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at 60 pc, tw hydra (tw hya) is the closest example of a star with a gas-rich protoplanetary disk, though tw hya may be relatively old (3-15 myr). as such, tw hya is especially appealing for testing our understanding of the interplay between stellar and disk evolution. we present a high-resolution near-infrared spectrum of tw hya obtained with the immersion grating infrared spectrometer (igrins) to re-evaluate the stellar parameters of tw hya. we compare these data to synthetic spectra of magnetic stars produced by moogstokes, and use sensitive spectral line profiles to probe the effective temperature, surface gravity, and magnetic field. a model with {t}{eff}=3800 k, {log} g=4.2, and b=3.0 kg best fits the near-infrared spectrum of tw hya. these results correspond to a spectral type of m0.5 and an age of 8 myr, which is well past the median life of gaseous disks. | characterizing tw hydra |
the presence of superconducting and superfluid components in the core of mature neutron stars calls for the rethinking of a number of key magnetohydrodynamical notions like resistivity, the induction equation, magnetic energy and flux-freezing. using a multifluid magnetohydrodynamics formalism, we investigate how the magnetic field evolution is modified when neutron star matter is composed of superfluid neutrons, type-ii superconducting protons and relativistic electrons. as an application of this framework, we derive an induction equation where the resistive coupling originates from the mutual friction between the electrons and the vortex/fluxtube arrays of the neutron and proton condensates. the resulting induction equation allows the identification of two time-scales that are significantly different from those of standard magnetohydrodynamics. the astrophysical implications of these results are briefly discussed. | magnetic field evolution in superconducting neutron stars |
radio emission from magnetars provides a unique probe of the relativistic, magnetized plasma within the near-field environment of these ultra-magnetic neutron stars. the transmitted waves can undergo birefringent and dispersive propagation effects that result in frequency-dependent conversions of linear to circularly polarized radiation and vice-versa, thus necessitating classification when relating the measured polarization to the intrinsic properties of neutron star and fast radio burst (frb) emission sites. we report the detection of such behavior in 0.7-4 ghz observations of the p = 5.54 s radio magnetar xte j1810$-$197 following its 2018 outburst. the phenomenon is restricted to a narrow range of pulse phase centered around the magnetic meridian. its temporal evolution is closely coupled to large-scale variations in magnetic topology that originate from either plastic motion of an active region on the magnetar surface or free precession of the neutron star crust. our model of the effect deviates from simple theoretical expectations for radio waves propagating through a magnetized plasma. birefringent self-coupling between the transmitted wave modes, line-of-sight variations in the magnetic field direction and differences in particle charge or energy distributions above the magnetic pole are explored as possible explanations. we discuss potential links between the immediate magneto-ionic environments of magnetars and those of frb progenitors. | linear to circular conversion in the polarized radio emission of a magnetar |
as a star spins-down during the main sequence, its wind properties are affected. in this work, we investigate how earth's magnetosphere has responded to the change in the solar wind. earth's magnetosphere is simulated using 3d magnetohydrodynamic models that incorporate the evolving local properties of the solar wind. the solar wind, on the other hand, is modelled in 1.5d for a range of rotation rates ω from 50 to 0.8 times the present-day solar rotation (ω⊙). our solar wind model uses empirical values for magnetic field strengths, base temperature, and density, which are derived from observations of solar-like stars. we find that for rotation rates ≃10 ω⊙, earth's magnetosphere was substantially smaller than it is today, exhibiting a strong bow shock. as the sun spins-down, the magnetopause standoff distance varies with ω-0.27 for higher rotation rates (early ages, ≥1.4 ω⊙) and with ω-2.04 for lower rotation rates (older ages, <1.4 ω⊙). this break is a result of the empirical properties adopted for the solar wind evolution. we also see a linear relationship between the magnetopause distance and the thickness of the shock on the subsolar line for the majority of the evolution (≤10 ω⊙). it is possible that a young fast rotating sun would have had rotation rates as high as 30-50 ω⊙. in these speculative scenarios, at 30 ω⊙, a weak shock would have been formed, but for 50 ω⊙, we find that no bow shock could be present around earth's magnetosphere. this implies that with the sun continuing to spin-down, a strong shock would have developed around our planet and remained for most of the duration of the solar main sequence. | the evolution of earth's magnetosphere during the solar main sequence |
sun-like and low-mass stars possess high-temperature coronae and lose mass in the form of stellar winds, which are driven by thermal pressure and complex magnetohydrodynamic processes. these magnetized outflows probably do not significantly affect the star’s structural evolution on the main sequence, but they brake the stellar rotation by removing angular momentum, a mechanism known as magnetic braking. previous studies have shown how the braking torque depends on the magnetic field strength and geometry, stellar mass and radius, mass-loss rate, and rotation rate of the star, assuming a fixed coronal temperature. for this study, we explore how different coronal temperatures can influence the stellar torque. we employ 2.5d, axisymmetric, magnetohydrodynamic simulations, computed with the pluto code, to obtain steady-state wind solutions from rotating stars with dipolar magnetic fields. our parameter study includes 30 simulations with different coronal temperatures and surface magnetic field strengths. we consider a parker-like (i.e., thermal-pressure-driven) wind, and therefore coronal temperature is the key parameter determining the velocity and acceleration profile of the flow. since the mass-loss rates for these types of stars are not well-constrained, we determine how the torque scales for a vast range of stellar mass-loss rates. hotter winds lead to faster acceleration, and we show that (for a given magnetic field strength and mass-loss rate) a hotter outflow leads to a weaker torque on the star. we derive new predictive torque formulae that quantify this effect over a range of possible wind acceleration profiles. | magnetic braking of sun-like and low-mass stars: dependence on coronal temperature |
small-scale magnetic flux ropes in the solar wind have been studied for decades via both simulation and observation. statistical analysis utilizing various in situ spacecraft measurements is the main observational approach, which helps investigate the generation and evolution of these small-scale structures. in this study, we extend the automated detection of small-scale flux ropes based on the grad-shafranov reconstruction to the complete data set of in situ measurements of the ulysses spacecraft. we first discuss the temporal variation of the bulk properties of 22,719 flux ropes found through our approach, namely, the average magnetic field and plasma parameters, etc., as functions of the heliographic latitudes and heliocentric radial distances. we then categorize all identified events into three groups based on event distributions in different latitudes separated by 30°, at different radial distances, and under different solar activities. with the detailed statistical analysis, we conclude the following: (1) the properties of flux ropes, such as the duration, scale size, etc., follow power-law distributions, but with different slope indices, especially for distributions at different radial distances. (2) they are also affected by the solar wind speed, which has different distributions under different solar activities, manifested as a latitudinal effect. (3) the main difference in flux rope properties between the low and high latitudes is attributed to possible alfvénic structures or waves and to flux ropes with relatively high alfvénicity. (4) flux ropes with longer durations and larger scale sizes occur more often at larger radial distances. (5) with a stricter walén slope threshold, more events are excluded at higher latitudes, which further reduces the latitudinal effects on flux rope properties. the entire database is published online at http://www.fluxrope.info. | analysis of small-scale magnetic flux ropes covering the whole ulysses mission |
the quark matter symmetry free energy and the thermodynamical properties of strange quark matter (sqm) in strong magnetic fields and non-zero temperature cases are discussed in this work within nambu-jona-lasinio (njl) model by considering two kinds of vector interactions. the properties of the proto-quark stars (pqss) are also studied by introducing a density-dependent magnetic field strength distribution and assuming two 'extreme' magnetic orientation cases inside the quark stars (qss). the results indicate that the strength and orientation distribution of the magnetic fields in the qss and the heating/cooling process during the star evolution may both influence the star mass of qss within su(3) njl model. | quark matter at finite temperature under strong magnetic fields within nambu-jona-lasinio model |
the pulsar wind model is updated by considering the effect of particle density and pulsar death. it can describe both the short-term and long-term rotational evolution of pulsars consistently. it is applied to model the rotational evolution of the crab pulsar. the pulsar is spun down by a combination of magnetic dipole radiation and particle wind. the parameters of the crab pulsar, including magnetic field, inclination angle, and particle density are calculated. the primary particle density in acceleration region is about 103 times the goldreich-julian charge density. the lower braking index between glitches is due to a larger outflowing particle density. this may be glitch induced magnetospheric activities in normal pulsars. evolution of braking index and the crab pulsar in p-dot{p} diagram are calculated. the crab pulsar will evolve from magnetic dipole radiation dominated case towards particle wind-dominated case. considering the effect of pulsar `death', the crab pulsar (and other normal pulsars) will not evolve to the cluster of magnetars but downwards to the death valley. different acceleration models are also considered. applications to other sources are also discussed, including pulsars with braking index measured, and the magnetar population. | rotational evolution of the crab pulsar in the wind braking model |
we study the simultaneous effects of the symmetry energy and temperature on the crust-core transition of a magnetar. the dynamical and the thermodynamical spinodals are used to calculate the transition region within a relativistic mean-field approach for the equation of state. quantizing magnetic fields with intensities in the range of 2 ×1015<b <5 ×1016g are considered. under these strong magnetic fields, the crust extension is very sensitive to the density dependence of the symmetry energy, and the properties that depend on the crust thickness could set a constraint on the equation of state. it is shown that the effect on the extension of the crust-core transition is washed out for temperatures above 109 k. however, for temperatures below that value, a noticeable effect exists that grows as the temperature decreases and which should be taken into account when the evolution of magnetars is studied. | crust-core transition of a neutron star: effects of the symmetry energy and temperature under strong magnetic fields |
cool stars with outer convective envelopes are observed to have magnetic fields with a variety of geometries, which on large scales are dominated by a combination of the lowest-order fields such as the dipole, quadrupole, and octupole modes. magnetized stellar wind outflows are primarily responsible for the loss of angular momentum from these objects during the main sequence. previous works have shown the reduced effectiveness of the stellar wind braking mechanism with increasingly complex but singular magnetic field geometries. in this paper, we quantify the impact of mixed dipolar and quadrupolar fields on the spin-down torque using 50 mhd simulations with mixed fields, along with 10 each of the pure geometries. the simulated winds include a wide range of magnetic field strength and reside in the slow-rotator regime. we find that the stellar wind braking torque from our combined geometry cases is well described by a broken power-law behavior, where the torque scaling with field strength can be predicted by the dipole component alone or the quadrupolar scaling utilizing the total field strength. the simulation results can be scaled and apply to all main-sequence cool stars. for solar parameters, the lowest-order component of the field (dipole in this paper) is the most significant in determining the angular momentum loss. | the effect of combined magnetic geometries on thermally driven winds. i. interaction of dipolar and quadrupolar fields |
we study the coupling of the force-free magnetosphere to the long-term internal evolution of a magnetar. we allow the relation between the poloidal and toroidal stream functions - that characterizes the magnetosphere - to evolve freely without constraining its particular form. we find that, on time-scales of the order of kyr, the energy stored in the magnetosphere gradually increases, as the toroidal region grows and the field lines expand outwards. this continues until a critical point is reached beyond which force-free solutions for the magnetosphere can no longer be constructed, likely leading to some large-scale magnetospheric reorganization. the energy budget available for such events can be as high as several 10^{45} erg for fields of 10^{14} g. subsequently, starting from the new initial conditions, the evolution proceeds in a similar manner. the time-scale to reach the critical point scales inversely with the magnetic field amplitude. allowing currents to pass through the last few metres below the surface, where the magnetic diffusivity is orders of magnitude larger than in the crust, should give rise to a considerable amount of energy deposition through joule heating. we estimate that the effective surface temperature could increase locally from {∼ } 0.1 kev to {∼ } 0.3-0.6 kev, in good agreement with observations. similarly, the power input from the interior into the magnetosphere could be as high as 10^{35}-10^{36} erg s-1, which is consistent with peak luminosities observed during magnetar outbursts. therefore, a detailed treatment of currents flowing through the envelope may be needed to explain the thermal properties of magnetars. | crust-magnetosphere coupling during magnetar evolution and implications for the surface temperature |
the magnetic interaction between a classical t tauri star and its surrounding accretion disk is thought to influence its rotational evolution. we use 2.5d magnetohydrodynamic, axisymmetric simulations of star-disk interaction, computed via the pluto code, to calculate the net torque acting on these stars. we divide the net torque into three contributions: accretion (spin-up), stellar winds (spin-down), and magnetospheric ejections (mes) (spin-up or down). in paper i, we explored interaction regimes in which the stellar magnetosphere truncates the inner disk at a location spinning faster than the star, resulting in a strong net spin-up contribution from accretion and mes ("steady accretion" regime). in this paper, we investigate interaction regimes in which the truncation radius gets closer to and even exceeds corotation, where it is possible for the disk material to gain angular momentum and be periodically ejected by the centrifugal barrier ("propeller" regime). this reduces the accretion torque, can change the sign of the me torque, and can result in a net stellar spin-down configuration. these results suggest it is possible to have a net spin-down stellar torque even for truncation radii within the corotation radius (r t ≳ 0.7r co). we fit semi-analytic functions for the truncation radius, and the torque associated with star-disk interaction (i.e., the sum of accretion and me torques) and stellar wind, allowing for the prediction of the net stellar torque for a parameter regime covering both net spin-up and spin-down configurations, as well as the possibility of investigating rotational evolution via 1d stellar evolution codes. | magnetic braking of accreting t tauri stars ii: torque formulation spanning spin-up and spin-down regimes |
tidal interaction between an exoplanet and its host star is a possible pathway to transfer angular momentum between the planetary orbit and the stellar spin. in cases where the planetary orbital period is shorter than the stellar rotation period, this may lead to angular momentum being transferred into the star's rotation, possibly counteracting the intrinsic stellar spin-down induced by magnetic braking. observationally, detecting altered rotational states of single, cool field stars is challenging, as precise ages for such stars are rarely available. here we present an empirical investigation of the rotation and magnetic activity of a sample of planet-hosting stars that are accompanied by wide stellar companions. without needing knowledge about the absolute ages of the stars, we test for relative differences in activity and rotation of the planet hosts and their co-eval companions, using x-ray observations to measure the stellar activity levels. employing three different tidal interaction models, we find that host stars with planets that are expected to tidally interact display elevated activity levels compared to their companion stars. we also find that those activity levels agree with the observed rotational periods for the host stars along the usual rotation-activity relationships, implying that the effect is indeed caused by a tidal interaction and not a purely magnetic interaction that would be expected to affect the stellar activity, but not necessarily the rotation. we conclude that massive, close-in planets have an impact on the stellar rotational evolution, while the smaller, more distant planets do not have a significant influence. | tidal star-planet interaction and its observed impact on stellar activity in planet-hosting wide binary systems |
