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we present the capabilities of our new code for obtaining doppler maps implementing the maximum likelihood approach. as test data, we used observations of the dwarf nova v2051 ophiuchi. the system was observed in quiescence at least 16 d before the onset of the next outburst. using doppler maps obtained for ten emissio... | spiral structures and temperature distribution in the quiescent accretion disc of the cataclysmic binary v2051 ophiuchi |
magnetorotational instability (mri) has the potential to generatevigorous turbulence in protoplanetary disks, although its turbulence strength and accretion stress remain debatable because of the uncertainty of mri with a low ionization fraction. we focus on the heating of electrons by strong electric fields, which amp... | electron heating and saturation of self-regulating magnetorotational instability in protoplanetary disks |
the magnetohydrodynamics (mhd) of protoplanetary disks are strongly subject to the nonideal mhd effects arising from the low ionization fraction of the disk gas. a strong electric field induced by gas motions can heat ionized gas particles and can thereby affect the ionization balance in the disks. our previous studies... | the generalized nonlinear ohm's law: how a strong electric field influences nonideal mhd effects in dusty protoplanetary disks |
we study the development of coherent structures in local simulations of the magnetorotational instability in accretion discs in regimes of on-off intermittency. in a previous paper, we have shown that the laminar and bursty states due to the on-off spatiotemporal intermittency in a one-dimensional model of non-linear w... | on-off intermittency and amplitude-phase synchronization in keplerian shear flows |
there are several peculiar long-period dwarf-nova-like objects that show rare, low-amplitude outbursts with highly ionized emission lines; 1swasp j162117+441254, bd pav, and v364 lib are among them. some researchers even doubt whether 1swasp j1621 and v364 lib have the same nature as normal dwarf novae. we studied the ... | on the nature of long-period dwarf novae with rare and low-amplitude outbursts |
context. long gamma ray bursts (grbs) originate from the collapse of massive, rotating stars. some of the grbs exhibit much stronger variability patterns in the prompt grb emission than the usual stochastic variations. we discuss the mechanisms able to account for this effect. aims we aim to model the process of stella... | self-gravitating collapsing star and black hole spin-up in long gamma ray bursts |
context. the thermodynamical evolution of gas during the collapse of the primordial star-forming cloud depends significantly on the initial degree of rotation.aims: however, there is no clear understanding of how the initial rotation can affect the heating and cooling process and hence the temperature that leads to the... | on the effects of rotation in primordial star-forming clouds |
we present our spectroscopic observations of v455 andromedae during the 2007 superoutburst. our observations cover this superoutburst from around the optical peak of the outburst to the post-superoutburst stage. during the early superhump phase, the emission lines of the balmer series, he i, he ii, bowen blend, and c i... | spectroscopic observations of v455 andromedae superoutburst in 2007: the most exotic spectral features in dwarf nova outbursts |
x-ray variabilities with spectral state transitions in bright low-mass x-ray binaries containing a neutron star are investigated by using the one-day bin light curves of maxi/gsc (gas slit camera) and swift/bat (burst alert telescope). four sources (4u 1636-536, 4u 1705-44, 4u 1608-52, and gs 1826-238) exhibited small-... | x-ray variability with spectral state transitions in ns-lmxbs observed with maxi/gsc and swift/bat |
we report long-term simulations of black hole-neutron star binary mergers where the neutron star possesses an asymmetric magnetic field dipole. focusing on the scenario where the neutron star is tidally disrupted by the black hole, we track the evolution of the binary up to ≈100 ms after the merger. we uncover more tha... | effects of magnetic field topology in black hole-neutron star mergers: long-term simulations |
i have run hydrodynamic simulations that follow the colliding wind structure of the massive binary system hd 166734 along its binary orbit. i show that close to periastron passage the secondary wind is suppressed and the secondary accretes mass from the primary wind. the system consists of two blue supergiants with mas... | wind collision and accretion simulations of the massive binary system hd 166734 |
white dwarfs are physically simple and numerous. their properties provide insight into stellar evolution and have applications to many astrophysical questions. in this dissertation, we present new measurements of white dwarf properties in two environments that help further our knowledge of the structure and evolution o... | fundamental properties of white dwarfs alone and in binaries |
