-axis orientations; any alignment in praesepe, as suggested by kovacs, is strongly disfavoured by the broad distribution of projected radii. | a search for radius inflation among active m-dwarfs in praesepe |
context. we study the time-resolved spectral properties of energetic gamma-ray bursts (grbs) with good high-energy photon statistics observed by the gamma-ray burst monitor (gbm) onboard the fermi gamma-ray space telescope.aims: we aim to constrain in detail the spectral properties of grb prompt emission on a time-resolved basis and to discuss the theoretical implications of the fitting results in the context of various prompt emission models.methods: our sample comprises eight grbs observed by the fermi gbm in its first five years of mission, with 1 kev-1 mev fluence f> 1.0 × 10-4 erg cm-2 and a signal-to-noise ratio level of s/n ≥ 10.0 above 900 kev. we performed a time-resolved spectral analysis using a variable temporal binning technique according to optimal s/n criteria, resulting in a total of 299 time-resolved spectra. we performed band function fits to all spectra and obtained the distributions for the low-energy power-law index α, the high-energy power-law index β, the peak energy in the observed νfν spectrum ep, and the difference between the low- and high-energy power-law indices δs = α - β. we also applied a physically motivated synchrotron model, which is a triple power-law with constrained power-law indices and a blackbody component, to test the prompt emission for consistency with a synchrotron origin and obtain the distributions for the two break energies eb,1 and eb,2, the middle segment power-law index β, and the planck function temperature kt.results: the band function parameter distributions are α = -0.73+0.16-0.21, β =ي-2.13+0.28-0.56, ep = 374.4+307.3-187.7 , kev (log10ep = 2.57+0.26-0.30), and δs = 1.38+0.54-0.31 , with average errors σα ~ 0.1, σβ ~ 0.2, and σep ~ 0.1ep. using the distributions of δs and β, the electron population index p is found to be consistent with the "moderately fast" scenario, in which fast- and slow-cooling scenarios cannot be distinguished. the physically motivated synchrotron-fitting function parameter distributions are eb,1 = 129.6+132.2-32.4 kev, eb,2 = 631.4+582.6-309.6 kev, β = -1.72+0.48-0.25 , and kt = 10.4+4.9-3.7 kev, with average errors σβ ~ 0.2, σeb,1 ~ 0.1eb,1, σeb,2 ~ 0.4eb,2, and σkt ~ 0.1kt. this synchrotron function requires the synchrotron injection and cooling break (i.e., emin and ecool) to be close to each other within a factor of ten, often in addition to a planck function.conclusions: a synchrotron model is found that is consistent with most of the time-resolved spectra for eight energetic fermi gbm bursts with good high-energy photon statistics as long as both the cooling and injection break are included and the leftmost spectral slope is lifted either by including a thermal component or when an evolving magnetic field is accounted for. appendix a is available in electronic form at http://www.aanda.org | synchrotron cooling in energetic gamma-ray bursts observed by the fermi gamma-ray burst monitor |
the x-ray transient xte j1701-462 was the first source observed to evolve through all known subclasses of low-magnetic-field neutron star low-mass x-ray binaries (ns-lmxbs), as a result of large changes in its mass accretion rate. to investigate to what extent similar evolution is seen in other ns-lmxbs we have performed a detailed study of the color-color and hardness-intensity diagrams (cds and hids) of cyg x-2, cir x-1, and gx 13+1—three luminous x-ray binaries, containing weakly magnetized neutron stars, known to exhibit strong secular changes in their cd/hid tracks. using the full set of rossi x-ray timing explorer proportional counter array data collected for the sources over the 16 year duration of the mission, we show that cyg x-2 and cir x-1 display cd/hid evolution with close similarities to xte j1701-462. although gx 13+1 shows behavior that is in some ways unique, it also exhibits similarities to xte j1701-462, and we conclude that its overall cd/hid properties strongly indicate that it should be classified as a z source, rather than as an atoll source. we conjecture that the secular evolution of cyg x-2, cir x-1, and gx 13+1—illustrated by sequences of cd/hid tracks we construct—arises from changes in the mass accretion rate. our results strengthen previous suggestions that within single sources cyg-like z source behavior takes place at higher luminosities and mass accretion rates than sco-like z behavior, and lend support to the notion that the mass accretion rate is the primary physical parameter distinguishing the various ns-lmxb subclasses. | common patterns in the evolution between the luminous neutron star low-mass x-ray binary subclasses |