magnetohydrodynamic (mhd) kink waves are ubiquitously observed in the solar atmosphere. the propagation and damping of these waves may play relevant roles in the transport and dissipation of energy in the solar atmospheric medium. however, in the atmospheric plasma dissipation of transverse mhd wave energy by viscosity or resistivity needs very small spatial scales to be efficient. here, we theoretically investigate the generation of small scales in nonuniform solar magnetic flux tubes due to phase mixing of mhd kink waves. we go beyond the usual approach based on the existence of a global quasi-mode that is damped in time due to resonant absorption. instead, we use a modal expansion to express the mhd kink wave as a superposition of alfvén continuum modes that are phase mixed as time evolves. the comparison of the two techniques evidences that the modal analysis is more physically transparent and describes both the damping of global kink motions and the building up of small scales due to phase mixing. in addition, we discuss that the processes of resonant absorption and phase mixing are closely linked. they represent two aspects of the same underlying physical mechanism: the energy cascade from large scales to small scales due to naturally occurring plasma and/or magnetic field inhomogeneities. this process may provide the necessary scenario for efficient dissipation of transverse mhd wave energy in the solar atmospheric plasma. | magnetohydrodynamic kink waves in nonuniform solar flux tubes: phase mixing and energy cascade to small scales |
turbulent mixing in the radiative regions of stars is usually either ignored or crudely accounted for in most stellar evolution models. however, there is growing evidence that such mixing is present and can affect various aspects of a star's life. here, we present a first attempt at quantifying mixing by horizontal shear instabilities in stars using direct numerical simulations. the shear is driven by a body force, and rapidly becomes unstable. at saturation, we find that several distinct dynamical regimes exist, depending on the relative importance of stratification and thermal diffusion. in each of the regimes identified, we propose a certain number of theoretically motivated scaling laws for the turbulent vertical eddy scale, the turbulent diffusion coefficient, and the amplitude of temperature fluctuations (among other quantities). based on our findings, we predict that the majority of stars should fall into one of two categories: high péclet number stratified turbulence, and low péclet number stratified turbulence. the latter is presented in a related paper by cope et al., while the former is discussed here. applying our results to the solar tachocline, we find that it should lie in the high péclet number stratified turbulence regime, and predict a substantial amount of vertical mixing for temperature, momentum, and composition. taken as is, the new turbulence model predictions are incompatible with the spiegel & zahn model of the solar tachocline. however, rotation and magnetic fields are likely to affect the turbulence, and need to be taken into account in future studies. | horizontal shear instabilities at low prandtl number |
the magnetorotational instability (mri) is key to physics in accretion disks and is widely considered to play some role in massive star core collapse. models of rotating massive stars naturally develop very strong shear at composition boundaries, a necessary condition for mri instability, and the mri is subject to triply diffusive destabilizing effects in radiative regions. we have used the mesa stellar evolution code to compute magnetic effects due to the spruit-tayler (st) mechanism and the mri, separately and together, in a sample of massive star models. we find that the mri can be active in the later stages of massive star evolution, leading to mixing effects that are not captured in models that neglect the mri. the mri and related magnetorotational effects can move models of given zero-age main sequence mass across "boundaries" from degenerate co cores to degenerate o/ne/mg cores and from degenerate o/ne/mg cores to iron cores, thus affecting the final evolution and the physics of core collapse. the mri acting alone can slow the rotation of the inner core in general agreement with the observed "initial" rotation rates of pulsars. the mri analysis suggests that localized fields ~1012 g may exist at the boundary of the iron core. with both the st and mri mechanisms active in the 20 m ⊙ model, we find that the helium shell mixes entirely out into the envelope. enhanced mixing could yield a population of yellow or even blue supergiant supernova progenitors that would not be standard sn iip. | the role of the magnetorotational instability in massive stars |
as another step towards understanding the long-term evolution of the magnetic field in neutron stars, we provide the first simulations of ambipolar diffusion in a spherical star. restricting ourselves to axial symmetry, we consider a charged-particle fluid of protons and electrons carrying the magnetic flux through a motionless, uniform background of neutrons that exerts a collisional drag force on the former. we also ignore the possible impact of β decays, proton superconductivity and neutron superfluidity. all initial magnetic field configurations considered are found to evolve on the analytically expected time-scales towards 'barotropic equilibria' satisfying the 'grad-shafranov equation', in which the magnetic force is balanced by the degeneracy pressure gradient, so ambipolar diffusion is choked. these equilibria are so-called 'twisted torus' configurations, which include poloidal and toroidal components, the latter restricted to the toroidal volumes in which the poloidal field lines close inside the star. in axial symmetry, they appear to be stable, although they are likely to undergo non-axially symmetric instabilities. | magnetic field evolution and equilibrium configurations in neutron star cores: the effect of ambipolar diffusion |
understanding how young stars gain their masses through disk-to-star accretion is of paramount importance in astrophysics. it affects our knowledge about the early stellar evolution, the disk lifetime and dissipation processes, the way the planets form on the smallest scales, or the connection to macroscopic parameters characterizing star-forming regions on the largest ones, among others. in turn, mass accretion rate estimates depend on the accretion paradigm assumed. for low-mass t tauri stars with strong magnetic fields there is consensus that magnetospheric accretion (ma) is the driving mechanism, but the transfer of mass in massive young stellar objects with weak or negligible magnetic fields probably occurs directly from the disk to the star through a hot boundary layer (bl). the intermediate-mass herbig ae/be (haebe) stars bridge the gap between both previous regimes and are still optically visible during the pre-main sequence phase, thus constituting a unique opportunity to test a possible change of accretion mode from ma to bl. this review deals with our estimates of accretion rates in haebes, critically discussing the different accretion paradigms. it shows that although mounting evidence supports that ma may extend to late-type haes but not to early-type hbes, there is not yet a consensus on the validity of this scenario versus the bl one. based on ma and bl shock modeling, it is argued that the ultraviolet regime could significantly contribute in the future to discriminating between these competing accretion scenarios. | on the mass accretion rates of herbig ae/be stars. magnetospheric accretion or boundary layer? |
hd189733 is an active k dwarf that is, with its transiting hot jupiter, among the most studied exoplanetary systems. in this first paper of the multiwavelength observations of an evaporating exoplanet and its star (moves) programme, we present a 2-yr monitoring of the large-scale magnetic field of hd189733. the magnetic maps are reconstructed for five epochs of observations, namely 2013 june-july, 2013 august, 2013 september, 2014 september and 2015 july, using zeeman-doppler imaging. we show that the field evolves along the five epochs, with mean values of the total magnetic field of 36, 41, 42, 32 and 37 g, respectively. all epochs show a toroidally dominated field. using previously published data of moutou et al. and fares et al., we are able to study the evolution of the magnetic field over 9 yr, one of the longest monitoring campaigns for a given star. while the field evolved during the observed epochs, no polarity switch of the poles was observed. we calculate the stellar magnetic field value at the position of the planet using the potential field source surface extrapolation technique. we show that the planetary magnetic environment is not homogeneous over the orbit, and that it varies between observing epochs, due to the evolution of the stellar magnetic field. this result underlines the importance of contemporaneous multiwavelength observations to characterize exoplanetary systems. our reconstructed maps are a crucial input for the interpretation and modelling of our moves multiwavelength observations. | moves - i. the evolving magnetic field of the planet-hosting star hd189733 |
the rate at which the solar wind extracts angular momentum (am) from the sun has been predicted by theoretical models for many decades, and yet we lack a conclusive measurement from in situ observations. in this letter we present a new estimate of the time-varying am flux in the equatorial solar wind, as observed by the wind spacecraft from 1994 to 2019. we separate the am flux into contributions from the protons, alpha particles, and magnetic stresses, showing that the mechanical flux in the protons is ∼3 times larger than the magnetic field stresses. we observe the tendency for the am flux of fast wind streams to be oppositely signed to the slow wind streams, as noted by previous authors. from the average total flux, we estimate the global am loss rate of the sun to be 3.3 × 1030 erg, which lies within the range of various magnetohydrodynamic wind models in the literature. this am loss rate is a factor of ∼2 weaker than required for a skumanich-like rotation period evolution ({{{ω }}}*\propto stellar age-1/2), which should be considered in studies of the rotation period evolution of sun-like stars. | direct detection of solar angular momentum loss with the wind spacecraft |
we consider the case of radio flares from black hole x-ray binaries in which the flare spectrum evolves from optically thick to optically thin, under the assumption that this is due to decreasing optical depth to synchrotron self-absorption. we are able to place upper and lower limits on the size of the emitting region associated with a radio flare, and determine the synchrotron source magnetic field and energy as a function of size. the energy has a clear minimum which occurs close to the condition that the magnetic field derived from synchrotron self-absorption equals that calculated from equipartition. this minimum energy estimate is independent of the rise time of the event, and so may be applied to any event for which the peak flux is measured and there is evidence for self-absorption. this is a much more accurate approach to minimum energy estimation than assuming expansion at close to the speed of light. we apply this method to four examples of optically thick radio flares and find that in each case either the filling factor of the synchrotron source is considerably less than unity, or the expansion speed is considerably less than the speed of light. the combination of unity filling factor and expansion speeds close to the speed of light is completely ruled out on energetic grounds for three of the four events we consider. the inferred slowed expansion is consistent with detailed modelling of such events, which has been recently reported in the literature. the minimum power requirements associated with the flares are found to be ∼1036 erg s-1, which are easily accommodated in the context of stellar mass black hole accretion at near-eddington levels, when these flares typically occur. however, the true jet power could still be orders of magnitude higher. | synchrotron self-absorption and the minimum energy of optically thick radio flares from stellar mass black holes |
black widows are millisecond pulsars with low-mass companions, a few per cent the mass of the sun, on orbits of several hours. these companions are presumably the remnants of main-sequence stars that lost their mass through a combination of roche lobe overflow and ablation by the host pulsar's high-energy radiation. while ablation itself is too weak to significantly reduce the mass of the companion star, the ablated wind couples to its magnetic field, removes orbital angular momentum, and thus maintains stable roche lobe overflow. we use the mesa stellar evolution code, complemented by analytical estimates, to track initially main-sequence companions as they are reduced to a fraction of their original mass by this ablation-driven magnetic braking. we argue that magnetic braking remains effective even for low-mass companions. a key ingredient of our model is that the irradiating luminosity of the pulsar lirr deposits energy in the companion's atmosphere and thereby slows down its kelvin-helmholtz cooling. we find that the high-energy luminosities measured by fermi $l_{\rm irr}=0.1\rm {-}3$ l⊙ can explain the span of black widow orbital periods. the same lirr range reproduces the companions' night-side temperatures, which cluster around 3000 k, as inferred from optical light curves. | black widow formation by pulsar irradiation and sustained magnetic braking |
off-centred dipole configurations have been suggested to explain different phenomena in neutron stars, such as natal kicks, irregularities in polarization of radio pulsars and properties of x-ray emission from millisecond pulsars. here, for the first time, we model magnetothermal evolution of neutron stars with crust-confined magnetic fields and off-centred dipole moments. we find that the dipole shift decays with time if the initial configuration has no toroidal magnetic field. the decay time-scale is inversely proportional to magnetic field. the octupole moment decreases much faster than the quadrupole. alternatively, if the initial condition includes strong dipolar toroidal magnetic field, the external poloidal magnetic field evolves from centred dipole to off-centred dipole. the surface thermal maps are very different for configurations with weak b = 1013 g and strong b = 1014 g magnetic fields. in the former case, the magnetic equator is cold while in the latter case, it is hot. we model light curves and spectra of our magnetothermal configurations. we found that in the case of cold equator, the pulsed fraction is small (below a few per cent in most cases) and spectra are well described with a single blackbody. under the same conditions, models with stronger magnetic fields produce light curves with pulsed fraction of tens of per cent. their spectra are significantly better described with two blackbodies. overall, the magnetic field strength has a more significant effect on bulk thermal emission of neutron stars than does the field geometry. | three-dimensional magnetothermal evolution of off-centred dipole magnetic field configurations in neutron stars |
aims: a large number of calcium infrared triplet (irt) spectra are expected from the gaia and carmenes missions. conversion of these spectra into known activity indicators will allow analysis of their temporal evolution to a better degree. we set out to find such a conversion formula and to determine its robustness.methods: we have compared 2274 ca ii irt spectra of active main-sequence f to k stars taken by the tigre telescope with those of inactive stars of the same spectral type. after normalizing and applying rotational broadening, we subtracted the comparison spectra to find the chromospheric excess flux caused by activity. we obtained the total excess flux, and compared it to established activity indices derived from the ca ii h and k lines, the spectra of which were obtained simultaneously to the infrared spectra.results: the excess flux in the ca ii irt is found to correlate well with r'hk and r+hk, as well as smwo, if the b - v-dependency is taken into account. we find an empirical conversion formula to calculate the corresponding value of one activity indicator from the measurement of another, by comparing groups of datapoints of stars with similar b - v. | the ca ii infrared triplet's performance as an activity indicator compared to ca ii h and k. empirical relations to convert ca ii infrared triplet measurements to common activity indices |
we present the results of the neutron star x-ray binary system cen x-3 performed by insight-hxmt with two observations during 2017 and 2018. during these two observations, the source reached a x-ray luminosity of ~1038 erg s-1 from 2-105 kev. the analysis of the broadband x-ray spectrum reports the presence of two cyclotron resonance scattering features (crsfs) with the fundamental line at ~ 28 kev and the harmonic line at ~47 kev. the multiple lines exist by fittings with different continuum models, like the absorbed negative and positive power-law with an exponential cutoff (npex) model and a power-law with high energy exponential cutoff model. this is the first time that both fundamental and harmonic lines are detected in cen x-3. we also show evidence of two cyclotron lines in the phase-resolved spectrum of cen x-3. the crsf and continuum spectral parameters show evolution with the pulse profile, and the two line centroid energy ratio does not change significantly and locates in a narrow value range of 1.6-1.7 over the pulse phase. the implications of the discovering two cyclotron absorption features and phase-resolved spectral properties are discussed. | discovery of two cyclotron resonance scattering features in x-ray pulsar cen x-3 by insight-hxmt |
we report on the presence of numerous tiny bright dots in and around an emerging flux region (an x-ray/coronal bright point) observed with solo's eui/hrieuv in 174 å. these dots are roundish and have a diameter of 675 ± 300 km, a lifetime of 50 ± 35 s, and an intensity enhancement of 30% ± 10% above their immediate surroundings. about half of the dots remain isolated during their evolution and move randomly and slowly (<10 km s-1). the other half show extensions, appearing as a small loop or surge/jet, with intensity propagations below 30 km s-1. many of the bigger and brighter hrieuv dots are discernible in the sdo/aia 171 å channel, have significant emissivity in the temperature range of 1-2 mk, and are often located at polarity inversion lines observed in sdo/hmi los magnetograms. although not as pervasive as in observations, a bifrost mhd simulation of an emerging flux region does show dots in synthetic fe ix/x images. these dots in the simulation show distinct doppler signatures-blueshifts and redshifts coexist, or a redshift of the order of 10 km s-1 is followed by a blueshift of similar or higher magnitude. the synthetic images of o v/vi and si iv lines, which represent transition region radiation, also show the dots that are observed in fe ix/x images, often expanded in size, or extended as a loop, and always with stronger doppler velocities (up to 100 km s-1) than that in fe ix/x lines. our observation and simulation results, together with the field geometry of dots in the simulation, suggest that most dots in emerging flux regions form in the lower solar atmosphere (at ≈ 1 mm) by magnetic reconnection between emerging and preexisting/emerged magnetic field. some dots might be manifestations of magnetoacoustic shocks through the line formation region of fe ix/x emission. | solo/eui observations of ubiquitous fine-scale bright dots in an emerging flux region: comparison with a bifrost mhd simulation |