young protostellar discs provide the initial conditions for planet formation. the properties of these discs may be different from those of late-phase (t tauri) discs due to continuing infall from the envelope and protostellar variability resulting from irregular gas accretion. we use a set of hydrodynamic simulations t... | the structure of young embedded protostellar discs |
we derive expressions for the local ideal magnetohydrodynamic (mhd) equations for a warped astrophysical disc using a warped shearing box formalism. a perturbation expansion of these equations to first order in the warping amplitude leads to a linear theory for the internal local structure of magnetized warped discs in... | a local model of warped magnetized accretion discs |
we study the stellar populations of bulges of milky way-like (mw-like) galaxies with the aim of identifying the physical processes involved in the formation of the bulge of our galaxy. we use the semi-analytic model of galaxy formation and evolution sag adapted to this aim; these kind of models can trace the properties... | stellar populations in a semi-analytic model i: bulges of milky way-like galaxies |
we conduct a linear stability calculation of an ideal keplerian flow on which a sinusoidal zonal flow is imposed. the analysis uses the shearing sheet model and is carried out both in isothermal and adiabatic conditions, with and without self-gravity (sg). in the non-sg regime, a structure in the potential vorticity (p... | non-axisymmetric instabilities in discs with imposed zonal flows |
rapidly rotating neutron stars in low mass x-ray binaries have been proposed as an interesting source of gravitational waves. in this chapter we present estimates of the gravitational wave emission for various scenarios, given the (electromagnetically) observed characteristics of these systems. first of all we focus on... | gravitational waves from rapidly rotating neutron stars |
we observed the 2014 superoutburst of the su uma-type intermediate polar cc scl. we detected superhumps with a mean period of 0.05998(2) d during the superoutburst plateau and during three nights after the fading. during the post-superoutburst stage after three nights, a stable superhump period of 0.059523(6) d was det... | cc sculptoris: eclipsing su uma-type intermediate polar |
the central molecular zone (cmz) of the milky way shows several peculiar properties: a large star formation rate (sfr), some of the most massive young star clusters and molecular clouds in the galaxy, and a twisted ring morphology in molecular gas. in this paper, i use smoothed particle hydrodynamics simulations to sho... | agn activity and nuclear starbursts: sgr a* activity shapes the central molecular zone |
we present an instability for exciting incompressible modes (e.g., gravity or rossby modes) at the surface of a star accreting through a boundary layer. the instability excites a stellar mode by sourcing an acoustic wave in the disk at the boundary layer, which carries a flux of energy and angular momentum with the opp... | incompressible modes excited by supersonic shear in boundary layers: acoustic cfs instability |
by using an n-body simulation of a bulge that was formed via a bar instability mechanism, we analyse the imprints of the initial (i.e. before bar formation) location of stars on the bulge kinematics, in particular on the heliocentric radial velocity distribution of bulge stars. four different latitudes were considered:... | a new look at the kinematics of the bulge from an n-body model |
in this paper we investigate effects of the r-mode instability on a newborn rapidly-rotating magnetar with fall-back accretion. such a magnetar could usually occur in core-collapse supernovae and gamma-ray bursts. we find that the magnetar's spin and r-mode evolution are influenced by accretion. if the magnetar is suff... | evolution of newborn rapidly rotating magnetars: effects of r-mode and fall-back accretion |
criterion of the jeans modes for self-gravitational systems in eddington-inspired born-infeld (eibi) gravity is revisited based on the framework of nonextensive statistics and kinetic theory. the results show the nonextensive parameter q has significant effects on the growth rate and critical wave-number of jeans insta... | jeans instability of eddington-inspired born-infield gravitational systems in the context of nonextensive statistics |
we study the local thermal stability of thin accretion disks. we present a full stability analysis in the presence of a magnetic field and, more importantly, wind. we use a general model suitable for adequately describing several kinds of winds. first, we explicitly show that the magnetic field, irrespective of the typ... | thermal instability of thin accretion disks in the presence of wind and a toroidal magnetic field |
we investigate the stability of a neutrino-dominated accretion flow (ndaf), which is expected to be formed in the gravitational collapse of a massive star or the merger of a neutron star binary, based on the variable-α prescription. recent magnetohydrodynamic simulations shows that the viscosity parameter α is proporti... | neutrino-dominated accretion flows with magnetic prandtl number-dependent mri-driven turbulence |