magnetars are highly magnetized rotating neutron stars that are predominantly observed as high-energy sources. six of this class of neutron star are known to also emit radio emission, so magnetars are a favoured model for the origin of at least some of the fast radio bursts (frbs). if magnetars, or neutron stars in general, are indeed responsible, sharp empirical constraints on the mechanism producing radio emission are required. here we report on the detection of polarized quasi-periodic substructure in the emission of all well-studied radio-detected magnetars. a correlation previously seen, relating substructure in pulsed emission of radio-emitting neutron stars to their rotational period, is extended and now shown to span more than six orders of magnitude in pulse period. this behaviour is not only seen in magnetars but in members of all classes of radio-emitting rotating neutron stars, regardless of their evolutionary history, their power source or their inferred magnetic field strength. if magnetars are responsible for frbs, it supports the idea of being able to infer underlying periods from sub-burst timescales in frbs. | quasi-periodic sub-pulse structure as a unifying feature for radio-emitting neutron stars |
we study the long-term quasi-steady evolution of the force-free magnetosphere of a magnetar coupled to its internal magnetic field. we find that magnetospheric currents can be maintained on long time-scales of the order of thousands of years. meanwhile, the energy, helicity and twist stored in the magnetosphere all gradually increase over the course of this evolution, until a critical point is reached, beyond which a force-free magnetosphere cannot be constructed. at this point, some large-scale magnetospheric rearrangement, possibly resulting in an outburst or a flare, must occur, releasing a large fraction of the stored energy, helicity and twist. after that, the quasi-steady evolution should continue in a similar manner from the new initial conditions. the time-scale for reaching this critical point depends on the overall magnetic field strength and on the relative fraction of the toroidal field. the energy stored in the force-free magnetosphere is found to be up to ∼30 per cent larger than the corresponding vacuum energy. this implies that for a 1014 g field at the pole, the energy budget available for fast magnetospheric events is of the order of a few 1044 erg. the spin-down rate is estimated to increase by up to ∼60 per cent, since the dipole content in the magnetosphere is enhanced by the currents present there. a rough estimate of the braking index n reveals that it is systematically n < 3 for the most part of the evolution, consistent with actual measurements for pulsars and early estimates for several magnetars. | long-term evolution of the force-free twisted magnetosphere of a magnetar |
we report on two nustar observations of the high-mass x-ray binary a 0535+26 taken toward the end of its normal 2015 outburst at very low 3-50 kev luminosities of 1.4 × 1036 erg s-1 and 5 × 1035 erg s-1, which are complemented by nine swift observations. the data clearly confirm indications seen in earlier data that the source's spectral shape softens as it becomes fainter. the smooth exponential rollover at high energies seen in the first observation evolves to a much more abrupt steepening of the spectrum at 20-30 kev. the continuum evolution can be nicely described with emission from a magnetized accretion column, modeled using the compmag model modified by an additional gaussian emission component for the fainter observation. between the two observations, the optical depth changes from 0.75 ± 0.04 to , the electron temperature remains constant, and there is an indication that the column decreases in radius. since the energy-resolved pulse profiles remain virtually unchanged in shape between the two observations, the emission properties of the accretion column reflect the same accretion regime. this conclusion is also confirmed by our result that the energy of the cyclotron resonant scattering feature (crsf) at 45 kev is independent of the luminosity, implying that the magnetic field in the region in which the observed radiation is produced is the same in both observations. finally, we also constrain the evolution of the continuum parameters with the rotational phase of the neutron star. the width of the crsf could only be constrained for the brighter observation. based on monte carlo simulations of crsf formation in single accretion columns, its pulse phase dependence supports a simplified fan beam emission pattern. the evolution of the crsf width is very similar to that of the crsf depth, which is, however, in disagreement with expectations. | looking at a 0535+26 at low luminosities with nustar |