we show that the performances of a stern-gerlach apparatus can be improved by using a magnetic field profile for the atomic spin evolution designed with the shortcut-to-adiabaticity technique. interestingly, it can be made more compact, for atomic beams propagating at a given velocity, and more resilient to a dispersion in velocity in comparison with the results obtained with a standard uniform rotation of the magnetic field. our results are obtained using a reverse-engineering approach based on lewis-riesenfeld invariants. we discuss quantitatively the advantages offered by our configuration in terms of the resources involved and show that it drastically enhances the fidelity of the quantum state transfer achieved by the stern-gerlach device. | shortcut to adiabaticity in a stern-gerlach apparatus |
we present the initial results of the large impact of magnetic fields on the evolution of hot stars (life) project. the focus of this project is the search for magnetic fields in evolved oba giants and supergiants with visual magnitudes between 4 and 8, with the aim to investigate how the magnetic fields observed in upper main-sequence (ms) stars evolve from the ms until the late post-ms stages. in this paper, we present spectropolarimetric observations of 15 stars observed using the espadons instrument of the canada-france-hawaii telescope. for each star, we have determined the fundamental parameters and have used stellar evolution models to calculate their mass, age, and radius. using the least-squared deconvolution technique, we have produced averaged line profiles for each star. from these profiles, we have measured the longitudinal magnetic field strength and have calculated the detection probability. we report the detection of magnetic fields in two stars of our sample: a weak field of bl = 1.0 ± 0.2 g is detected in the post-ms a5 star 19 aur and a stronger field of bl = -230 ± 10 g is detected in the ms/post-ms b8/9 star hr 3042. | first results from the life project: discovery of two magnetic hot evolved stars |
the role of slow-mode magnetohydrodynamic (mhd) shocks in magnetic reconnection is of great importance for energy conversion and transport, but in many astrophysical plasmas the plasma is not fully ionised. in this paper, we use numerical simulations to investigate the role of collisional coupling between a proton-electron, charge-neutral fluid and a neutral hydrogen fluid for the one-dimensional (1d) riemann problem initiated in a constant pressure and density background state by a discontinuity in the magnetic field. this system, in the mhd limit, is characterised by two waves. the first is a fast-mode rarefaction wave that drives a flow towards a slow-mode mhd shock wave. the system evolves through four stages: initiation, weak coupling, intermediate coupling, and a quasi-steady state. the initial stages are characterised by an over-pressured neutral region that expands with characteristics of a blast wave. in the later stages, the system tends towards a self-similar solution where the main drift velocity is concentrated in the thin region of the shock front. because of the nature of the system, the neutral fluid is overpressured by the shock when compared to a purely hydrodynamic shock, which results in the neutral fluid expanding to form the shock precursor. once it has formed, the thickness of the shock front is proportional to ξ i-1.2 , which is a smaller exponent than would be naively expected from simple scaling arguments. one interesting result is that the shock front is a continuous transition of the physical variables of subsonic velocity upstream of the shock front (a c-shock) to a sharp jump in the physical variables followed by a relaxation to the downstream values for supersonic upstream velocity (a j-shock). the frictional heating that results from the velocity drift across the shock front can amount to ~2 per cent of the reference magnetic energy. | the formation and evolution of reconnection-driven, slow-mode shocks in a partially ionised plasma |
we present submillimeter array 880 μm dust polarization observations of six massive dense cores in the dr21 filament. the dust polarization shows complex magnetic field structures in the massive dense cores with sizes of 0.1 pc, in contrast to the ordered magnetic fields of the parsec-scale filament. the major axes of the massive dense cores appear to be aligned either parallel or perpendicular to the magnetic fields of the filament, indicating that the parsec-scale magnetic fields play an important role in the formation of the massive dense cores. however, the correlation between the major axes of the cores and the magnetic fields of the cores is less significant, suggesting that during the core formation, the magnetic fields below 0.1 pc scales become less important than the magnetic fields above 0.1 pc scales in supporting a core against gravity. our analysis of the angular dispersion functions of the observed polarization segments yields a plane-of-sky magnetic field strength of 0.4-1.7 mg for the massive dense cores. we estimate the kinematic, magnetic, and gravitational virial parameters of the filament and the cores. the virial parameters show that the gravitational energy in the filament dominates magnetic and kinematic energies, while the kinematic energy dominates in the cores. our work suggests that although magnetic fields may play an important role in a collapsing filament, the kinematics arising from gravitational collapse must become more important than magnetic fields during the evolution from filaments to massive dense cores. | magnetic fields in the massive dense cores of the dr21 filament: weakly magnetized cores in a strongly magnetized filament |
context. forming massive stars launch outflows of magnetic origin, which in fact serve as a marker for finding sites of massive star formation. however, both the theoretical and observational study of the mechanisms that intervene in the formation and propagation of such magnetically driven outflows has been possible only until recent years.aims: with this work, we aim to study the mechanisms that drive highly collimated outflows from early stages of the formation of a massive star, in detail, and how those processes are impacted by the properties of the natal environment of the forming massive star.methods: we performed a series of 31 simulations with the aim of building a unified theoretical picture of these mechanisms, and determined how the impact of different environments alter their morphology and momentum output. the magnetohydrodynamical simulations also consider ohmic dissipation as a nonideal effect, self-gravity, and diffusive radiation transport for thermal absorption and emission by the dust and gas. we started from a collapsing cloud core that is threaded by an initially uniform magnetic field and which is slowly rotating. we utilized a two-dimensional axisymmetric grid in spherical coordinates.results: in the simulations, we can clearly distinguish a fast, magneto-centrifugally launched and collimated jet (of speeds ≳100 km s−1), from a wider magnetic tower flow driven by magnetic pressure which broadens in time. we analyze the acceleration of the flow, in detail, and its recollimation by magnetic forces happening at distances of several hundreds of astronomical units. we quantify the impact of magnetic braking in the outflows, which narrows the outflow cavity for the late evolution of the system. we find that despite the nonscalability of self-gravity and the thermodynamics of the medium, our results scale with the mass of the cloud core and can, in principle, be used with a range of values for such mass. we observe the presence of the same jet-driving mechanisms for a wide range of assumptions on the natal environment of the massive protostar, but with changes to their morphology and mechanical feedback into larger scales over time. | modeling disks and magnetic outflows around a forming massive star. ii. dynamics of jets from massive protostars |
ɛ lupi a is a binary system consisting of two main-sequence early b-type stars aa and ab in a short period, moderately eccentric orbit. the close binary pair is the only doubly magnetic massive binary currently known. using photometric data from the brite constellation we identify a modest heartbeat variation. combining the photometry with radial velocities of both components we determine a full orbital solution including empirical masses and radii. these results are compared with stellar evolution models as well as interferometry and the differences discussed. we also find additional photometric variability at several frequencies, finding it unlikely these frequencies can be caused by tidally excited oscillations. we do, however, determine that these signals are consistent with gravity mode pulsations typical for slowly pulsating b stars. finally we discuss how the evolution of this system will be affected by magnetism, determining that tidal interactions will still be dominant. | ɛ lupi: measuring the heartbeat of a doubly magnetic massive binary with brite constellation |
energetic outflows from main sequence stars accreting mass at very high rates might account for the powering of some eruptive objects, such as merging main sequence stars, major eruptions of luminous blue variables, e.g., the great eruption of eta carinae, and other intermediate luminosity optical transients (ilots; red novae; red transients). these powerful outflows could potentially also supply the extra energy required in the common envelope process and in the grazing envelope evolution of binary systems. we propose that a massive outflow/jets mediated by magnetic fields might remove energy and angular momentum from the accretion disk to allow such high accretion rate flows. by examining the possible activity of the magnetic fields of accretion disks, we conclude that indeed main sequence stars might accrete mass at very high rates, up to ≈ 10-2 m ⊙ yr-1 for solar type stars, and up to ≈ 1 m ⊙ yr-1 for very massive stars. we speculate that magnetic fields amplified in such extreme conditions might lead to the formation of massive bipolar outflows that can remove most of the disk's energy and angular momentum. it is this energy and angular momentum removal that allows the very high mass accretion rate onto main sequence stars. | binary interactions with high accretion rates onto main sequence stars |
accretion on to a highly magnetized neutron star runs through a magnetospheric flow, where the plasma follows the magnetic field lines in the force-free regime. the flow entering the magnetosphere is accelerated by the gravity of the star and then abruptly decelerated in a shock located above the surface of the star. for large enough mass accretion rates, most of the radiation comes from the radiation-pressure-dominated region below the shock, known as accretion column. though the one-dimensional, stationary structure of this flow has been studied for many years, its global dynamics was hardly ever considered before. considering the time-dependent structure of an accretion column allows us to test the stability of the existing stationary analytic solution, as well as its possible variability modes, and check the validity of its boundary conditions. using a conservative scheme, we perform one-dimensional time-dependent simulations of an ideal radiative mhd flow inside an aligned dipolar magnetosphere. whenever thermal pressure locally exceeds magnetic pressure, the flow is assumed to lose mass. position of the shock agrees well with the theoretical predictions below a limit likely associated with advection effects: if more than $2/3$ of the released power is advected with the flow, the analytic solution becomes self-inconsistent, and the column starts leaking at a finite height. depending on the geometry, this breakdown may broaden the column, mass load the field lines, and produce radiation-driven, mildly relativistic ejecta. evolving towards the equilibrium position, the shock front experiences damped oscillations at a frequency close to the inverse sound propagation time. | simulating the shock dynamics of a neutron star accretion column |
we investigate the effect of non-ideal magnetohydrodynamics (mhd) on the formation of binary stars using a suite of three-dimensional smoothed particle magnetohydrodynamics simulations of the gravitational collapse of 1 m⊙, rotating, perturbed molecular-cloud cores. alongside the role of ohmic resistivity, ambipolar diffusion and the hall effect, we also examine the effects of magnetic field strength, orientation and amplitude of the density perturbation. when modelling sub-critical cores, ideal mhd models do not collapse whereas non-ideal mhd models collapse to form single protostars. in supercritical ideal mhd models, increasing the magnetic field strength or decreasing the initial-density perturbation amplitude decreases the initial binary separation. strong magnetic fields initially perpendicular to the rotation axis suppress the formation of binaries and yield discs with magnetic fields ∼10 times stronger than if the magnetic field was initially aligned with the rotation axis. when non-ideal mhd is included, the resulting discs are larger and more massive, and the binary forms on a wider orbit. small differences in the supercritical cores caused by non-ideal mhd effects are amplified by the binary interaction near periastron. overall, the non-ideal effects have only a small impact on binary formation and early evolution, with the initial conditions playing the dominant role. | the impact of non-ideal magnetohydrodynamics on binary star formation |
both observations and numerical simulations suggest that alfvénic waves may carry sufficient energy to sustain the hot temperatures of the solar atmospheric plasma. however, the thermalization of wave energy is inefficient unless very short spatial scales are considered. phase mixing is a mechanism that can take energy down to dissipation lengths, but it operates over too long a timescale. here, we study how turbulence, driven by the nonlinear evolution of phase-mixed torsional alfvén waves in coronal loops, is able to take wave energy down to the dissipative scales much faster than the theory of linear phase mixing predicts. we consider a simple model of a transversely nonuniform cylindrical flux tube with a constant axial magnetic field. the flux tube is perturbed by the fundamental mode of standing torsional alfvén waves. we solved the three-dimensional ideal magnetohydrodynamics equations numerically to study the temporal evolution. initially, torsional alfvén waves undergo the process of phase mixing because of the transverse variation of density. after only few periods of torsional waves, azimuthal shear flows generated by phase mixing eventually trigger the kelvin-helmholtz instability (khi), and the flux tube is subsequently driven to a turbulent state. turbulence is very anisotropic and develops transversely only to the background magnetic field. after the onset of turbulence, the effective reynolds number decreases in the flux tube much faster than in the initial linear stage governed by phase mixing alone. we conclude that the nonlinear evolution of torsional alfvén waves, and the associated khi, is a viable mechanism for the onset of turbulence in coronal loops. turbulence can significantly speed up the generation of small scales. enhanced deposition rates of wave energy into the coronal plasma are therefore expected. movies are available at https://www.aanda.org | transition to turbulence in nonuniform coronal loops driven by torsional alfvén waves |
tidal disruption events (tdes) occur when a star gets torn apart by the strong tidal forces of a supermassive black hole, which results in the formation of a debris stream that partly falls back towards the compact object. this gas moves along inclined orbital planes that intersect near pericentre, resulting in a so-called 'nozzle shock'. we perform the first dedicated study of this interaction, making use of a two-dimensional simulation that follows the transverse gas evolution inside a given section of stream. this numerical approach circumvents the lack of resolution encountered near pericentre passage in global three-dimensional simulations using particle-based methods. as it moves inward, we find that the gas motion is purely ballistic, which near pericentre causes strong vertical compression that squeezes the stream into a thin sheet. dissipation takes place at the resulting nozzle shock, inducing a rise in pressure that causes the collapsing gas to bounce back, although without imparting significant net expansion. as it recedes to larger distances, this matter continues to expand while remaining thin despite the influence of pressure forces. this gas evolution specifies the strength of the subsequent self-crossing shock, which we find to be more affected by black hole spin than previously estimated. we also evaluate the impact of general relativistic effects, viscous dissipation, magnetic fields, and radiative processes on the nozzle shock. this study represents an important step forward in the theoretical understanding of tdes, bridging the gap between our robust knowledge of the fallback rate and the more complex following stages, during which most of the emission occurs. | the nozzle shock in tidal disruption events |
we perform the first 3d fully coupled magneto-thermal simulations of neutron stars (including the most realistic background structure and microphysical ingredients so far) applied to a very complex initial magnetic field topology in the crust, similar to what was recently obtained by proto-neutron stars dynamo simulations. in such configurations, most of the energy is stored in the toroidal field, while the dipolar component is a few per cent of the mean magnetic field. this initial feature is maintained during the long-term evolution (~106 yr), since the hall term favours a direct cascade (compensating for ohmic dissipation) rather than a strong inverse cascade, for such an initial field topology. the surface dipolar component, responsible for the dominant electromagnetic spin-down torque, does not show any increase in time, when starting from this complex initial topology. this is in contrast to the timing properties of young pulsars and magnetars which point to higher values of the surface dipolar fields. a possibility is that the deep-seated magnetic field (currents in the core) is able to self-organize in large scales (during the collapse or in the early life of a neutron star). alternatively, the dipolar field might be lower than is usually thought, with magnetosphere substantially contributing to the observed high spin-down, via e.g. strong winds or strong coronal magnetic loops, which can also provide a natural explanation to the tiny surface hotspots inferred from x-ray data. | 3d evolution of neutron star magnetic fields from a realistic core-collapse turbulent topology |