we develop a framework for magnetohydrodynamical (mhd) simulations in a local cylindrical shearing box by extending the formulation of the cartesian shearing box. we construct shearing-periodic conditions at the radial boundaries of a simulation box from the conservation relations of the basic mhd equations, taking int... | magnetohydrodynamics in a cylindrical shearing box |
a turbulent state of spectrally stable shear flows can be developed and sustained according to the bypass scenario of transition. if it works in non-magnetized boundless and homogeneous quasi-keplerian flow, then transiently growing shearing vortices should supply turbulence with energy. employing the large shearing bo... | transient growth of perturbations on scales beyond the accretion disc thickness |
dwarf novæ (dne) and low-mass x-ray binaries (lmxbs) are compact binaries showing variability on time-scales from years to less than seconds. here, we focus on explaining part of the rapid fluctuations in dne, following the framework of recent studies on the monthly eruptions of dne that use a hybrid disc composed of a... | qpos in compact binaries from small-scale eruptions in an inner magnetized disc |
accretion disks in binary systems can experience hydrodynamical influences at both their inner and outer edges. the former is typical for protoplanetary disks around young t tauri stars, while the latter is typical for circumstellar disks in close binaries. this influence excites perturbations with various scales and a... | excitation of turbulence in accretion disks of binary stars by non-linear perturbations |
while collisional accumulation is nearly universally accepted as the formation mechanism of rock and ice worlds, the situation regarding gas giant planet formation is more nuanced. gas accretion by solid cores formed by collisional accumulation is the generally favored mechanism, but observations increasingly suggest t... | flux-limited diffusion approximation models of giant planet formation by disk instability. ii. quadrupled spatial resolution |
we carry out a series of local, shearing-box simulations of the outer regions of protoplanetary disks, where ambipolar diffusion is important due to low ionization levels, to better characterize the nature of turbulence and angular momentum transport in these disks. these simulations are divided into two groups, one wi... | the nature of turbulence in the outer regions of protoplanetary disks |
context. the rather elusive high-frequency quasi-periodic oscillations (hfqpos) observed in the x-ray light curve of black holes have been seen in a wide range of frequencies, even within one source. also notable is the detection of "pairs" of hfqpos with a close-to-integer ratio between the frequencies.aims: the aim o... | rossby wave instability and high-frequency quasi-periodic oscillations in accretion discs orbiting around black holes |
x-ray pulsars shine thanks to the conversion of the gravitational energy of accreted material to x-ray radiation. the accretion rate is modulated by geometrical and hydrodynamical effects in the stellar wind of the pulsar companions and/or by instabilities in accretion disks. wind-driven flows are highly unstable close... | x-ray pulsars |
a set of strongly magnetized accreting white dwarfs (polars) shows quasi-periodic oscillations (qpos) with frequency about a hz in their optical luminosity. these hz-frequency qpos are thought to be generated by intensity variations of the emitted radiation originating at the post-shock accretion column. thermal instab... | quasi-periodic oscillations from post-shock accretion column of polars |
stunted outbursts are ∼0.ͫ6 eruptions, typically lasting 5-10 days, which are found in some novalike cataclysmic variables, including uu aqr. the mechanism responsible for stunted outbursts is uncertain but is likely related to an accretion disk instability or to variations in the mass transfer rate. a campaign to moni... | orbital light curves of uu aquarii in stunted outburst |
the prototype dwarf nova ss cyg unexpectedly exhibited an anomalous event in its light curve in the early few months of 2021 in which regular dwarf nova-type outbursts stopped, and only small-amplitude fluctuations occurred. inspired by this event, we have performed numerical simulations of light curves of ss cyg by va... | the light curve simulations of the 2021 anomalous event in ss cygni |
we analytically study the dynamical and thermal properties of the optically thin gases at the parsec-scale when they are spherically accreted on to low luminous active galactic nuclei. the falling gases are irradiated by the central x-ray radiation with the compton temperature of 5-15 × 107 k. the radiative heating/coo... | dynamical and thermal properties of the parsec-scale gases spherically accreted on to low luminous active galactic nuclei |