a theory of potential vorticity (pv) mixing in a disordered (tangled) magnetic field is presented. the analysis is in the context of β-plane mhd, with a special focus on the physics of momentum transport in the stably stratified, quasi-2d solar tachocline. a physical picture of mean pv evolution by vorticity advection and tilting of magnetic fields is proposed. in the case of weak field perturbations, quasi-linear theory predicts that the reynolds and magnetic stresses balance as turbulence alfvénizes for a larger mean magnetic field. jet formation is explored quantitatively in the mean field-resistivity parameter space. however, since even a modest mean magnetic field leads to large magnetic perturbations for large magnetic reynolds number, the physically relevant case is that of a strong but disordered field. we show that numerical calculations indicate that the reynolds stress is modified well before alfvénization—i.e., before fluid and magnetic energies balance. to understand these trends, a double-average model of pv mixing in a stochastic magnetic field is developed. calculations indicate that mean-square fields strongly modify reynolds stress phase coherence and also induce a magnetic drag on zonal flows. the physics of transport reduction by tangled fields is elucidated and linked to the related quench of turbulent resistivity. we propose a physical picture of the system as a resisto-elastic medium threaded by a tangled magnetic network. applications of the theory to momentum transport in the tachocline and other systems are discussed in detail. | potential vorticity mixing in a tangled magnetic field |
angular momentum is a key property regulating star formation and evolution. however, the physics driving the distribution of the stellar rotation rates of early-type main-sequence stars is as yet poorly understood. using our catalog of 40,034 early-type stars with homogeneous $v\sin i$ parameters, we review the statistical properties of their stellar rotation rates. we discuss the importance of possible contaminants, including binaries and chemically peculiar stars. upon correction for projection effects and rectification of the error distribution, we derive the distributions of our sample's equatorial rotation velocities, which show a clear dependence on stellar mass. stars with masses less than 2.5 m⊙ exhibit a unimodal distribution, with the peak velocity ratio increasing as stellar mass increases. a bimodal rotation distribution, composed of two branches of slowly and rapidly rotating stars, emerges for more massive stars (m > 2.5 m⊙). for stars more massive than 3.0 m⊙, the gap between the bifurcated branches becomes prominent. for the first time, we find that metal-poor ([m/h] < -0.2 dex) stars only exhibit a single branch of slow rotators, while metal-rich ([m/h] > 0.2 dex) stars clearly show two branches. the difference could be attributed to unexpectedly high spin-down rates and/or in part strong magnetic fields in the metal-poor subsample. | exploring the stellar rotation of early-type stars in the lamost medium-resolution survey. ii. statistics |
using physics-informed neural networks (pinns) to solve a specific boundary value problem is becoming more popular as an alternative to traditional methods. however, depending on the specific problem, they could be computationally expensive and potentially less accurate. the functionality of pinns for real-world physical problems can significantly improve if they become more flexible and adaptable. to address this, our work explores the idea of training a pinn for general boundary conditions and source terms expressed through a limited number of coefficients, introduced as additional inputs in the network. although this process increases the dimensionality and is computationally costly, using the trained network to evaluate new general solutions is much faster. our results indicate that pinn solutions are relatively accurate, reliable, and well behaved. we applied this idea to the astrophysical scenario of the magnetic field evolution in the interior of a neutron star connected to a force-free magnetosphere. solving this problem through a global simulation in the entire domain is expensive due to the elliptic solver's needs for the exterior solution. the computational cost with a pinn was more than an order of magnitude lower than the similar case solved with a finite difference scheme, arguably at the cost of accuracy. these results pave the way for the future extension to three-dimensional of this (or a similar) problem, where generalized boundary conditions are very costly to implement. | modelling force-free neutron star magnetospheres using physics-informed neural networks |