strong magnetic fields play an important role in powering the emission of neutron stars. nevertheless, a full understanding of the interior configuration of the field remains elusive. in this work, we present general relativistic magnetohydrodynamics (mhd) simulations of the magnetic field evolution in neutron stars lasting ${\sim } {880}\,$ms (~6.5 alfvén crossing periods) and up to resolutions of $0.1155\,$km using athena++. we explore two different initial conditions, one with purely poloidal magnetic field and the other with a dominant toroidal component, and study the poloidal and toroidal field energies, the growth times of the various instability-driven oscillation modes, and turbulence. we find that the purely poloidal setup generates a toroidal field, which later decays exponentially reaching $1{{\ \rm per\ cent}}$ of the total magnetic energy, showing no evidence of reaching equilibrium. the initially stronger toroidal field setup, on the other hand, loses up to 20 per cent of toroidal energy and maintains this state till the end of our simulation. we also explore the hypothesis, drawn from previous mhd simulations, that turbulence plays an important role in the quasi-equilibrium state. an analysis of the spectra in our higher resolution setups reveals, however, that in most cases we are not observing turbulence at small scales, but rather a noisy velocity field inside the star. we also observe that the majority of the magnetic energy gets dissipated as heat increasing the internal energy of the star, while a small fraction gets radiated away as electromagnetic radiation. | long-term general relativistic magnetohydrodynamics simulations of magnetic field in isolated neutron stars |
context. understanding the collapse of dense molecular cloud cores to stellar densities and the subsequent evolution of the protostar is of importance to model the feedback effects such an object has on its surrounding environment, as well as describing the conditions with which it enters the stellar evolutionary track. this process is fundamentally multi-scale, both in density and in spatial extent, and requires the inclusion of complex physical processes such as self-gravity, turbulence, radiative transfer, and magnetic fields. as such, it necessitates the use of robust numerical simulations.aims: we aim to model the birth and early evolution of a low-mass protostar. we also seek to describe the interior structure of the protostar and the radiative behavior of its accretion shock front.methods: we carried out a high resolution numerical simulation of the collapse of a gravitationally unstable 1 m⊙ dense molecular cloud core to stellar densities using 3d radiation hydrodynamics under the gray flux-limited diffusion approximation. we followed the initial isothermal phase, the first adiabatic contraction, the second gravitational collapse triggered by the dissociation of h2 molecules, and ≈247 days of the subsequent main accretion phase.results: we find that the subcritical radiative behavior of the protostar's shock front causes it to swell as it accretes matter. we also find that the protostar is turbulent from the moment of its inception despite its radiative stability. this turbulence causes significant entropy mixing inside the protostar, which regulates the swelling. furthermore, we find that the protostar is not fully ionized at birth, but the relative amount of ionized material within it increases as it accretes matter from its surroundings. finally, we report in the appendix the results of the first 3d calculations involving a frequency-dependent treatment of radiative transfer, which has not produced any major differences with its gray counterpart. | the birth and early evolution of a low-mass protostar |
solar-type stars, which shed angular momentum via magnetized stellar winds, enter the main sequence with a wide range of rotational periods prot. this initially wide range of rotational periods contracts and has mostly vanished by a stellar age $t\sim {0.6}\, {\rm gyr}$, after which solar-type stars spin according to the skumanich relation $p_\text{rot}\propto \sqrt{t}$. magnetohydrodynamic stellar wind models can improve our understanding of this convergence of rotation periods. we present wind models of 15 young solar-type stars aged ~24 myr to ~0.13 gyr. with our previous wind models of stars aged ~0.26 and ~0.6 gyr we obtain 30 consistent three-dimensional wind models of stars mapped with zeeman-doppler imaging - the largest such set to date. the models provide good cover of the pre-skumanich phase of stellar spin-down in terms of rotation, magnetic field, and age. we find the mass-loss rate $\dot{m}\propto \phi ^{{0.9\pm 0.1}}$ with a residual spread of ~150 per cent and the wind angular momentum loss rate $\dot{j}\propto {}p_\text{rot}^{-1} \phi ^{1.3\pm 0.2}$ with a residual spread of ~500 per cent where φ is the unsigned surface magnetic flux. when comparing different magnetic field scalings for each single star we find a gradual reduction in the power-law exponent with increasing magnetic field strength. | the winds of young solar-type stars in the pleiades, ab doradus, columba, and β pictoris |
we present the implementation of general-relativistic resistive magnetohydrodynamics solvers and three divergence-free handling approaches adopted in the general-relativistic multigrid numerical (gmunu) code. in particular, implicit-explicit runge-kutta schemes are used to deal with the stiff terms in the evolution equations for small resistivity. the three divergence-free handling methods are (i) hyperbolic divergence cleaning (also known as the generalized lagrange multiplier), (ii) staggered-meshed constrained transport schemes, and (iii) elliptic cleaning through a multigrid solver, which is applicable in both cell-centered and face-centered (stagger grid) magnetic fields. the implementation has been tested with a number of numerical benchmarks from special-relativistic to general-relativistic cases. we demonstrate that our code can robustly recover from the ideal magnetohydrodynamics limit to a highly resistive limit. we also illustrate the applications in modeling magnetized neutron stars, and compare how different divergence-free handling methods affect the evolution of the stars. furthermore, we show that the preservation of the divergence-free condition of the magnetic field when using staggered-meshed constrained transport schemes can be significantly improved by applying elliptic cleaning. | an extension of gmunu: general-relativistic resistive magnetohydrodynamics based on staggered-meshed constrained transport with elliptic cleaning |
context. for moderate and slow rotation, the magnetic activity of solar-like stars is observed to strongly depend on rotation, while for rapid rotation, only a very weak or no dependency is detected. these observations do not yet have a solid explanation in terms of dynamo theory.aims: we aim to find such an explanation by numerically investigating the rotational dependency of dynamo drivers in solar-like stars, that is, stars that have a convective envelope of similar thickness to that of the sun.methods: we ran semi-global convection simulations of stars with rotation rates from 0 to 30 times the solar value, corresponding to coriolis numbers, co, of 0 to 110. we measured the turbulent transport coefficients contributing to the magnetic field evolution with the help of the test-field method, and compared with the dynamo effect arising from the differential rotation that is self-consistently generated in the models.results: the trace of the α tensor increases for moderate rotation rates with co0.5 and levels off for rapid rotation. this behavior is in agreement with the kinetic α based on the kinetic helicity, if one takes into account the decrease of the convective scale with increasing rotation. the α tensor becomes highly anisotropic for co ≳ 1. furthermore, αrr dominates for moderate rotation (1 < co < 10), and αϕϕ for rapid rotation (co ≳ 10). the effective meridional flow, taking into account the turbulent pumping effects, is markedly different from the actual meridional circulation profile. hence, the turbulent pumping effect is dominating the meridional transport of the magnetic field. taking all dynamo effects into account, we find three distinct regimes. for slow rotation, the α and rädler effects are dominating in the presence of anti-solar differential rotation. for moderate rotation, α and ω effects are dominant, indicative of αω or α2ω dynamos in operation, producing equatorward-migrating dynamo waves with a qualitatively solar-like rotation profile. for rapid rotation, an α2 mechanism with an influence from the rädler effect appears to be the most probable driver of the dynamo.conclusions: our study reveals the presence of a large variety of dynamo effects beyond the classical αω mechanism, which need to be investigated further to fully understand the dynamos of solar-like stars. the highly anisotropic α tensor might be the primary reason for the change of axisymmetric to non-axisymmetric dynamo solutions in the moderate rotation regime. | rotational dependence of turbulent transport coefficients in global convective dynamo simulations of solar-like stars |
we study the general-relativistic dynamics of matter being accreted on to and ejected by a magnetized and non-rotating neutron star. the dynamics is followed in the framework of fully general relativistic magnetohydrodynamics (grmhd) within the ideal-mhd limit and in two spatial dimensions. more specifically, making use of the numerical code bhac, we follow the evolution of a geometrically thick matter torus driven into accretion by the development of a magnetorotational instability. by making use of a number of simulations in which we vary the strength of the stellar dipolar magnetic field, we can determine self-consistently the location of the magnetospheric (or alfvén) radius rmsph and study how it depends on the magnetic moment μ and on the accretion rate. overall, we recover the analytic newtonian scaling relation, i.e. rmsph ∝ b4/7, but also find that the dependence on the accretion rate is very weak. furthermore, we find that the material torque correlates linearly with the mass-accretion rate, although both of them exhibit rapid fluctuations. interestingly, the total torque fluctuates drastically in strong magnetic field simulations and these unsteady torques observed in the simulations could be associated with the spin fluctuations observed in x-ray pulsars. | grmhd simulations of accreting neutron stars i: non-rotating dipoles |
context. magnetic fields influence the formation and evolution of stars and impact the observed stellar properties. magnetic a-type stars (ap stars) are a prime example of this. access to precise and accurate determinations of their stellar fundamental properties, such as masses and ages, is crucial to understand the origin and evolution of fossil magnetic fields.aims: we propose using the radii and luminosities determined from interferometric measurements, in addition to seismic constraints when available, to infer fundamental properties of 14 ap stars préviously characterised.methods: we used a grid-based modelling approach, employing stellar models computed with the cestam stellar evolution code, and the parameter search performed with the aims optimisation method. the stellar model grid was built using a wide range of initial helium abundances and metallicities in order to avoid any bias originating from the initial chemical composition. the large frequency separations (δν) of hr 1217 (hd 24712) and α cir (hd 128898), two rapidly oscillating ap stars of the sample, were used as seismic constraints.results: we inferred the fundamental properties of the 14 stars in the sample. the overall results are consistent within 1σ with previous studies, however, the stellar masses inferred in this study are higher. this trend likely originates from the broader range of chemical compositions considered in this work. we show that the use of δν in the modelling significantly improves our inferences, allowing us to set reasonable constraints on the initial metallicity which is, otherwise, unconstrained. this gives an indication of the efficiency of atomic diffusion in the atmospheres of roap stars and opens the possibility of characterising the transport of chemical elements in their interiors. | fundamental properties of a selected sample of ap stars: inferences from interferometric and asteroseismic constraints |
as followup to our recent detection of a pre-transit signal around hd 189733 b, we obtained full pre-transit phase coverage of a single planetary transit. the pre-transit signal is again detected in the balmer lines but with variable strength and timing, suggesting that the bow shock geometry reported in our previous work does not describe the signal from the latest transit. we also demonstrate the use of the ca ii h and k residual core flux as a proxy for the stellar activity level throughout the transit. a moderate trend is found between the pre-transit absorption signal in the 2013 data and the ca ii h flux. this suggests that some of the 2013 pre-transit hydrogen absorption can be attributed to varying stellar activity levels. a very weak correlation is found between the ca ii h core flux and the balmer line absorption in the 2015 transit, hinting at a smaller contribution from stellar activity compared to the 2013 transit. we simulate how varying stellar activity levels can produce changes in the balmer line transmission spectra. these simulations show that the strength of the 2013 and 2015 pre-transit signals can be reproduced by stellar variability. if the pre-transit signature is attributed to circumplanetary material, its evolution in time can be described by accretion clumps spiraling toward the star, although this interpretation has serious limitations. further high-cadence monitoring at hα is necessary to distinguish between true absorption by transiting material and short-term variations in the stellar activity level. | variation in the pre-transit balmer line signal around the hot jupiter hd 189733b |
we have studied numerically the evolution of magnetic fields in barotropic neutron stars, by performing non-linear magnetohydrodynamical simulations with the code pluto. for both initially predominantly poloidal and toroidal fields, with varying strengths, we find that the field settles down to a mixed poloidal-toroidal configuration, where the toroidal component contributes between ${\rm 10}$ and $20 {{\ \rm per\ cent}}$ of the total magnetic energy. this is, however, not a strict equilibrium, as the instability leads to the development of turbulence, which, in turn, gives rise to an inverse helicity cascade, which determines the final 'twisted torus' setup. the final field configuration is thus dictated by the non-linear saturation of the instability, and is not stationary. the average energy of the poloidal and toroidal components, however, is approximately stable in our simulations, and a complex multipolar structure emerges at the surface, while the magnetic field is dipolar at the exterior boundary, outside the star. | magnetic field configurations in neutron stars from mhd simulations |
we present the results obtained from the analysis of high-mass x-ray binary pulsar 4u 1909+07 using nustar and astrosat observations in july 2015 and 2017, respectively. x-ray pulsations at ≈604 s are clearly detected in our study. based on the long-term spin-frequency evolution, the source is found to spun-up in the last 17 yr. we observed a strongly energy-dependent pulse profile that evolved from a complex broad structure in soft x-rays into a profile with a narrow emission peak followed by a plateau in energy ranges above 20 kev. this behaviour ensured a positive correlation between the energy and pulse fraction. the pulse profile morphology and its energy evolution are almost similar during both the observations, suggesting a persistent emission geometry of the pulsar over time. the broad-band energy spectrum of the pulsar is approximated by an absorbed high-energy exponential cut-off power-law model with iron emission lines. in contrast to the previous report, we found no statistical evidence for the presence of cyclotron absorption features in the x-ray spectra. we performed phase-resolved spectroscopy using data from the nustar observation. our results showed a clear signature of absorbing material at certain pulse phases of the pulsar. these findings are discussed in terms of stellar wind distribution and its effect on the beam geometry of this wind-fed accreting neutron star. we also reviewed the subsonic quasi-spherical accretion theory and its implication on the magnetic field of 4u 1909+07 depending on the global spin-up rate. | revisiting the spectral and timing properties of 4u 1909+07 with nustar and astrosat |
in a previous paper, we reported simulations of the evolution of the magnetic field in neutron star (ns) cores through ambipolar diffusion, taking the neutrons as a motionless uniform background. however, in real nss, neutrons are free to move, and a strong composition gradient leads to stable stratification (stability against convective motions) both of which might impact on the time-scales of evolution. here, we address these issues by providing the first long-term two-fluid simulations of the evolution of an axially symmetric magnetic field in a neutron star core composed of neutrons, protons, and electrons with density and composition gradients. again, we find that the magnetic field evolves towards barotropic 'grad-shafranov equillibria', in which the magnetic force is balanced by the degeneracy pressure gradient and gravitational force of the charged particles. however, the evolution is found to be faster than in the case of motionless neutrons, as the movement of charged particles (which are coupled to the magnetic field, but are also limited by the collisional drag forces exerted by neutrons) is less constrained, since neutrons are now allowed to move. the possible impact of non-axisymmetric instabilities on these equilibria, as well as beta decays, proton superconductivity, and neutron superfluidity, are left for future work. | two-fluid simulations of the magnetic field evolution in neutron star cores in the weak-coupling regime |