intermediate polars (ips) are cataclysmic variables with mildly magnetized white dwarfs (wds). this analysis of the long-term optical activity of five examples of ips with accretion discs used data from the catalina real-time transient survey, digital access to a sky century @ harvard (dasch) and the american associati... | a study of the long-term activity of five intermediate polars with accretion discs |
the accreting millisecond x-ray pulsar swift j1756.9-2508 launched into an outburst in april 2018 and june 2019 - 8.7 years after the previous period of activity. we investigated the temporal, timing, and spectral properties of these two outbursts using data from nicer, xmm-newton, nustar, integral, swift, and insight-... | broadband x-ray spectra and timing of the accreting millisecond pulsar swift j1756.9-2508 during its 2018 and 2019 outbursts |
the presence of a surface at the inner boundary, such as in a neutron star or a white dwarf, allows the existence of a standing shock in steady spherical accretion. the standing shock can become unstable in 2d or 3d; this is called the standing accretion shock instability (sasi). two mechanisms - advective-acoustic and... | magnetized sasi: its mechanism and possible connection to some qpos in xrbs |
fourteen years after its eruption as a classical nova (cn), v1047 cen (nova cen 2005) began an unusual re-brightening in 2019 april. the amplitude of the brightening suggests that this is a dwarf nova (dn) eruption in a cn system. very few cne have had dn eruptions within decades of the main cn outburst. the 14 yr sepa... | infrared spectroscopy of the recent outburst in v1047 cen (nova centauri 2005) |
circumstellar discs are thought to be self-gravitating at very early times. if the disc is relatively cool, extended and accreting sufficiently rapidly, it can fragment into bound objects of order a few jupiter masses and upwards. given that the fragment's initial angular momentum is non-zero, and it will continue to a... | daughter fragmentation is unlikely to occur in self-gravitating circumstellar discs |
one of the currently favored scenarios for the formation of globular clusters (gcs) with multiple stellar populations is that an initial massive stellar system forms (`first generation', fg), subsequently giving rise to gaseous ejecta which is converted into a second generation (sg) of stars to form a gc. we investigat... | formation of globular clusters with multiple stellar populations from massive gas clumps in high-z gas-rich dwarf galaxies |
numerical models of gas inflow towards a supermassive black hole (smbh) show that star formation may occur in such an environment through the growth of a gravitationally unstable gas disc. we consider the effect of nuclear activity on such a scenario. we present the first three-dimensional grid-based radiative hydrodyn... | gas inflow and star formation near supermassive black holes: the role of nuclear activity |
we analyse the time evolution of the luminosity of a cluster of population iii protostars formed in the early universe. we argue from the jeans criterion that primordial gas can collapse to form a cluster of first stars that evolve relatively independently of one another (i.e. with negligible gravitational interaction)... | the luminosity of population iii star clusters |
while it has been observed that the parameters intrinsic to the type c low-frequency quasi-periodic oscillations are related in a nonlinear manner among themselves, there has been, up to now, no model to explain or reproduce how the frequency, the fwhm, and the rms amplitude of the type c low-frequency quasi-periodic o... | reproducing the correlations of type c low-frequency quasi-periodic oscillation parameters in xte j1550-564 with a spiral structure |
we have analysed a series of suzaku data and one data set of xmm-newton of the su uma type dwarf nova vw hyi in optical quiescence. the observed spectra in the 0.2-10 kev band are moderately well represented by multitemperature thermal plasma emission models with a maximum temperature of 5-9 kev and bolometric luminosi... | variation of mass accretion rate on to the white dwarf in the dwarf nova vw hyi in quiescence |
context. the most successful scenario for the origin of astrophysical jets requires a large-scale magnetic field anchored in a rotating object (black hole or star) and/or its surrounding accretion disk. platform jet simulations, where the mass load onto the magnetic field is not computed by solving the vertical equilib... | numerical simulations of mhd jets from keplerian accretion disks. i. recollimation shocks |
shear flows have a significant impact on the dynamics in an assortment of different astrophysical objects, including accretion discs and stellar interiors. investigating shear flow instabilities in a polytropic atmosphere provides a fundamental understanding of the motion in stellar interiors where turbulent motions, m... | shear instabilities in a fully compressible polytropic atmosphere |