we present a first 3d magnetohydrodynamic (mhd) simulation of oxygen, neon, and carbon shell burning in a rapidly rotating $16\hbox{-}\mathrm{m}_\odot$ core-collapse supernova progenitor. we also run a purely hydrodynamic simulation for comparison. after $\mathord \approx 180\mathrm{s}$ ($\mathord \approx$ 15 and 7 convective turnovers, respectively), the magnetic fields in the oxygen and neon shells achieve saturation at 1011 and 5 × 1010 g. the strong maxwell stresses become comparable to the radial reynolds stresses and eventually suppress convection. the suppression of mixing by convection and shear instabilities results in the depletion of fuel at the base of the burning regions, so that the burning shell eventually move outward to cooler regions, thus reducing the energy generation rate. the strong magnetic fields efficiently transport angular momentum outwards, quickly spinning down the rapidly rotating convective oxygen and neon shells and forcing them into rigid rotation. the hydrodynamic model shows complicated redistribution of angular momentum and develops regions of retrograde rotation at the base of the convective shells. we discuss implications of our results for stellar evolution and for the subsequent core-collapse supernova. the rapid redistribution of angular momentum in the mhd model casts some doubt on the possibility of retaining significant core angular momentum for explosions driven by millisecond magnetars. however, findings from multidimensional models remain tentative until stellar evolution calculations can provide more consistent rotation profiles and estimates of magnetic field strengths to initialize multidimensional simulations without substantial numerical transients. we also stress the need for longer simulations, resolution studies, and an investigation of non-ideal effects. | 3d simulations of magnetoconvection in a rapidly rotating supernova progenitor |
the bright nova cygni 1975 is a rare nova on a magnetic white dwarf (wd). later it was found to be an asynchronous polar, now called v1500 cyg. our multisite photometric campaign occurring 40 yr post eruption covered 26 nights (2015-2017). the reflection effect from the heated donor has decreased but still dominates the optical radiation with an amplitude ∼1{^m.}5. the 0{^m.}3 residual reveals cyclotron emission and ellipsoidal variations. mean brightness modulation from night-to-night is used to measure the 9.6-d spin-orbit beat period that is due to changing accretion geometry including magnetic pole-switching of the flow. by subtracting the orbital and beat frequencies, spin-phase dependent light curves are obtained. the amplitude and profile of the wd spin light curves track the cyclotron emitting accretion regions on the wd and they vary systematically with beat phase. a weak intermittent signal at 0.137613 d is likely the spin period, which is 1.73(1) min shorter than the orbital period. the o-c diagram of light-curve maxima displays phase jumps every one-half beat period, a characteristic of asynchronous polars. the first jump we interpret as pole switching between regions separated by 180°. then the spot drifts during ∼0.1 beat phase before undergoing a second phase jump between spots separated by less than 180°. we trace the cooling of the still hot wd as revealed by the irradiated companion. the post nova evolution and spin-orbit asynchronism of v1500 cyg continues to be a powerful laboratory for accretion flows on to magnetic wds. | asynchronous polar v1500 cyg: orbital, spin, and beat periods |
context. determining the effects of an accretion disk is crucial to understanding the evolution of young stars. during the combined evolution, stellar and disk parameters influence one another, which motivated us to develop a combined stellar and disk model. this makes a combined numerical model, with the disk evolving alongside the star, the next logical step in the progress of studying early stellar evolution.aims: we aim to understand the effects of metallicity on the accretion disk and the stellar spin evolution during the t tauri phase.methods: we combined the numerical treatment of a hydrodynamic disk with stellar evolution, including a stellar spin model and allowing a self-consistent calculation of the back-reactions between the individual components.results: we present the self-consistent theoretical evolution of t tauri stars coupled to a stellar disk. we find that disks in low-metallicity environments are heated differently and have shorter lifetimes compared to their solar-metallicity counterparts. differences in stellar radii, the contraction rate of the stellar radius, and the shorter disk lifetimes result in low-metallicity stars rotating more rapidly.conclusions: we present an additional explanation for the observed short disk lifetimes in low-metallicity clusters. a combination of our model with those of previous studies (e.g., a metallicity-based photo-evaporation) could help us understand disk evolution and dispersal at different metallicities. furthermore, our stellar spin evolution model includes several important effects that had previously been ignored (e.g., the stellar magnetic field strength and a realistic calculation of the disk lifetime). we encourage others to include our results as initial or input parameters in further spin evolution models that cover the stellar evolution toward and during the main sequence. | the influence of metallicity on a combined stellar and disk evolution |
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