the theory of the formation of the first stars in the universe, the so-called population iii (pop iii), has until now largely neglected the impact of magnetic fields. complementing a series of recent studies of the magnetohydrodynamic (mhd) aspects of pop iii star formation, we here carry out a suite of idealized numerical experiments where we ascertain how the fragmentation properties of primordial protostellar discs are modified if mhd effects are present. specifically, starting from cosmological initial conditions, we focus on the central region in a select minihalo at redshift z ~ 25, inserting a magnetic field at an intermediate evolutionary stage, normalized to a fraction of the equipartition value. to explore parameter space, we consider different field geometries, including uniform, radial, toroidal, and poloidal field configurations, with the toroidal configuration being the most realistic. the collapse of the gas is followed for ~8 orders of magnitude in density after the field was inserted, until a maximum of $10^{15} {\rm \, cm}^{-3}$ is reached. we find that the magnetic field leads to a delay in the collapse of the gas. moreover, the toroidal field has the strongest effect on the collapse as it inhibits the fragmentation of the emerging disc surrounding the central core and leads to the formation of a more massive core. the full understanding of the formation of pop iii stars and their mass distribution thus needs to take into account the effect of magnetic fields. we further conclude that ideal mhd is only a first step in this endeavour, to be followed up with a comprehensive treatment of dissipative effects, such as ambipolar diffusion and ohmic dissipation. | impact of magnetic fields on population iii star formation |
fast radio bursts (frbs) are mysterious radio transients with millisecond durations. recently, ~16 days of periodic activity and ~159 days of possible periodicity were detected to arise from frb 180916.j0158+65 and frb 121102, respectively, and the spin period of a slow-rotation magnetar was further considered to be one of the possible explanations of the periodic activities of repeating frbs. for isolated neutron stars, the spin evolution suggests that it has difficulty reaching several hours. in this work, we mainly focus on the possible maximum spin period of isolated nss/magnetars dominated by an interaction between a star's magnetic field and the disk. we find that the disk wind plays an important role in spin evolution, whose influence varies the power-law index in the evolution equation of mass flow rate. for a magnetar without disk wind, the longest spin period is tens of hours. when the disk wind with a classical parameter is involved, the maximum spin period can reach hundreds of hours. but for an extremely large index of mass flow rate due to disk wind or other angular momentum extraction processes, a spin period of ~(16-160) days is still possible. | do the periodic activities of repeating fast radio bursts represent the spins of neutron stars? |
recent numerical simulations have demonstrated that transverse coronal loop oscillations are susceptible to the kelvin-helmholtz (kh) instability due to the counterstreaming motions at the loop boundary. we present the first analytical model of this phenomenon. the region at the loop boundary where the shearing motions are greatest is treated as a straight interface separating time-periodic counterstreaming flows. in order to consider a twisted tube, the magnetic field at one side of the interface is inclined. we show that the evolution of the displacement at the interface is governed by mathieu’s equation, and we use this equation to study the stability of the interface. we prove that the interface is always unstable and that, under certain conditions, the magnetic shear may reduce the instability growth rate. the result, that the magnetic shear cannot stabilize the interface, explains the numerically found fact that the magnetic twist does not prevent the onset of the kh instability at the boundary of an oscillating magnetic tube. we also introduce the notion of the loop σ-stability. we say that a transversally oscillating loop is σ-stable if the kh instability growth time is larger than the damping time of the kink oscillation. we show that even relatively weakly twisted loops are σ-stable. | an analytical model of the kelvin-helmholtz instability of transverse coronal loop oscillations |
magnetic fields and mass accretion processes create dark and bright spots on the surface of young stars. these spots manifest as surface thermal inhomogeneities, which alter the global temperature measured on the stars. to understand the effects and implications of these starspots, we conducted a large ishell high-resolution infrared spectroscopic survey of t tauri stars in taurus-auriga and ophiuchus star-forming regions. from the k-band spectra, we measured stellar temperatures and magnetic field strengths using a magnetic radiative transfer code. we compared our infrared-derived parameters against literature optical temperatures and found (a) a systematic temperature difference between optical and infrared observations, and (b) a positive correlation between the magnetic field strengths and the temperature differences. the discrepant temperature measurements imply significant differences in the inferred stellar masses from stellar evolutionary models. to discern which temperature better predicts the mass of the star, we compared our model-derived masses against dynamical masses measured from atacama large millimeter/submillimeter array and the plateau de bure interferometer for a subsample of our sources. from this comparison we conclude that, in the range of stellar masses from 0.3 to 1.3 m ⊙, neither infrared nor optical temperatures perfectly reproduce the stellar dynamical masses. but, on average, infrared temperatures produce more precise and accurate stellar masses than optical ones. | the effects of starspots on spectroscopic mass estimates of low-mass young stars |
we present an observational study of the plasma dynamics at the base of a solar coronal jet, using high resolution extreme ultraviolet imaging data taken by the extreme ultraviolet imager on board solar orbiter, and by the atmospheric imaging assembly on board solar dynamics observatory. we observed multiple plasma ejection events over a period of ∼1 h from a dome-like base that is ca. 4 mm wide and is embedded in a polar coronal hole. within the dome below the jet spire, multiple plasma blobs with sizes around 1−2 mm propagate upwards to the dome apex with speeds of the order of the sound speed (ca. 120 km s−1). upon reaching the apex, some of these blobs initiate flows with similar speeds towards the other footpoint of the dome. at the same time, high speed super-sonic outflows (∼230 km s−1) are detected along the jet spire. these outflows as well as the intensity near the dome apex appear to be repetitive. furthermore, during its evolution, the jet undergoes many complex morphological changes, including transitions between the standard and blowout type eruption. these new observational results highlight the underlying complexity of the reconnection process that powers these jets and they also provide insights into the plasma response when subjected to rapid energy injection. movies associated to figs. 1, 2, and 4 are available at https://www.aanda.org | a highly dynamic small-scale jet in a polar coronal hole |
stellar magnetism plays an important role in stellar evolution theory. approximatively 10 per cent of observed main sequence (ms) and pre-main-sequence (pms) radiative stars exhibit surface magnetic fields above the detection limit, raising the question of their origin. these stars host outer radiative envelopes, which are stably stratified. therefore, they are assumed to be motionless in standard models of stellar structure and evolution. we focus on rapidly rotating, radiative stars which may be prone to the tidal instability, due to an orbital companion. using direct numerical simulations in a sphere, we study the interplay between a stable stratification and the tidal instability, and assess its dynamo capability. we show that the tidal instability is triggered regardless of the strength of the stratification (brunt-väisälä frequency). furthermore, the tidal instability can lead to both mixing and self-induced magnetic fields in stably stratified layers (provided that the brunt-väisälä frequency does not exceed the stellar spin rate in the simulations too much). the application to stars suggests that the resulting magnetic fields could be observable at the stellar surfaces. indeed, we expect magnetic field strengths up to several gauss. consequently, tidally driven dynamos should be considered as a (complementary) dynamo mechanism, possibly operating in radiative ms and pms stars hosting orbital companions. in particular, tidally driven dynamos may explain the observed magnetism of tidally deformed and rapidly rotating vega-like stars. | magnetic fields driven by tidal mixing in radiative stars |
context. the sub-millimetre polarisation of dust emission from star-forming clouds carries information on grain properties and on the effects that magnetic fields have on cloud evolution.aims: using observations of a dense filamentary cloud g035.39-00.33, we aim to characterise the dust emission properties and the variations of the polarisation fraction.methods: jcmt scuba-2/pol-2 observations at 850 μm were combined with planck 850 μm(353 ghz) data to map polarisation fraction at small and large scales. with previous total intensity scuba-2 observations (450 and 850 μm) and herschel data, the column densities were determined via modified black-body fits and via radiative transfer modelling. models were constructed to examine how the observed polarisation angles and fractions depend on potential magnetic field geometries and grain alignment processes.results: pol-2 data show clear changes in the magnetic field orientation. these are not in contradiction with the uniform orientation and almost constant polarisation fraction seen by planck, because of the difference in the beam sizes and the pol-2 data being affected by spatial filtering. the filament has a peak column density of n(h2) 7 × 1022 cm-2, a minimum dust temperature of t 12 k, and a mass of 4300 m⊙ for the area n(h2) > 5 × 1021 cm-2. the estimated average value of the dust opacity spectral index is β 1.9. the ratio of sub-millimetre and j-band optical depths is τ (250 μm)/τ (j) 2.5 × 10-3, more than four times the typical values for diffuse medium. the polarisation fraction decreases as a function of column density to p 1% in the central filament. because of noise, the observed decrease of p(n) is significant only at n(h2) > 2 × 1022 cm-2. the observations suggest that the grain alignment is not constant. although the data can be explained with a complete loss of alignment at densities above 104 cm-3 or using the predictions of radiative torques alignment, the uncertainty of the field geometry and the spatial filtering of the scuba-2 data prevent strong conclusions.conclusions: the g035.39-00.33 filament shows strong signs of dust evolution and the low polarisation fraction is suggestive of a loss of polarised emission from its densest parts. | dust spectrum and polarisation at 850 μm in the massive irdc g035.39-00.33 |
magnetic fields (b-fields) play a key role in the formation and evolution of protoplanetary disks, but their properties are poorly understood due to the lack of observational constraints. using canaricam at the 10.4 m gran telescopio canarias, we have mapped out the mid-infrared polarization of the protoplanetary disk around the herbig ae star ab aur. we detect ∼0.44% polarization at 10.3 μm from ab aur's inner disk (r < 80 au), rising to ∼1.4% at larger radii. our simulations imply that the mid-infrared polarization of the inner disk arises from dichroic emission of elongated particles aligned in a disk b-field. the field is well ordered on a spatial scale, commensurate with our resolution (∼50 au), and we infer a poloidal shape tilted from the rotational axis of the disk. the disk of ab aur is optically thick at 10.3 μm, so polarimetry at this wavelength is probing the b-field near the disk surface. our observations therefore confirm that this layer, favored by some theoretical studies for developing magneto-rotational instability and its resultant viscosity, is indeed very likely to be magnetized. at radii beyond ∼80 au, the mid-infrared polarization results primarily from scattering by dust grains with sizes up to ∼1 μm, a size indicating both grain growth and, probably, turbulent lofting of the particles from the disk mid-plane. | an ordered magnetic field in the protoplanetary disk of ab aur revealed by mid-infrared polarimetry |
we report on magnetic field measurements of 157 chemically peculiar a/b stars (ap/bp) based on resolved, magnetically split absorption lines present in h-band spectra provided by the sloan digital sky survey (sdss)/apache point observatory galactic evolution experiment (apogee) survey. these stars represent the extreme magnetic end of a still-growing sample of >900 ap/bp stars selected among the apogee telluric standard stars as those with ce iii absorption lines and/or literature ap/bp classifications. the lines most frequently resolved into their split components for these stars in the h-band pertain primarily pertain to ce iii, cr ii, fe i, mn ii, si i, and ca ii, in addition to one or more unidentified ions. using mean magnetic field modulus (< b> ) estimates for transitions with known landé factors, we estimate effective landé factors for 5 ce iii lines and 15 unknown lines and proceed to measure < b> of 157 stars, only 3 of which have previous literature estimates of < b> . this 183% increase in the number of ap/bp stars for which < b> has been measured is a result of the large number of stars observed by sdss/apogee, extension of high-resolution ap/bp star observations to fainter magnitudes, and the advantages of long wavelengths for resolving magnetically split lines. with < b> ∼ 25 kg, the star 2mass j02563098+4534239 is currently the most magnetic star of the sdss/apogee sample. effective landé factors, representative line profiles, and magnetic field moduli are presented. the validity of the results is supported using optical, high-resolution, follow-up spectra for 29 of the stars. | discovery of resolved magnetically split lines in sdss/apogee spectra of 157 ap/bp stars |
in the standard scenario for spin evolution of isolated neutron stars, a young pulsar slows down with a surface magnetic field b that does not change. thus the pulsar follows a constant b trajectory in the phase space of spin period and spin period time derivative. such an evolution predicts a braking index n = 3 while the field is constant and n > 3 when the field decays. this contrasts with all nine observed values being n < 3. here we consider a magnetic field that is buried soon after birth and diffuses to the surface. we use a model of a growing surface magnetic field to fit observations of the three pulsars with lowest n: psr j0537-6910 with n = -1.5, psr b0833-45 (vela) with n = 1.4, and psr j1734-3333 with n = 0.9. by matching the age of each pulsar, we determine their magnetic field and spin period at birth and confirm the magnetar-strength field of psr j1734-3333. our results indicate that all three pulsars formed in a similar way to central compact objects (ccos), with differences due to the amount of accreted mass. we suggest that magnetic field emergence may play a role in the distinctive glitch behaviour of low braking index pulsars, and we propose glitch behaviour and characteristic age as possible criteria in searches for cco descendants. | magnetic field growth in young glitching pulsars with a braking index |
the model for the generation of magnetic fields in a neutron star, based on the magnetic field instability caused by the electroweak interaction between electrons and nucleons, is developed. using the methods of the quantum field theory, the helicity flip rate of electrons in their scattering off protons in dense matter of a neutron star is calculated. the influence of the electroweak interaction between electrons and background nucleons on the process of the helicity flip is studied. the kinetic equation for the evolution of the chiral imbalance is derived. the obtained results are applied for the description of the magnetic fields evolution in magnetars. | relaxation of the chiral imbalance and the generation of magnetic fields in magnetars |
it has recently been established that the evolution of protoplanetary disks is primarily driven by magnetized disk winds, requiring a large-scale magnetic flux threading the disks. the size of such disks is expected to shrink with time, as opposed to the conventional scenario of viscous expansion. we present the first global 2d non-ideal magnetohydrodynamic simulations of protoplanetary disks that are truncated in the outer radius, aiming to understand the interaction of the disk with the interstellar environment, as well as the global evolution of the disk and magnetic flux. we find that as the system relaxes, the poloidal magnetic field threading the disk beyond the truncation radius collapses toward the midplane, leading to a rapid reconnection. this process removes a substantial amount of magnetic flux from the system and forms closed poloidal magnetic flux loops encircling the outer disk in quasi-steady state. these magnetic flux loops can drive expansion beyond the truncation radius, corresponding to substantial mass loss through a magnetized disk outflow beyond the truncation radius analogous to a combination of viscous spreading and external photoevaporation. the magnetic flux loops gradually shrink over time, the rates of which depend on the level of disk magnetization and the external environment, which eventually governs the long-term disk evolution. | global non-ideal magnetohydrodynamic simulations of protoplanetary disks with outer truncation |