this work presents a linear analytical calculation on the stability and evolution of a compressible, viscous self-gravitating (sg) keplerian disc with both horizontal thermal diffusion and a constant cooling time-scale when an axisymmetric structure is present and freely evolving. the calculation makes use of the shear... | zonal flow evolution and overstability in accretion discs |
x-ray emission associated to accretion onto compact objects displays important levels of photometric and spectroscopic time-variability. when the accretor orbits a supergiant star, it captures a fraction of the supersonic radiatively-driven wind which forms shocks in its vicinity. the amplitude and stability of this gr... | wind accretion onto compact objects |
the dynamical evolution of protoplanetary disks is of key interest for building a comprehensive theory of planet formation and to explain the observational properties of these objects. using the magnetohydrodynamics code athena++, with an isothermal shearing box setup, we study the boundary between the active and dead ... | transition region from turbulent to dead zone in protoplanetary disks: local shearing box simulations |
close binaries containing a white dwarf accreting from a disk that receives material from a late stellar companion show an interesting variety of behaviors that depend on their accretion rates and the changes in this rate. the highest accretion rates are evident during the bright states of novalikes, as well as the sup... | insights from multi-wavelength observations during high and low states of non-magnetic cvs |
the persistent low-luminosity neutron star x-ray binary 4u 1812-12 is a potential member of the scarce family of ultra-compact systems. we performed deep photometric and spectroscopic optical observations with the 10.4 m gran telescopio canarias in order to investigate the chemical composition of the accreted plasma, w... | optical spectroscopy of 4u 1812-12. an ultra-compact x-ray binary seen through an h ii region |
hete j1900.1-2455 is an ultra-compact low-mass x-ray binary that underwent a long-lasting (about 10 yr) active state. the analysis presented here of its activity uses the observations of rxte/asm, swift/bat, and iss/maxi for investigating this active state and the relation of time evolution of fluxes in the hard and me... | x-ray outbursts and high-state episodes of hete j1900.1-2455 |
gro j1744-28 (the bursting pulsar) is a neutron star low-mass x-ray binary which shows highly structured x-ray variability near the end of its x-ray outbursts. in this letter we show that this variability is analogous to that seen in transitional millisecond pulsars such as psr j1023+0038: `missing link' systems consis... | the bursting pulsar gro j1744-28: the slowest transitional pulsar? |
the structured jets are postulated to account for emission properties of high energy sources across the mass scale, from stellar mass black holes in grbs to supermassive black holes in agn. their active cores contain magnetized accretion disks and rotation of the kerr black hole provides mechanism for jet launching. th... | can mad accretion disks launching structured jets explain both grb and agn engines? |
the classical alpha-disc model assumes that the turbulent stress scales linearly with - and responds instantaneously to - the pressure. it is likely, however, that the stress possesses a non-negligible relaxation time and will lag behind the pressure on some time-scale. to measure the size of this lag we carry out unst... | the stress-pressure lag in mri turbulence and its implications for thermal instability in accretion discs |
this ph.d. thesis explores the modeling of dynamics in magnetospheres around compact objects (black holes and neutron stars), and their implications in the formation of high energy phenomena such as magnetar flares and the highly variable teraelectron volt (tev) emission of some active galactic nuclei, by means of nume... | dynamics in the magnetospheres of compact objects |
accretion disks around young binary stars are subject to strong forces exerted by the system components. gas-dynamical interactions excite strong non-linear perturbations in the disk, which can give rise to turbulence. this study considers a mechanism for the excitation of turbulence based on the instability of non-lin... | excitation of turbulence in protoplanetary disks around binary stars |
the spectra of disc-dominated cataclysmic variables (cvs) often deviate from the spectra of accretion disc models; in particular, the balmer jump and absorption lines are found to be shallower in the observations than in the models. we carried out a combined ultraviolet-optical spectral analysis of two dwarf novae (dne... | the hydrogen balmer lines and jump in absorption in accretion disc modelling - an ultraviolet-optical spectral analysis of the dwarf novae uz serpentis and cy lyrae |
context. self-gravitational rotating bodies do not have spherically symmetric geometries. the study of physical events appearing in fast-spinning compact stars and accretion disks, for example those due to localized thermonuclear ignitions in white dwarfs or to the role played by hydrodynamic instabilities in stars and... | self-gravitating barotropic equilibrium configurations of rotating bodies with smoothed particle hydrodynamics |
