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At least 613 records · Page 34
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

NUCLEAR DOMINATED ACCRETION FLOWS IN TWO DIMENSIONS. I. TORUS EVOLUTION WITH PARAMETRIC MICROPHYSICS

We explore the evolution of radiatively inefficient accretion disks in which nuclear reactions are dynamically important ('Nuclear Dominated Accretion Flows' or NuDAFs). Examples of such disks are those generated by the merger of a white dwarf with a neutron star or black hole, or by the collapse of a rotating star. Here, we present two-dimensional hydrodynamic simulations that systematically explore the effect of adding a single nuclear reaction to a viscous torus. The equation of state, anomalous shear stress, and nuclear reactions are given parametric forms. Our results point to the existence of two qualitatively different regimes of NuDAF evolution: (1) steady accretion with quiescent burning or (2) detonation of the disk. These outcomes are controlled primarily by the ratio {Psi} of the nuclear energy released to the enthalpy at the burning radius. Disks detonate if {Psi} exceeds a critical value {Psi}{sub crit} {approx} 1, and if burning occurs in regions where neutrino cooling is unimportant. Thermonuclear runaways are seeded by the turbulent mixing of hot ash with cold fuel at the burning front. Disks with {Psi} < {Psi}{sub crit} do not explode, but instead power a persistent collimated outflow of unbound material composed primarily of ash, with a mass-loss rate that increases with {Psi}. We discuss the implications of our results for supernova-like counterparts from astrophysical events in the NuDAF regime. In particular, detonations following a white dwarf-neutron star merger could account for some subluminous Type Ia supernovae, such as the class defined by SN 2002cx.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THE TRANSITION LAYER BETWEEN THE THIN ACCRETION DISK AND THE HOT CORONA: CONSTRAINTS FROM THE THERMAL X-RAY LINE EMISSION

The observed continuum spectra in the low-luminosity active galactic nuclei, M81, can be reproduced by the advection-dominated accretion flow (ADAF)+thin disk/corona accretion flow model. We find that the best fit to the spectral energy distribution of M81 requires accretion rate m-dot =3 Multiplication-Sign 10{sup -4} for R {sub tr} = 60 R {sub S}. The observed H-like and He-like iron lines may probably be emitted from the transition layer between the vertically connected hot corona and cold thin disk. We model a power-law function of height z for the electron temperature in the transition layer, and calculate the thermal H-like and He-like iron line emission from M81. Combining the observations with our results, we find that the thickness of the transition layer is {approx}14%-23% of the thickness of the corona at the same radius. We discuss that the observed stronger H-like line emission than He-like line emission cannot be explained with the pure ADAF model. Our results provide useful constraints on the theoretical model for disk-corona connection.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

TURBULENCE IN THE OUTER REGIONS OF PROTOPLANETARY DISKS. I. WEAK ACCRETION WITH NO VERTICAL MAGNETIC FLUX

We use local numerical simulations to investigate the strength and nature of magnetohydrodynamic (MHD) turbulence in the outer regions of protoplanetary disks, where ambipolar diffusion is the dominant non-ideal MHD effect. The simulations include vertical stratification and assume zero net vertical magnetic flux. We employ a super time-stepping technique to ameliorate the Courant restriction on the diffusive time step. We find that in idealized stratified simulations, with a spatially constant ambipolar Elsasser number Am, turbulence driven by the magnetorotational instability (MRI) behaves in a similar manner as in prior unstratified calculations. Turbulence dies away for Am {<=} 1, and becomes progressively more vigorous as ambipolar diffusion is decreased. Near-ideal MHD behavior is recovered for Am {>=} 10{sup 3}. In the intermediate regime (10 {<=} Am {<=} 10{sup 3}) ambipolar diffusion leads to substantial increases in both the period of the MRI dynamo cycle and the characteristic scales of magnetic field structures. To quantify the impact of ambipolar physics on disk accretion, we run simulations at 30 AU and 100 AU that include a vertical Am profile based upon far-ultraviolet (FUV) ionized disk models. These models develop a vertically layered structure analogous to the Ohmic dead zone that is present at smaller radii. We find that, although the levels of surface turbulence can be strong (and consistent with constraints on turbulent line widths at these radii), the inferred accretion rates are at least an order of magnitude smaller than those observed in T Tauri stars. This discrepancy is very likely due to the assumption of zero vertical magnetic field in our simulations and suggests that vertical magnetic fields are essential for MRI-driven accretion in the outer regions of protoplanetary disks.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THE STRUCTURE OF GAS-ACCRETING PROTOPLANETS AND THE CONDITION OF THE CRITICAL CORE MASS