context. relativistic jets are ubiquitous in the universe. in microquasars, especially in high-mass x-ray binaries, the interaction of jets with the strong winds driven by the massive and hot companion star in the vicinity of the compact object is fundamental for understanding the jet dynamics, nonthermal emission, and long-term stability. however, the role of the jet magnetic field in this process is unclear. in particular, it is still debated whether the magnetic field favors jet collimation or triggers more instabilities that can jeopardize the jet evolution outside the binary.aims: we study the dynamical role of weak and moderate to strong toroidal magnetic fields during the first several hundred seconds of jet propagation through the stellar wind, focusing on the magnetized flow dynamics and the mechanisms of energy conversion.methods: we developed the code lóstrego v1.0, a new 3d relativistic magnetohydrodynamics code to simulate astrophysical plasmas in cartesian coordinates. using this tool, we performed the first 3d relativistic magnetohydrodynamics numerical simulations of relativistic magnetized jets propagating through the clumpy stellar wind in a high-mass x-ray binary. to highlight the effect of the magnetic field in the jet dynamics, we compared the results of our analysis with those of previous hydrodynamical simulations.results: the overall morphology and dynamics of weakly magnetized jet models is similar to previous hydrodynamical simulations, where the jet head generates a strong shock in the ambient medium and the initial overpressure with respect to the stellar wind drives one or more recollimation shocks. on the timescales of our simulations (i.e., t < 200 s), these jets are ballistic and seem to be more stable against internal instabilities than jets with the same power in the absence of fields. however, moderate to strong toroidal magnetic fields favor the development of current-driven instabilities and the disruption of the jet within the binary. a detailed analysis of the energy distribution in the relativistic outflow and the ambient medium reveals that magnetic and internal energies can both contribute to the effective acceleration of the jet. moreover, we verified that the jet feedback into the ambient medium is highly dependent on the jet energy distribution at injection, where hotter, more diluted and/or more magnetized jets are more efficient. this was anticipated by feedback studies in the case of jets in active galaxies. | 3d rmhd simulations of jet-wind interactions in high-mass x-ray binaries |
recent observations of the low-mass (0.1-0.6 {m}⊙ ) rotation distributions of the pleiades and praesepe clusters have revealed a ubiquitous correlation between mass and rotation, such that late m dwarfs rotate an order-of-magnitude faster than early m dwarfs. in this paper, we demonstrate that this mass-rotation correlation is present in the 10 myr upper scorpius association, as revealed by new k2 rotation measurements. using rotational evolution models, we show that the low-mass rotation distribution of the 125 myr pleiades cluster can only be produced if it hosted an equally strong mass-rotation correlation at 10 myr. this suggests that physical processes important in the early pre-main sequence (pms; star formation, accretion, disk-locking) are primarily responsible for the m dwarf rotation morphology, and not quirks of later angular momentum (am) evolution. such early mass trends must be taken into account when constructing initial conditions for future studies of stellar rotation. finally, we show that the average m star loses ∼25%-40% of its am between 10 and 125 myr, a figure accurately and generically predicted by modern solar-calibrated wind models. their success rules out a lossless pms and validates the extrapolation of magnetic wind laws designed for solar-type stars to the low-mass regime at early times. | m dwarf rotation from the k2 young clusters to the field. i. a mass-rotation correlation at 10 myr |
context. the importance of magnetic fields at the onset of star formation related to the early fragmentation and collapse processes is largely unexplored today.aims: we want to understand the magnetic field properties at the earliest evolutionary stages of high-mass star formation.methods: the atacama large millimeter array is used at 1.3 mm wavelength in full polarization mode to study the polarized emission, and, using this, the magnetic field morphologies and strengths of the high-mass starless region irdc 18310-4.results: polarized emission is clearly detected in four sub-cores of the region; in general it shows a smooth distribution, also along elongated cores. estimating the magnetic field strength via the davis-chandrasekhar-fermi method and following a structure function analysis, we find comparably large magnetic field strengths between 0.3-5.3 mg. comparing the data to spectral line observations, the turbulent-to-magnetic energy ratio is low, indicating that turbulence does not significantly contribute to the stability of the gas clump. a mass-to-flux ratio around the critical value 1.0 - depending on column density - indicates that the region starts to collapse, which is consistent with the previous spectral line analysis of the region.conclusions: while this high-mass region is collapsing and thus at the verge of star formation, the high magnetic field values and the smooth spatial structure indicate that the magnetic field is important for the fragmentation and collapse process. this single case study can only be the starting point for larger sample studies of magnetic fields at the onset of star formation. | magnetic fields at the onset of high-mass star formation |
one of the most important problems in the context of cataclysmic variables (cvs) is the lack of observations of systems with periods between 2 and 3.12 hr, known as the period gap. the orbital evolution of cvs with periods shorter than those in the gap is dominated by gravitational radiation, while for periods exceeding those of the gap it is dominated by magnetic braking of the secondary star. spruit & ritter showed that as periods approach 3 hr and secondary stars become fully convective a sharp decline in magnetic dynamo and braking efficiency would result in such a gap. recent x-ray observations finding coronal magnetic energy dissipation is similar in fully convective and partly radiative m dwarfs cast this theory into doubt. in this work, we use zeeman-doppler imaging observations culled from the literature to show that the complexity of the surface magnetic fields of rapidly rotating m dwarfs increases with decreasing rotation period. garraffo et al. have shown that the efficiency of angular momentum loss of cool stars declines strongly with increasing complexity of their surface magnetic field. we explore the idea of taam & spruit that magnetic complexity might then explain the period gap. by generating synthetic cv populations using a schematic cv evolutionary approach, we show that the cv period gap can naturally arise as a consequence of a rise in secondary star magnetic complexity near the long-period edge of the gap that renders a sharp decline in their angular-momentum-loss rate. | the magnetic nature of the cataclysmic variable period gap |
large-scale morphology and time evolution are investigated for the unified model of bipolar outflows outlined in shang et al. (2006), where an outflow forms by a radially directed, wide-angle magnetized wind interacting with magnetized isothermal toroids in various quasistatic states. the primary wide-angle wind is toroidally magnetized and maintains a cylindrically stratified density profile, mimicking the asymptotic solution of the cold x-wind model. we explore the interplay between the toroidally magnetized primary wind and the surrounding toroids threaded by poloidal magnetic fields and examine how the jet and shell morphology and fine structures within them vary with the physical parameters. the variation in flatness of the density distribution in the ambient isothermal toroids helps shape the varieties of lobe morphology and collimation. the presence of a stronger ambient poloidal field helps shape the outflow, forming a magnetic cocoon and nested multilayered cavities surrounding the wind-filled lobe, which is most evident in the more open configurations of the ambient toroids. the wind-toroid interface is prone to substantial shear and thus unstable to the kelvin-helmholtz instability. magnetic forces in the compressed toroidally magnetized high-velocity wind can generate vorticity, leading to nonlinear patterns within the extended magnetized mixing layers. magnetic disturbances generated by the interplay could modulate lobe shapes, density, and velocities, giving rise to visual impressions of thicker and rugged shells and apparent episodic distribution of matter. the system maintains a quasi-self-similar evolution in time, which serves as a proxy for understanding the underlying physical mechanisms driving it. | a unified model for bipolar outflows from young stars: the interplay of magnetized wide-angle winds and isothermal toroids |
we study the long-term evolution of the centroid energy of cyclotron lines - often referered to as cyclotron resonance scattering features (crsfs) - in her x-1, vela x-1, and cen x-3, using survey observations of the burst alert telescope onboard swift. we find a significant decrease of the fundamental crsf energy in her x-1 and the first harmonic line energy in vela x-1 since the launch of swift in 2004 and until 2010 and 2012, respectively. in both sources, the decreases stopped at some time, with a quite stable centroid energy thereafter. unlike in her x-1 and vela x-1, the crsf energy in cen x-3 does not show a long-term decrease. it is observed not to change for at least the past 14 yr. the long-term variation of the line energy is a direct way to investigate the magnetic field structure in the polar regions of pulsars. our results may stimulate the development of theoretical models, especially regarding how the accreted mass accumulates in the accretion mound or how the magnetic field distorts around the polar cap. | long-term evolutions of the cyclotron line energies in her x-1, vela x-1, and cen x-3 as observed with swift/bat |
this article briefly reviews our current understanding of the evolution of magnetic fields in neutron stars, which basically defines the evolutionary pathways between different observational classes of neutron stars. the emphasis here is on the evolution in binary systems and the newly emergent classes of millisecond pulsars. | magnetic fields of neutron stars |
results are presented of a first study of collisionless magnetic reconnection starting from a recently found exact nonlinear force-free vlasov-maxwell equilibrium. the initial state has a harris sheet magnetic field profile in one direction and a non-uniform guide field in a second direction, resulting in a spatially constant magnetic field strength as well as a constant initial plasma density and plasma pressure. it is found that the reconnection process initially resembles guide field reconnection, but that a gradual transition to anti-parallel reconnection happens as the system evolves. the time evolution of a number of plasma parameters is investigated, and the results are compared with simulations starting from a harris sheet equilibrium and a harris sheet plus constant guide field equilibrium. | particle-in-cell simulations of collisionless magnetic reconnection with a non-uniform guide field |
we explore the thermal and magnetic field structure of a late-stage proto-neutron star (proto-ns). we find the dominant contribution to the entropy in different regions of the star, from which we build a simplified equation of state (eos) for the hot neutron star (ns). with this, we numerically solve the stellar equilibrium equations to find a range of models, including magnetic fields and rotation up to keplerian velocity. we approximate the eos as a barotrope, and discuss the validity of this assumption. for fixed magnetic field strength, the induced ellipticity increases with temperature; we give quantitative formulae for this. the keplerian velocity is considerably lower for hotter stars, which may set a de facto maximum rotation rate for non-recycled nss well below 1 khz. magnetic fields stronger than around 1014 g have qualitatively similar equilibrium states in both hot and cold nss, with large-scale simple structure and the poloidal field component dominating over the toroidal one; we argue this result may be universal. we show that truncating magnetic field solutions at low multipoles leads to serious inaccuracies, especially for models with rapid rotation or a strong toroidal-field component. | magnetic fields in late-stage proto-neutron stars |
instabilities in a neutron star can generate alfvén waves in its magnetosphere. propagation along the curved magnetic field lines strongly shears the wave, boosting its electric current j a. we derive an analytic expression for the evolution of the wavevector k and the growth of j a. in the strongly sheared regime, j a may exceed the maximum current j 0 that can be supported by the background e ± plasma. we investigate these charge-starved waves, first using a simplified two-fluid analytic model, then with first-principles kinetic simulations. we find that the alfvén wave is able to propagate successfully even when κ ≡ j a/j 0 ≫ 1. it sustains j a by compressing and advecting the plasma along the magnetic field lines with an increasing lorentz factor, γ ≳ κ 1/2. the simulations show how plasma instabilities lead to gradual dissipation of the wave energy. our results suggest that an extremely high charge-starvation parameter κ ≳ 104 may be required in order for this mechanism to power the observed fast radio bursts (frbs) from sgr 1935+2154. however, cosmological frbs with much higher luminosities are unlikely to be a result of charge-starvation. | relativistic alfvén waves entering charge-starvation in the magnetospheres of neutron stars |
accreting x-ray pulsars undergo luminous x-ray outbursts during which the luminosity-dependent spectral and timing features of the neutron star's emission can be analyzed in detail, thus shedding light on the accretion regime at work. we took advantage of a monitoring campaign that was performed with nustar, swift/xrt, astrosat and nicer to follow the be/x-ray binary 2s 1553-542 along one of its rare outbursts, and thus trace its spectral and timing evolution. we report the discovery of a luminosity-dependent cyclotron line energy for the first time in this source. the pulse profiles and pulsed fraction also show variability along the outburst, which is consistent with the interpretation that the source transitions from the subcritical to the supercritical accretion regime, separated by a critical luminosity of l crit ≈ 4 × 1037 erg s-1. | accreting on the edge: a luminosity-dependent cyclotron line in the be/x-ray binary 2s 1553-542 accompanied by accretion regimes transition |
we present radio spectra spanning 0.1-10 ghz for the sample of heavily obscured luminous quasars with extremely red mid-infrared-optical colors and compact radio emission. the spectra are constructed from targeted 10 ghz observations and archival radio survey data that together yield 6-11 flux-density measurements for each object. our primary result is that most (62%) of the sample have peaked or curved radio spectra and many (37%) could be classified as gigahertz-peaked spectrum (gps) sources. this indicates compact emission regions likely arising from recently triggered radio jets. assuming synchrotron self-absorption (ssa) generates the peaks, we infer compact source sizes (3-100 pc) with strong magnetic fields (6-100 mg) and young ages (30-104 yr). conversely, free-free absorption (ffa) could also create peaks due to the high column densities associated with the deeply embedded nature of the sample. however, we find no correlations between the existence or frequency of the peaks and any parameters of the mir emission. the high-frequency spectral indices are steep (α ≈ -1) and correlate, weakly, with the ratio of mir photon energy density to magnetic energy density, suggesting that the spectral steepening could arise from inverse compton scattering off the intense mir photon field. this study provides a foundation for combining multifrequency and mixed-resolution radio survey data for understanding the impact of young radio jets on the ism and star-formation rates of their host galaxies. fagithub | radio spectra of luminous, heavily obscured wise-nvss selected quasars |
we report on our investigation of the evolution of a system of spark discharges in the inner acceleration region (iar) above the pulsar polar cap. the surface of the polar cap is heated to temperatures of around 106 k and forms a partially screened gap (psg), due to thermionic emission of positively charged ions from the stellar surface. the spark lags behind corotation speed during their lifetimes due to variable e × b drift. in a psg, spark discharges arise in locations where the surface temperatures go below the critical level (ti ) for ions to freely flow from the surface. the spark commences due to the large drop in potential developing along the magnetic field lines in these lower temperature regions and subsequently back-streaming particles heat the surface to ti . regulation of the temperature requires the polar cap to be tightly filled with sparks and a continuous presence of sparks is required around its boundary since no heating is possible from the closed field line region. we estimate the time evolution of the spark system in the iar, which shows a gradual shift in the spark formation along two distinct directions resembling clockwise and anticlockwise motions in two halves of the polar cap. due to the differential shift of the spark pattern in the two halves, a central spark develops representing the core emission. the temporal evolution of the spark process was simulated for different orientations of a non-dipolar polar cap and reproduced the diverse observational features associated with subpulse drifting. | two-dimensional configuration and temporal evolution of spark discharges in pulsars |
we made near-infrared polarimetric observations toward serpens south. this region contains three dense filaments that are roughly parallel to one another. using the histogram of relative orientations, the three filaments are found to be roughly perpendicular to the global magnetic field. the morphology of the plane-of-sky (pos) magnetic field and molecular gas suggests that the magnetic field plays an important role in the filament formation and evolution. applying the davis-chandrasekhar-fermi method, the pos magnetic field strengths are estimated to be 10-80 μg. the evaluated mass-to-flux ratios indicate that the center filament is magnetically supercritical, while the others are approximately magnetically critical. we speculate that the filaments are formed by fragmentation of a sheet-like cloud that was created through the gravitational contraction of a magnetized, turbulent cloud. | magnetic field structure in serpens south |
galactic winds are a crucial player in galaxy formation and evolution, but observations of them have proven extraordinarily difficult to interpret, leaving large uncertainties even in basic quantities such as mass outflow rates. here we present an analysis of the wind of the nearby dwarf starburst galaxy m82 using a semi-analytic model that is able to take advantage of the full three-dimensional information present in position-position-velocity data cubes measured in the h i 21-cm line, the co j = 2 → 1 line, and the hα line. our best-fitting model produces position-dependent spectra in good agreement with the observations, and shows that the total wind mass flux in the atomic and molecular phases is ≈10 m⊙ yr-1 (corresponding to a mass loading factor of ≈2-3), with less than a factor of 2 uncertainty; the mass flux in the warm ionized phase is more poorly constrained, and may be comparable to or smaller than this. at least over the few kpc off the plane for which we trace the outflow, it appears to be a wind escaping the galaxy, rather than a fountain that falls back. our fits require that clouds of cool gas entrained into the wind expand only modestly, suggesting they are confined by magnetic fields, radiative cooling, or a combination of both. finally, we demonstrate that attempts to model the wind using simplifying assumptions such as instantaneous acceleration and a constant terminal wind speed can yield significantly erroneous results. | the observable properties of cool winds from galaxies, agn, and star clusters - ii. 3d models for the multiphase wind of m82 |