linearized and flattened light curves (29 in b band and 29 in v band) of the recurent nova rs oph, taken in 2008--2017 by 5 telescopes, are analyzed. the purposes are (i) characterizing of the flickering phenomenon by various ways and (ii) justification of the "resonance" distribution of the modes of the flickering qua... | intra-night flickering of rs ophiuchi: ii. exponentially distributed quasi-period modes |
in this study, we addressed the question of how the growth rate of the magnetorotational instability is modified when the radial component of the stellar dipole magnetic field is taken into account in addition to the vertical component. considering a fiducial radius in the disc where diamagnetic currents are pronounced... | magnetorotational instability in diamagnetic, misaligned protostellar discs |
a simple look at the steady high-energy universe reveals a clear correlation with outflows generated around compact objects (winds and jets). in the case of relativistic jets, they are thought to be produced as a consequence of the extraction of rotational energy from a kerr black hole (blandford-znajek), or from the d... | propagation and stability of relativistic jets |
accreting millisecond x-ray pulsars (amxps) are an important subclass of low-mass x-ray binaries (lmxbs), in which coherent millisecond x-ray pulsations can be observed during outburst states. they have dual characteristics of lmxbs and millisecond pulsars, providing a direct confirmation for the recycling scenario. ho... | formation of accreting millisecond x-ray pulsars |
context. enhanced angular momentum transfer through the boundary layer near the surface of weakly magnetised accreting star is required in order to explain the observed accretion timescales in low-mass x-ray binaries, cataclysmic variables, or young stars with massive protoplanetary discs. the accretion disc boundary l... | subcritical transition to turbulence in accretion disc boundary layer |
a number of white dwarf stars show absoption lines of heavy elements in their spectra. many of them also exhibit infra-red excess in their spectral energy distribution. these observations prove that these white dwarfs are surrounded by an orbiting debris disk resulting from the disruption of rocky planetesimals, remnan... | fingering convection and its consequences for accreting white dwarfs |
3xmm j031820.8-663034, first detected by rosat in ngc 1313, is one of a few known transient ultraluminous x-ray sources (ulxs). in this paper, we present decades of x-ray data of this source from rosat, xmm-newton, chandra, and the neil gehrels swift observatory. we find that its x-ray emission experienced four outburs... | recurrent outbursts revealed in 3xmm j031820.8-663034 |
recent numerical simulations of magnetized accretion discs show that the radial-azimuthal component of the stress tensor due to the magnetorotational instability is well represented by a power-law function of the gas pressure rather than a linear relation that has been used in most of the accretion disc studies. the ex... | properties of an accretion disc with a power-law stress-pressure relationship |
we observed the deeply eclipsing su uma-type dwarf nova v4140 sgr and established the very short supercycle of 69.7(3) d. there were several short outbursts between superoutbursts. these values, together with the short orbital period (0.06143 d), were similar to, but not as extreme as, those of er uma-type dwarf novae.... | detection of the supercycle in v4140 sagittarii: first eclipsing er ursae majoris-like object |
we use an n-body simulation, constructed using gadget-2, to investigate an accretion flow onto an astrophysical disk that is in the opposite sense to the disk's rotation. in order to separate dynamics intrinsic to the counter-rotating flow from the impact of the flow onto the disk, we consider an initial condition in w... | instability of counter-rotating stellar disks |
> in this paper, we analyse the linear stability of a stellar accretion disk having a stratified morphology. the study is performed in the framework of ideal magneto-hydrodynamics and therefore results in a characterization of the linear unstable magneto-rotational modes. the peculiarity of the present scenario cons... | morphology of two-dimensional mri in axial symmetry |
we report the analysis of time series of infrared jhks photometry of the dwarf nova v2051 oph in quiescence. we modelled the ellipsoidal variations caused by the distorted mass-donor star to infer its jhks fluxes. from its infrared colours, we estimate a spectral type of m(8.0 ± 1.5) and an equivalent blackbody tempera... | infrared photometry of the dwarf nova v2051 ophiuchi - i. the mass-donor star and the distance |
supermassive black holes (smbhs) with m• ∼ 109 m⊙ at z > 6 likely originate from massive seed black holes (bhs). we investigate the consequences of seeding smbhs with direct collapse bhs (dcbhs) (m_{\bullet }=10^{4-6} m_⊙) on proto-galactic disc growth. we show that even in the absence of direct feedback effects, th... | how black holes stop their host galaxy from growing without agn feedback |