In the core accretion model for the formation of gas giant planets, runaway gas accretion onto a core is the primary requisite, triggered when the core mass reaches a critical value. The recently revealed wide diversity of the extrasolar giant planets suggests the necessity to further the understanding of the conditions resulting in the critical core mass that initiates runaway accretion. We study the internal structure of protoplanets under hydrostatic and thermal equilibria represented in terms of a polytropic equation of state to investigate what factors determine and affect the critical core mass. We find that the protoplanets, embedded in protoplanetary disks, have the same configuration as red giants, characterized by the envelope of the centrally condensed type solution. Applying the theory of stellar structure with homology invariants, we demonstrate that there are three types of criteria for the critical core mass depending on the stiffness of polytrope and the nature of outer boundary condition. For the stiff polytropes of index N {<=} 3 with the Bondi radius as the outer boundary, the criterion governing the critical core mass occurs at the surface. For stiff polytropes with the Hill outer boundary and for soft polytropes of N > 3, this criterion acts at the bottom of gaseous envelope. Further, we elucidate the roles and effects of coexistent radiative and convective zones in the envelope of critical core mass. Based on the results, we discuss the relevance of Bondi and Hill surface conditions and explore the parameter dependences of critical core mass.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

MAGNETIZED ACCRETION AND DEAD ZONES IN PROTOSTELLAR DISKS

The edges of magnetically dead zones in protostellar disks have been proposed as locations where density bumps may arise, trapping planetesimals and helping form planets. Magneto-rotational turbulence in magnetically active zones provides both accretion of gas on the star and transport of mass to the dead zone. We investigate the location of the magnetically active regions in a protostellar disk around a solar-type star, varying the disk temperature, surface density profile, and dust-to-gas ratio. We also consider stellar masses between 0.4 and 2 M{sub Sun }, with corresponding adjustments in the disk mass and temperature. The dead zone's size and shape are found using the Elsasser number criterion with conductivities including the contributions from ions, electrons, and charged fractal dust aggregates. The charged species' abundances are found using the approach proposed by Okuzumi. The dead zone is in most cases defined by the ambipolar diffusion. In our maps, the dead zone takes a variety of shapes, including a fish tail pointing away from the star and islands located on and off the midplane. The corresponding accretion rates vary with radius, indicating locations where the surface density will increase over time, and others where it will decrease. We show that density bumps do not readily grow near the dead zone's outer edge, independently of the disk parameters and the dust properties. Instead, the accretion rate peaks at the radius where the gas-phase metals freeze out. This could lead to clearing a valley in the surface density, and to a trap for pebbles located just outside the metal freezeout line.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