it has been suggested that a non-repeating fast radio burst (frb) represents the final signal of a magnetized neutron star collapsing to a black hole. in this model, a supramassive neutron star supported by rapid rotation, will collapse to a black hole several thousand to million years after its birth, as a result of spin-down. the collapse violently snaps the magnetic field lines anchored on the stellar surface, thus producing an electromagnetic pulse that will propagate outward and accelerate electrons, thus producing a massive radio burst, i.e., a “blitzar.” we present a systematic study of the gravitational collapse of rotating and magnetized neutron stars, with special attention to far-field evolution at late times after the collapse. by considering a series of neutron stars with rotation ranging from zero to millisecond periods and different magnetic-field strengths, we show that the blitzar emission is very robust and always characterized by a series sub-millisecond pulses decaying exponentially in amplitude. the luminosity and energy released when the magnetosphere is destroyed are well-reproduced by a simple expression in terms of the stellar magnetic field and radius. finally, we assess the occurrence of pair production during a blitzar scenario. we conclude that, for typical magnetic-field strengths of 1012 g and spin frequencies of a few hz, pair production is suppressed. overall, the very good match between the results of the simulations and the luminosities normally observed for frbs lends credibility to the blitzar model as a simple yet plausible explanation for the phenomenology of non-repeating frbs. | electromagnetic emission from blitzars and its impact on non-repeating fast radio bursts |
black hole x-ray binaries undergo occasional outbursts caused by changing inner accretion flows. here we report high angular resolution radio observations of the 2013 outburst of the black hole candidate x-ray binary system xte j1908+094, using data from the very long baseline array and european vlbi network. we show that following a hard-to-soft state transition, we detect moving jet knots that appear asymmetric in morphology and brightness, and expand to become laterally resolved as they move away from the core, along an axis aligned approximately -11° east of north. we initially see only the southern component, whose evolution gives rise to a 15-mjy radio flare and generates the observed radio polarization. this fades and becomes resolved out after 4 days, after which a second component appears to the north, moving in the opposite direction. from the timing of the appearance of the knots relative to the x-ray state transition, a 90° swing of the inferred magnetic field orientation, the asymmetric appearance of the knots, their complex and evolving morphology, and their low speeds, we interpret the knots as working surfaces where the jets impact the surrounding medium. this would imply a substantially denser environment surrounding xte j1908+094 than has been inferred to exist around the microquasar sources grs 1915+105 and gro j1655-40. | resolved, expanding jets in the galactic black hole candidate xte j1908+094 |
magnetically arrested disks (mads) appear when accretion flows are supplied with a sufficient amount of magnetic flux. in this work, we use results of magnetohydrodynamic simulations to set the configuration of the magnetic field and investigate the dynamics and radiative properties of the resulting accretion flow (i.e., without that of the jet) of mads. the method developed here is applied to both the mad and the standard and normal evolution (sane) accretion flow with or without large-scale magnetic fields. for the radiative processes, we include synchrotron, bremsstrahlung, and compton scattering. we find that, in general, accretion flows of mads have similar spectra to those of the sane, which complicates the task of distinguishing mads from sanes. at the same accretion rates, mads are systematically brighter than sanes. however, the critical accretion rate above which the hot solution ceases to exist is lower in mads. consequently, the maximum luminosity an mad can reach is comparable to but slightly lower than that of sane, and the dependence on the magnetic flux is weak. we then discuss the implications of our results for active galactic nuclei and accreting black hole binaries. | radiative properties of magnetically arrested disks |
we present a statistical analysis for the characteristics and radial evolution of linear magnetic holes (lmhs) in the solar wind from 0.166 to 0.82 au using parker solar probe observations of the first two orbits. it is found that the lmhs mainly have a duration less than 25 s and the depth is in the range from 0.25 to 0.7. the durations slightly increase and the depths become slightly deeper with the increasing heliocentric distance. both the plasma temperature and the density for about 50% of all events inside the holes are higher than the ones surrounding the holes. the average occurrence rate is 8.7 events day-1, much higher than that of the previous observations. the occurrence rate of the lmhs has no clear variation with the heliocentric distance (only a slight decreasing trend with the increasing heliocentric distance), and has several enhancements around ∼0.525 and ∼0.775 au, implying that there may be new locally generated lmhs. all events are segmented into three parts (i.e., 0.27, 0.49, and 0.71 au) to investigate the geometry evolution of the linear magnetic holes. the results show that the geometry of lmhs are prolonged both across and along the magnetic field direction from the sun to the earth, while the scales across the field extend a little faster than along the field. the present study could help us understand the evolution and formation mechanism of the lmhs in the solar wind. | characteristics of magnetic holes in the solar wind revealed by parker solar probe |
treatment of the vortex motion in the superfluids of the inner crust and the outer core of neutron stars is a key ingredient in modelling a number of pulsar phenomena, including glitches and magnetic field evolution. after recalculating the microscopic vortex velocity in the inner crust, we evaluate the velocity for the vortices in the outer core for the first time. the vortex motion between pinning sites is found to be substantially faster in the inner crust than in the outer core, v_0^crust ∼ 107{ cm s^{-1}} ≫ v_0^core ∼ 1{ cm s^{-1}}. one immediate result is that vortex creep is always in the nonlinear regime in the outer core in contrast to the inner crust, where both nonlinear and linear regimes of vortex creep are possible. other implications for pulsar glitches and magnetic field evolution are also presented. | microscopic vortex velocity in the inner crust and outer core of neutron stars |
we report on quasi-simultaneous observations from radio to x-ray frequencies of the neutron star x-ray binary aql x-1 over accretion state transitions during its 2016 outburst. all the observations show radio to millimetre spectra consistent with emission from a jet, with a spectral break from optically thick to optically thin synchrotron emission that decreases from 100 ghz to <5.5 ghz during the transition from a hard to a soft accretion state. the 5.5 ghz radio flux density as the source reaches the soft state, 0.82 ± 0.03 mjy, is the highest recorded to date for this source. during the decay of the outburst, the jet spectral break is detected again at a frequency of 30-100 ghz. the flux density is 0.75 ± 0.03 mjy at 97.5 ghz at this stage. this is the first time that a change in the frequency of the jet break of a neutron star x-ray binary has been measured, indicating that the processes at play in black holes are also present in neutron stars, supporting the idea that the internal properties of the jet rely most critically on the conditions of the accretion disc and corona around the compact object, rather than the black hole mass or spin or the neutron star surface or magnetic field. | the evolving jet spectrum of the neutron star x-ray binary aql x-1 in transitional states during its 2016 outburst |
pulsars are famous for their rotational stability. most of them steadily spin-down and display a highly repetitive pulse shape. but some pulsars experience timing irregularities such as nulling, intermittency, mode changing and timing noise. as changes in the pulse shape are often correlated with timing irregularities, precession is a possible cause of these phenomena. whereas pulsar magnetospheres are filled with plasma, most pulsar precession studies were carried out within the vacuum approximation and neglected the effects of magnetospheric currents and charges. recent numerical simulations of plasma-filled pulsar magnetospheres provide us with a detailed quantitative description of magnetospheric torques exerted on the pulsar surface. in this paper, we present the study of neutron star evolution using these new torque expressions. we show that they lead to (1) much slower long-term evolution of pulsar parameters and (2) much less extreme solutions for these parameters than the vacuum magnetosphere models. to facilitate the interpretation of observed pulsar timing residuals, we derive an analytic model that (1) describes the time evolution of non-spherical pulsars and (2) translates the observed pulsar timing residuals into the geometrical parameters of the pulsar. we apply this model to two pulsars with very different temporal behaviours. for the pulsar b1828-11, we demonstrate that the timing residual curves allow two pulsar geometries: one with stellar deformation pointing along the magnetic axis and one along the rotational axis. for the crab pulsar, we use the model show that the recent observation of its magnetic and rotational axes moving away from each other can be explained by precession. | evolution of non-spherical pulsars with plasma-filled magnetospheres |
neutron stars radiate in a broad-band spectrum from radio wavelengths up to very high energies. they have been sorted into several classes depending on their respective place in the p-\dot{p} diagram and depending on spectral/temporal properties. fundamental physical parameters such as their characteristic age and magnetic field strength are deduced from these primary observables. however, this deduction relies mostly on interpretations based on simple vacuum or force-free rotating dipole models that are unrealistic. in this paper, we show that the computation of the stellar surface magnetic field is poorly estimated or even erroneous if multipolar components and particle loading are neglected. we show how quadrupolar magnetic field and monopolar winds alter field estimates and characteristic ages in the p-\dot{p} diagram. corrections brought by general relativity are also discussed. we derive some important parameters of pulsar physics such as the wind lorentz factor (γ) times the pair multiplicity (κ) to be around γ κ ≈ 108-10^{10}. therefore, the standard magnetodipole radiation losses formula must be used with caution to reckon neutron star surface magnetic fields and related secular evolution parameters. depending on models we found that all field strengths, both for magnetars and for pulsars lie below the quantum critical value of bc ≈ 4, 4 × 109 t. | the illusion of neutron star magnetic field estimates |
we study breaking stress of deformed coulomb crystals in a neutron star crust, taking into account electron plasma screening of ion-ion interaction; calculated breaking stress is fitted as a function of electron screening parameter. we apply the results for analysing torsional oscillation modes in the crust of a non-magnetic star. we present exact analytical expression for the fundamental frequencies of such oscillations and show that the frequencies of all torsional oscillations are insensitive to the presence of the outer neutron star crust. the results can be useful in theoretical modelling of processes involving deformed coulomb crystals in the crust of neutron stars, such as magnetic field evolution, torsional crustal, or magneto-elastic quasi-periodic oscillations of flaring soft gamma-ray repeaters, pulsar glitches. the applicability of the results to soft gamma-ray repeaters is discussed. | deformed crystals and torsional oscillations of neutron star crust |
intermittency and anisotropy are two important aspects of plasma turbulence, which the solar wind provides a natural laboratory to investigate. however, their forms and nature are still under debate, making it difficult to achieve a consensus in the theoretical interpretation. here, we perform higher-order statistics for the observations in the fast solar wind at 1.48 au obtained by ulysses and in the slow solar wind at 0.17 au obtained by parker solar probe (psp). we find that two subranges clearly exist in the inertial range and they present distinct features with regard to the intermittency and anisotropy. the subrange 1 with smaller scale has a multifractal scaling with the second index $\xi(2) \sim 2/3$ and the subrange 2 with larger scale is also multifractal but with $\xi(2) \sim 1/2$. the break between two subranges locates at the same spatial scale for both ulysses and psp observations. subrange 1 is multifractal in the direction perpendicular to the local magnetic field with $\xi_{\perp}(2) \sim 2/3$ and seems to be monoscaling in the parallel direction with $\xi_{\parallel}(2) \sim 1$. subrange 2 is multifractal in both parallel and perpendicular directions with $\xi_{\perp}(2) \sim 1/2$ and $\xi_{\parallel}(2) \sim 2/3$. both subrange 1 and subrange 2 present power and wavevector anisotropies. the distinct features of two subranges suggest that a transition from weak to strong turbulence may occur and the spatial scale of the break may not evolve with the solar wind expansion. these new results update our knowledge of the inertial range and provide strong observational constraints on the understanding of intermittency and anisotropy in solar wind turbulence. | on the scaling and anisotropy of two subranges in the inertial range of solar wind turbulence |
context. the question of the origin and evolution of magnetic fields in stars possessing a radiative envelope, like the a-type stars, is still regarded as a challenge for stellar physics. those zones are likely to be differentially rotating, which suggests that strong interactions between differential rotation and magnetic fields could be at play in such regions.aims: we would like to analyse in detail the evolution of magnetic fields in a differentially rotating stellar radiative zone and the possible presence of magnetic instabilities.methods: we numerically compute the joint evolution of the magnetic and velocity fields in a 3d spherical shell starting from an initial profile for the poloidal magnetic field and differential rotation. in order to characterise the nature of the magnetic instabilities that may be expected, we use the predictions of a local linear analysis.results: the poloidal magnetic field is initially wound up by the differential rotation to produce a toroidal field which becomes unstable. we find that different types of instabilities may occur, depending on the balance between the shear strength and the magnetic field intensity. in the particular setup studied here where the differential rotation is dominant, the instability is not of the tayler-type but is the magneto-rotational instability. the growth rate of the instability depends mainly on the initial rotation rate, while the background state typically oscillates over a poloidal alfvén time. we thus find that the axisymmetric magnetic configuration is strongly modified by the instability only if the ratio between the poloidal alfvén frequency and the rotation rate is sufficiently small. an enhanced transport of angular momentum is found in the most unstable cases: the typical time to flatten the rotation profile is then much faster than the decay time associated with the phase-mixing mechanism, which also occurs in the stable cases. when the instability saturates before reaching a significant amplitude, the magnetic configuration relaxes into a stable axisymmetric equilibrium formed by several twisted tori.conclusions: we conclude that the magneto-rotational instability is always favoured (over the tayler instability) in unstratified spherical shells when an initial poloidal field is sheared by a sufficiently strong cylindrical differential rotation. a possible application to the magnetic desert observed among a stars is given. we argue that the dichotomy between stars exhibiting strong axisymmetric fields (ap stars) and those harbouring a sub-gauss magnetism could be linked to the threshold for the instability. | three-dimensional evolution of magnetic fields in a differentially rotating stellar radiative zone |
magnetic activity is known to be correlated to the rotation period for moderately active main-sequence solar-like stars. in turn, the stellar rotation period evolves as a result of magnetized stellar winds that carry away angular momentum. understanding the interplay between magnetic activity and stellar rotation is therefore a central task for stellar astrophysics. angular momentum evolution models typically employ spin-down torques that are formulated in terms of the surface magnetic field strength. however, these formulations fail to account for the magnetic field geometry, unlike those that are expressed in terms of the open flux, i.e. the magnetic flux along which stellar winds flow. in this work, we model the angular momentum evolution of main-sequence solar-mass stars using a torque law formulated in terms of the open flux. this is done using a potential field source surface model in conjunction with the zeeman-doppler magnetograms of a sample of roughly solar-mass stars. we explore how the open flux of these stars varies with stellar rotation and choice of source surface radii. we also explore the effect of field geometry by using two methods of determining the open flux. the first method only accounts for the dipole component while the second accounts for the full set of spherical harmonics available in the zeeman-doppler magnetogram. we find only a small difference between the two methods, demonstrating that the open flux, and indeed the spin-down, of main-sequence solar-mass stars is likely dominated by the dipolar component of the magnetic field. | the open flux evolution of a solar-mass star on the main sequence |