we follow the development of nonaxisymmetric instabilities of self-gravitating disks from the linear regime to the nonlinear regime. particular attention is paid to comparison of nonlinear simulation results with previous linear and quasi-linear modeling results to study the mass and angular momentum transport driven b... | nonaxisymmetric instabilities in self-gravitating disks iii. angular momentum transport |
the nature of accretion in symbiotic binaries, in which the red giant transfers material to a white dwarf (wd) companion, has been difficult to uncover. the accretion flows in a symbiotic binary are most clearly observable, however, when there is no quasi-steady shell burning on the wd to hide them. through observation... | accretion disks in symbiotic stars |
magnetorotational instability (mri), also known as the balbus-hawley instability, is thought to have an important role on the initiation of turbulence and angular momentum transport in accretion disks. in this work, we investigate the effect of the magnetic field gradient in the azimuthal direction on mri. we solve the... | magnetic field gradient effects on the magnetorotational instability |
active states in white dwarfs are usually associated with light curve's effects that concern to the bursts, flickering or flare-up occurrences. it is common that a gas-dynamics source exists for each of these processes there. we consider the white dwarf binary stars with accretion disc around the primary. we suggest a ... | active states and structure transformations in accreting white dwarfs |
3d numerical simulations demonstrate the formation of precessional spiral density waves in accretion disks of close binary stars. the precesional wave occurs in the keplerian disk as a result of gravitational action of the donor-star. the wave causes the appearance of strong density and velocity gradients in the disk. ... | precessional density wave as a reason of turbulence in accretion disks of non-magnetic close binary stars |
the outburst activity of the symbiotic system ag dra has been studied using extensive spectroscopic observational material. high luminosity and temperature of the hot component of ag dra indicate that quasi-steady thermonuclear shell burning takes place on the surface of the white dwarf. the major (cool) outbursts at t... | outburst activity of the symbiiotic binary ag dra |
high-energy radiation from the central t tauri and protostars plays an important role in shaping protoplanetary disks and influences their evolution. such radiation, in particular x-rays and extreme-ultraviolet (euv) radiation, is predominantly generated in unstable stellar magnetic fields (e.g., the stellar corona), b... | ionization and heating by x-rays and cosmic rays |
by performing multi-dimensional radiation-mhd simulations, we reveal the inflow-outflow structure around the black holes (bhs) and the neutron stars (nss). in both cases, the quasi-steady, super-eddington disk forms around the central objects. the disk is supported by the strong radiation pressure and the radiatively-d... | numerical simulations of sueper-eddington accretion flows and outflows around black holes and neutron stars |
population iii accretion disks are prone to gravitational instabilities that can cause them to fragment. studies of present-day star formation find that magnetic fields can help to stabilize protostellar accretion disks and suppress fragmentation. however, the role of magnetic fields during pop iii star formation is un... | stabilizing population iii accretion disks with magnetic fields |
we propose a 100ks observation of the unique outbursting seyfert galaxyic 3599, to be triggered if it flares again in 2019. two giant-amplitudex-ray outbursts (factor>100) in 1990 and 2010 were accompanied by dramaticemission-line response never seen before in any agn, but high-resolutionx-ray spectra where missed. ... | the nature of the x-ray outbursts in ic 3599 |
we develop a semi-analytic formalism for the determination of the evolution of the stellar mass accretion rate for specified density and velocity profiles that emerge from the runaway collapse of a prestellar cloud core. in the early phase, when the infall of matter from the surrounding envelope is substantial, mass is... | a semi-analytic model for the temporal evolution of the episodic disk-to-star accretion rate during star formation |
kepler revealed the common existence of tightly-packed super-earth systems around solar-type stars, existing entirely inside the orbit of our venus. those systems must be stable for the ages of their host stars (~10^9 years); their formation mechanism must provide inter-planet spacings that permit long-term stability. ... | consolidating and crushing exoplanets: did it happen here? |