X-RAY SPECTRA FROM MAGNETOHYDRODYNAMIC SIMULATIONS OF ACCRETING BLACK HOLES

We present the results of a new global radiation transport code coupled to a general relativistic magnetohydrodynamic simulation of an accreting, non-rotating black hole. For the first time, we are able to explain from first principles in a self-consistent way all the components seen in the X-ray spectra of stellar-mass black holes, including a thermal peak and all the features associated with strong hard X-ray emission: a power law extending to high energies, a Compton reflection hump, and a broad iron line. Varying only the mass accretion rate, we are able to reproduce a wide range of X-ray states seen in most galactic black hole sources. The temperature in the corona is T{sub e} {approx} 10 keV in a boundary layer near the disk and rises smoothly to T{sub e} {approx}> 100 keV in low-density regions far above the disk. Even as the disk's reflection edge varies from the horizon out to Almost-Equal-To 6M as the accretion rate decreases, we find that the shape of the Fe K{alpha} line is remarkably constant. This is because photons emitted from the plunging region are strongly beamed into the horizon and never reach the observer. We have also carried out a basic timing analysis of the spectra and find that the fractional variability increases with photon energy and viewer inclination angle, consistent with the coronal hot spot model for X-ray fluctuations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

ACCRETION DISK TEMPERATURES OF QSOs: CONSTRAINTS FROM THE EMISSION LINES

We compare QSO emission-line spectra to predictions based on theoretical ionizing continua of accretion disks. The observed line intensities do not show the expected trend of higher ionization with theoretical accretion disk temperature as predicted from the black hole mass and accretion rate. Consistent with earlier studies, this suggests that the inner disk does not reach temperatures as high as expected from standard disk theory. Modified radial temperature profiles, taking account of winds or advection in the inner disk, achieve better agreement with observation. The emission lines of radio-detected and radio-undetected sources show different trends as a function of the theoretically predicted disk temperature.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

DEUTERIUM BURNING IN MASSIVE GIANT PLANETS AND LOW-MASS BROWN DWARFS FORMED BY CORE-NUCLEATED ACCRETION

Using detailed numerical simulations, we study the formation of bodies near the deuterium-burning limit according to the core-nucleated giant planet accretion scenario. The objects, with heavy-element cores in the range 5-30 M{sub Circled-Plus }, are assumed to accrete gas up to final masses of 10-15 Jupiter masses (M{sub Jup}). After the formation process, which lasts 1-5 Myr and which ends with a ''cold-start'', low-entropy configuration, the bodies evolve at constant mass up to an age of several Gyr. Deuterium burning via proton capture is included in the calculation, and we determined the mass, M{sub 50}, above which more than 50% of the initial deuterium is burned. This often-quoted borderline between giant planets and brown dwarfs is found to depend only slightly on parameters, such as core mass, stellar mass, formation location, solid surface density in the protoplanetary disk, disk viscosity, and dust opacity. The values for M{sub 50} fall in the range 11.6-13.6 M{sub Jup}, in agreement with previous determinations that do not take the formation process into account. For a given opacity law during the formation process, objects with higher core masses form more quickly. The result is higher entropy in the envelope at the completion of accretion, yielding lower values of M{sub 50}. For masses above M{sub 50}, during the deuterium-burning phase, objects expand and increase in luminosity by one to three orders of magnitude. Evolutionary tracks in the luminosity versus time diagram are compared with the observed position of the companion to Beta Pictoris.

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DOE OSTI2013

PHOTOMETRIC DETERMINATION OF THE MASS ACCRETION RATES OF PRE-MAIN-SEQUENCE STARS. IV. RECENT STAR FORMATION IN NGC 602

We have studied the young stellar populations in NGC 602, in the Small Magellanic Cloud, using a novel method that we have developed to combine Hubble Space Telescope photometry in the V, I, and Hα bands. We have identified about 300 pre-main-sequence (PMS) stars, all of which are still undergoing active mass accretion, and have determined their physical parameters (effective temperature, luminosity, age, mass, and mass accretion rate). Our analysis shows that star formation has been present in this field over the last 60 Myr. In addition, we can recognize at least two clear, distinct, and prominent episodes in the recent past: one about 2 Myr ago, but still ongoing in regions of higher nebulosity, and one (or more) older than 30 Myr, encompassing both stars dispersed in the field and two smaller clusters located about 100'' north of the center of NGC 602. The relative locations of younger and older PMS stars do not imply a causal effect or triggering of one generation on the other. The strength of the two episodes appears to be comparable, but the episodes occurring more than 30 Myr ago might have been even stronger than the current one. We have investigated the evolution of the mass accretion rate, M-dot{sub acc}, as a function of the stellar parameters finding that log M-dot{sub acc}≅-0.6 log t + log m + c, where t is the age of the star, m is its mass, and c is a decreasing function of the metallicity.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The outflows accelerated by the magnetic fields and radiation force of accretion disks