subvirial gravitational collapse is one mechanism by which star clusters may form. here we investigate whether this mechanism can be inferred from observations of young clusters. to address this question, we have computed smoothed particle hydrodynamics simulations of the initial formation and evolution of a dynamically young star cluster through cold (subvirial) collapse, starting with an ellipsoidal, turbulently seeded distribution of gas, and forming sink particles representing (proto)stars. while the initial density distributions of the clouds do not have large initial mass concentrations, gravitational focusing due to the global morphology leads to cluster formation. we use the resulting structures to extract observable morphological and kinematic signatures for the case of subvirial collapse. we find that the signatures of the initial conditions can be erased rapidly as the gas and stars collapse, suggesting that kinematic observations need to be made early in cluster formation and/or at larger scales, away from the growing cluster core. our results emphasize that a dynamically young system is inherently evolving on short timescales, so that it can be highly misleading to use current-epoch conditions to study aspects such as star formation rates as a function of local density. our simulations serve as a starting point for further studies of collapse including other factors such as magnetic fields and stellar feedback. | signatures of star cluster formation by cold collapse |
while it has been suggested that there is a connection between the magnetic properties and the internal structure of young stars, there have not been enough magnetic measurements to firmly establish such a correlation at the earliest ages. here, we contribute to this endeavor by presenting stellar parameters and magnetic field strength measurements of bp tau and v347 aur, both stars observed with the near-infrared spectrograph ishell. we first test the accuracy of our method by fitting synthetic stellar spectra to a sample of nine main and post-main-sequence stars. we report uncertainties of {σ }teff} = 91 k in temperature and {σ }log(g)} = 0.14 in gravity. we then apply the modeling technique to bp tau and measure a surface magnetic field strength of < b> = {2.5}-0.16+0.15 kg, confirming literature results. for this star, however, we obtain a much lower temperature value than previous optical studies ({{δ }}t∼ 400 k) and interpret this significant temperature difference as due to the relatively higher impact of starspots at near-infrared wavelengths than at optical wavelengths. we further apply this technique to the class i protostellar source v347 aur and measure for the first time its magnetic field strength < b> = {1.36}-0.05+0.06 kg and its surface gravity log g = 3.25{}-0.14+0.14. lastly, we combine our measurements with pre-main-sequence stellar evolutionary models and illustrate the effects produced by starspots on the retrieved masses and ages of young stars. | measuring the magnetic field of young stars using ishell observations: bp tau and v347 aur |
context. the role of magnetic fields during the formation of high-mass stars is not yet fully understood, and the processes related to the early fragmentation and collapse are as yet largely unexplored. the high-mass star forming region g9.62+0.19 is a well known source, presenting several cores at different evolutionary stages.aims: we seek to investigate the magnetic field properties at the initial stages of massive star formation. we aim to determine the magnetic field morphology and strength in the high-mass star forming region g9.62+0.19 to investigate its relation to the evolutionary sequence of the cores.methods: we made use of atacama large millimeter array (alma) observations in full polarisation mode at 1 mm wavelength (band 7) and we analysed the polarised dust emission. we estimated the magnetic field strength via the davis-chandrasekhar-fermi and structure function methods.results: we resolve several protostellar cores embedded in a bright and dusty filamentary structure. the polarised emission is clearly detected in six regions: two in the northern field and four in the southern field. moreover the magnetic field is orientated along the filament and appears perpendicular to the direction of the outflows. the polarisation vectors present ordered patterns and the cores showing polarised emission are less fragmented. we suggest an evolutionary sequence of the magnetic field, and the less evolved hot core exhibits a stronger magnetic field than the more evolved hot core. an average magnetic field strength of the order of 11 mg was derived, from which we obtain a low turbulent-to-magnetic energy ratio, indicating that turbulence does not significantly contribute to the stability of the clump. we report a detection of linear polarisation from thermal line emission, probably from methanol or carbon dioxide, and we tentatively compared linear polarisation vectors from our observations with previous linearly polarised oh masers observations. we also compute the spectral index, column density, and mass for some of the cores.conclusions: the high magnetic field strength and smooth polarised emission indicate that the magnetic field could play an important role in the fragmentation and the collapse process in the star forming region g9.62+019 and that the evolution of the cores can be magnetically regulated. one core shows a very peculiar pattern in the polarisation vectors, which can indicate a compressed magnetic field. on average, the magnetic field derived by the linear polarised emission from dust, thermal lines, and masers is pointing in the same direction and has consistent strength. a copy of the reduced images is 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/qcat?j/a+a/626/a36 | alma reveals the magnetic field evolution in the high-mass star forming complex g9.62+0.19 |
this report presents a three-dimensional (3d) numerical magnetohydrodynamics (mhd) model of the white-light coronagraph observational phenomena known as coronal inflows and in/out pairs. coronal inflows in the large angle and spectrometric coronagraph/c2 field of view (approximately $2\mbox{--}6\,{r}_{\odot }$ ) were thought to arise from the dynamic and intermittent release of solar wind plasma associated with the helmet streamer belt as the counterpart to outward-propagating streamer blobs, formed by magnetic reconnection. this interpretation was essentially confirmed with the subsequent identification of in/out pairs and the multispacecraft observations of their 3d structure. the mhd simulation results show relatively narrow lanes of density depletion form high in the corona and propagate inward with sinuous motion that has been characterized as "tadpole-like" in coronagraph imagery. the height-time evolution and velocity profiles of the simulation inflows and in/out pairs are compared to their corresponding observations and a detailed analysis of the underlying magnetic field structure associated with the synthetic white-light and mass density evolution is presented. understanding the physical origin of this structured component of the slow solar wind's intrinsic variability could make a significant contribution to solar wind modeling and the interpretation of remote and in situ observations from parker solar probe and solar orbiter. | a model for coronal inflows and in/out pairs |
gro j1008-57, as a be/x-ray transient pulsar, is considered to have the highest magnetic field in known neutron star x-ray binary systems. observational data of the x-ray outbursts in gro j1008-57 from 2017 to 2020 were collected by the insight-hxmt satellite. in this work, the spin period of the neutron star in gro j1008-57 was determined to be about 93.28 seconds in august 2017, 93.22 seconds in february 2018, 93.25 seconds in june 2019 and 93.14 seconds in june 2020. gro j1008-57 evolved in the spin-up process with a mean rate of - (2.10 ± 0.05) ×10-4 s/d from 2009 - 2018, and turned into a spin down process with a rate of (6.7 ± 0.6) ×10-5 s/d from feb 2018 to june 2019. during the type ii outburst of 2020, gro j1008-57 had the spin-up torque again. during the torque reversals, the pulse profiles and continuum x-ray spectra did not change significantly, and the cyclotron resonant scattering feature around 80 kev was only detected during the outbursts in 2017 and 2020. based on the observed mean spin-up rate, we estimated the inner accretion disk radius in gro j1008-57 (about 1 - 2 times of the alfvén radius) by comparing different accretion torque models of magnetic neutron stars. during the spin-down process, the magnetic torque should dominate over the matter accreting inflow torque, and we constrained the surface dipole magnetic field b ≥ 6 ×1012 g for the neutron star in gro j1008-57, which is consistent with the magnetic field strength obtained by cyclotron line centroid energy. | accretion torque reversals in gro j1008-57 revealed by insight-hxmt |
failed supernovae (fsn) are a possible channel for the formation of heavy stellar-mass black holes (m bh > ~30 m ⊙). however, the effects of metallicity, rotation, and magnetic field on the islands of explodabilty of massive stars are not clear. here, we simulate the stellar structure and evolution in the mass range between 6 and 55 m ⊙ with different initial rotational velocities, metallicities, and magnetic fields from zero-age main sequence (zams) to pre-collapse. we find that the rapid rotating stars can remain lower 12c mass fraction at the time of c ignition, which allows the transition, from convective carbon burning to radiative burning, to occur at lower m zams than those from stars without rotation. however, the rapid rotation is unfavorable for fsn occurring but is conducive to long gamma-ray bursts (lgrbs) because it results in the specific angular momentum in the co core being greater than the last stable orbit at core collapse. the increasing metallicity does not affect fsn islands, but high metallicity inhibits rotational mixing and is unfavorable for producing lgrbs. a magnetic field can constrain the mass-loss rate even for rapid rotating stars, resulting in higher mass at pre-collapse. the magnetic braking triggered by the magnetic field can reduce the rotation velocity for high-metallicity models, which decreases the specific angular momentum in the co core and is favorable for fsn occurring. we suggest that the heavy-mass black holes detected by ligo may originate from rapidly rotating massive stars with strong magnetic fields, rather than those with very low metallicity. | the effects of rotation, metallicity, and magnetic field on the islands of failed supernovae |
a novel scheme has been developed to show that the observed phase behavior associated with subpulse drifting from two pulsars, j1034-3224 and j1720-2933, can be used to obtain the magnetic field configuration in the partially screened gap (psg). the outflowing plasma along the open magnetic field line region of pulsars is generated as a result of spark discharges in an inner acceleration region (iar) above the polar cap. the iar has been modeled as a psg with a steady supply of positively charged ions emitted from the heated polar cap surface dominated by strong non-dipolar magnetic fields. in a psg the sparks are tightly packed and constrained to be present along the polar cap boundary. the sparks lag behind the rotation of the star during their lifetimes. as a result, the sparking pattern evolves along two different directions in a clockwise and counterclockwise manner around a stationary central spark and can be associated with the observed phenomenon of subpulse drifting. psr j1034-3224 has four prominent components and exhibits bi-drifting where alternate components show the opposite sense of drifting, while psr j1720-2933 has a single component profile and shows systematically coherent drift bands. we show that the differences in their drifting behavior can be directly linked to different natures of the non-dipolar surface magnetic field configurations. | estimating the evolution of sparks in the partially screened gap of pulsars from subpulse drifting |
context. close-by planets can excite various kinds of oscillations in their host stars through their time-varying tidal potential.aims: magnetostrophic oscillations with a frequency much smaller than the stellar rotation frequency have recently been proposed to account for the spin-orbit commensurability observed in several planet-hosting stars. in principle, they can be resonantly excited in an isolated slender magnetic flux tube by a fourier component of the time-varying tidal potential with a very low frequency in the reference frame rotating with the host. however, due to the weakness of such high-order tidal components, a mechanism is required to lock the oscillations in phase with the forcing for long time intervals (103-107 yr) in order to allow the oscillation amplitude to grow.methods: we propose that the locking mechanism is an auto-resonance produced by the nonlinear dependence of the oscillation frequency on its amplitude. we suggest that the angular momentum loss rate is remarkably reduced in hosts entering auto-resonance and this contributes to maintaining those systems in that regime for a long time.results: we apply our model to a sample of ten systems that show spin-orbit commensurability and estimate the maximum drifts of the relevant tidal potential frequencies that allow them to enter the auto-resonant regime. such drifts are compared with the drifts expected from the tidal evolution of the planetary orbits and the stellar angular momentum loss in the magnetized winds, and we find that auto-resonance is a viable mechanism in eight systems, at least in our idealized model.conclusions: the duration of the auto-resonant regime and the associated spin-orbit commensurability may be comparable with the main-sequence lifetimes of the host stars, indicating that gyrochronology may not be applicable to those hosts. | tidal excitation of auto-resonant oscillations in stars with close-by planets |
in this paper, we present the detailed analysis of recurrent homologous jets originating from an emerging negative magnetic flux at the edge of an active region. the observed jets show multithermal features. their evolution shows high consistence with the characteristic parameters of the emerging flux, suggesting that with more free magnetic energy, the eruptions tend to be more violent, frequent, and blowout-like. the average temperature, average electron number density, and axial speed are found to be similar for different jets, indicating that they should have been formed by plasmas from similar origins. statistical analysis of the jets and their footpoint region conditions reveals a strong positive relationship between the footpoint region total 131 å intensity enhancement and jets’ length/width. stronger linearly positive relationships also exist between the total intensity enhancement/thermal energy of the footpoint regions and jets’ mass/kinetic/thermal energy, with higher cross-correlation coefficients. all the above results together confirm the direct relationship between the magnetic reconnection and the jets and validate the important role of magnetic reconnection in transporting large amounts of free magnetic energy into jets. it is also suggested that there should be more free energy released during the magnetic reconnection of blowout than of standard jet events. | on the magnetic and energy characteristics of recurrent homologous jets from an emerging flux |
we analyze the evolutionary status of recently discovered long-period radio sources psr j0901-4046, gleam-x j1627-52, and gpm j1839-10. we discuss the hypothesis that all three sources are radio pulsars. in the framework of standard scenarios, it is often accepted that the pulsar mechanism is switched off when an external matter can penetrate the light cylinder. if the matter is stopped outside the light cylinder then the neutron star is at the ejector stage. we demonstrate that for realistic parameters of the interstellar medium, the 76-second pulsar psr j0901-4046 might be at this stage. however, sources gleam-x j1627-52 and gpm j1839-10 with periods $\gtrsim 1000$ s can be ejectors only in the case of unrealistically large dipolar fields $\gtrsim 10^{16}$ g. also, we show that neutron stars with spin periods $\sim 100$ s and dipolar magnetic fields $\lesssim 10^{13}$ g cannot be ejectors in a typical interstellar medium. thus, we predict that long-period pulsars with standard fields will not be discovered. | evolutionary status of long-period radio pulsars |
we show that magnetar models for ultraluminous x-ray sources (ulxs) have serious internal inconsistencies. the magnetic fields required to increase the limiting luminosity for radiation pressure above the observed (assumed isotropic) luminosities are completely incompatible with the spin-up rates observed for pulsing ulxs. we note that at least one normal be-star + neutron star system, with a standard (non-magnetar) field, is observed to become a ulx during a large outburst and return to its previous be-star binary state afterwards. we note further that recent polarimetric observations of the well-studied binary cyg x-3 reveal that it produces strong emission directed away from the observer, in line with theoretical predictions of its total accretion luminosity from evolutionary arguments. we conclude that the most likely explanation for ulx behaviour involves radiation beaming by accretion disc winds. a large fraction of x-ray binaries must pass through a ulx state in the course of their evolution. | ultraluminous x-ray sources are beamed |
we revisit the various approximations employed to study the long-term evolution of the magnetic field in neutron star cores and discuss their limitations and possible improvements. a recent controversy on the correct form of the induction equation and the relevant evolution time-scale in superconducting neutron star cores is addressed and clarified. we show that this ambiguity in the estimation of time-scales arises as a consequence of nominally large terms that appear in the induction equation, but which are, in fact, mostly irrotational. this subtlety leads to a discrepancy by many orders of magnitude when velocity fields are absent or ignored. even when internal velocity fields are accounted for, only the solenoidal part of the electric field contributes to the induction equation, which can be substantially smaller than the irrotational part. we also argue that stationary velocity fields must be incorporated in the slow evolution of the magnetic field as the next level of approximation. | on the magnetic field evolution time-scale in superconducting neutron star cores |
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