we will attempt to confirm the detection of an accreting protoplanet detected after the cycle 30 deadline with the magao-x instrument in a narrow-band h-alpha filter. the observations were taken under exceptional conditions, and it is highly unlikely that similar conditions will be met during a second epoch observation... | confirming an accreting planet in a very young protoplanetary disk |
the origin of the soft x-ray excess in luminous active galactic nuclei (agn) such as type 1 seyfert galaxies is thought to be the warm compton region with a temperature of 106-107 k. this temperature is higher than that of the optically thick standard disk around a supermassive black hole and lower than the optically t... | radiation magnetohydrodynamic simulations of sub-eddington accretion flows in agn |
short-period super-earths and mini-neptunes have been shown to be common, yet it is still not understood how and where inside protoplanetary discs they could have formed. to form these planets at the short periods at which they are detected, the inner regions of protoplanetary discs must be enriched in dust. dust could... | mri-accreting inner regions of protoplanetary discs |
in the early universe, we observe supermassive black holes with masses of up to a billion times the mass of the sun, accreting at or even above the eddington limit. these high-redshift quasars are some of the most luminous objects in the universe, and raise many questions about the formation and growth of the first bla... | the host galaxies of high-redshift quasars |
all giant planets in the solar system have two types of moons as defined by their orbits and mode of origin. the first type, referred to as the regular moons, has tight circular orbits close to the equatorial plane of the host, implying primordial accretion in the circum-planetary disc. the second type, called the irre... | an in-situ formation for triton and nereid |
v630 cas is one of several long-period cataclysmic variables withrare, long-duration, dwarf nova outbursts. the characteristics of theseoutbursts are likely shaped by the large physical size of the accretiondisks. moreover, they often are luminous (> 10e33 ergs/s) x-ray sourcesin quiescence, severely challenging the... | probing accretion disks on intermediate size scales: the case of v630 cas |
the work is devoted to synthetic light curves modeling for the observed binary stellar systems and an interpretation of their characteristic features including pre-eclipse humps, asymmetry in the vicinity of the eclipse and variability of light curves from revolution to revolution. the mathematical model of the accreti... | mathematical modeling of inclined accretion disks in cataclysmic variables |
we report an extensive campaign of v-band time-series photometry of the new x-ray transient maxi j1820+070 = asassn-18ey. about 90 days into its outburst, the star developed large-amplitude photometric waves with a period of 0.690(2) days. these are likely to be superhumps characteristic of an eccentric instability in ... | asassn-18ey (maxi j1820+070): king of the black-hole superhumps |
close binaries containing a white dwarf accreting from a disk that receives material from a late stellar companion show an interesting variety of behavior that depends on their accretion rate and the changes in this rate. the highest accretion rates are evident during the bright states of novalikes, as well as the supe... | insights from multi-wavelength observations during high and low accretion states of non-magnetic cvs |
active galactic nuclei (agns) and x-ray binaries (xrbs) contain at their cores supermassive black holes (smbh), and stellar mass black holes or neutron stars, respectively. these objects bind matter gravitationally, leading to the formation of accretion disks. the accretion of matter leads to efficient conversion of gr... | the effects of winds on accretion disks and spectra of x-ray binaries and active galactic nuclei |
we illustrate the peculiar x-ray variability displayed by the accreting millisecond x-ray pulsar igr j00291+5934 in a 80 ks-long joint nustar and xmm-newton observation performed during the source outburst in 2015. the lightcurve of the source is characterized by a flaring behavior, with typical rise and decay timescal... | a soft mhz quasi periodic oscillation in the fastest accreting millisecond pulsar. |
protoplanetary disks are observed to undergo episodes of powerful eruptions known as fuor- and exor-type outbursts. thermal instability has been proposed as one of the underlying mechanisms behind this outburst activity, which can produce self-regulating accretion events. this instability sets in at high temperatures, ... | links between water and episodic accretion in young stars |
gravitational instability has been extensively studied in contexts of protoplanetary disks as well as agn accretion disks around supermassive blackholes (smbhs). when a gravitationally stable standard shakura-sunyaev accretion disk, with its accretion parameter alpha driven primarily by magnetorotational instability (m... | radiation hydrodynamic simulations of gravitational instability in agn accretion disks: the radiation-modified fragmentation boundary |
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