The inner region of a luminous accretion disk is radiation-pressure-dominated. We estimate the surface temperature of a radiation-pressure-dominated accretion disk, Θ=c{sub s}{sup 2}/r{sup 2}Ω{sub K}{sup 2}≪(H/r){sup 2}, which is significantly lower than that of a gas-pressure-dominated disk, Θ ∼ (H/r){sup 2}. This means that the outflow can be launched magnetically from the photosphere of the radiation-pressure-dominated disk only if the effective potential barrier along the magnetic field line is extremely shallow or no potential barrier is present. For the latter case, the slow sonic point in the outflow will probably be in the disk, which leads to a slow circular dense flow above the disk. This implies that hot gas (probably in the corona) is necessary for launching an outflow from the radiation-pressure-dominated disk, which provides a natural explanation for the observational evidence that the relativistic jets are related to hot plasma in some X-ray binaries and active galactic nuclei. We investigate the outflows accelerated from the hot corona above the disk by the magnetic field and radiation force of the accretion disk. We find that with the help of the radiation force, the mass loss rate in the outflow is high, which leads to a slow outflow. This may be why the jets in radio-loud narrow-line Seyfert galaxies are in general mildly relativistic compared with those in blazars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Evidence for large temperature fluctuations in quasar accretion disks from spectral variability

The well-known bluer-when-brighter trend observed in quasar variability is a signature of the complex processes in the accretion disk and can be a probe of the quasar variability mechanism. Using a sample of 604 variable quasars with repeat spectra in the Sloan Digital Sky Survey-I/II (SDSS), we construct difference spectra to investigate the physical causes of this bluer-when-brighter trend. The continuum of our composite difference spectrum is well fit by a power law, with a spectral index in excellent agreement with previous results. We measure the spectral variability relative to the underlying spectra of the quasars, which is independent of any extinction, and compare to model predictions. We show that our SDSS spectral variability results cannot be produced by global accretion rate fluctuations in a thin disk alone. However, we find that a simple model of an inhomogeneous disk with localized temperature fluctuations will produce power-law spectral variability over optical wavelengths. We show that the inhomogeneous disk will provide good fits to our observed spectral variability if the disk has large temperature fluctuations in many independently varying zones, in excellent agreement with independent constraints from quasar microlensing disk sizes, their strong UV spectral continuum, and single-band variability amplitudes. Our results provide an independent constraint on quasar variability models and add to the mounting evidence that quasar accretion disks have large localized temperature fluctuations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Radiation magnetohydrodynamic simulations of the formation of hot accretion disk coronae

A new mechanism to form a magnetic pressure supported, high temperature corona above the photosphere of an accretion disk is explored using three dimensional radiation magnetohydrodynamic (MHD) simulations. The thermal properties of the disk are calculated self-consistently by balancing radiative cooling through the surfaces of the disk with heating due to dissipation of turbulence driven by magneto-rotational instability (MRI). As has been noted in previous work, we find the dissipation rate per unit mass increases dramatically with height above the mid-plane, in stark contrast to the α-disk model which assumes this quantity is a constant. Thus, we find that in simulations with a low surface density (and therefore a shallow photosphere), the fraction of energy dissipated above the photosphere is significant (about 3.4% in our lowest surface density model), and this fraction increases as surface density decreases. When a significant fraction of the accretion energy is dissipated in the optically thin photosphere, the gas temperature increases substantially and a high temperature, magnetic pressure supported corona is formed. The volume-averaged temperature in the disk corona is more than 10 times larger than at the disk mid-plane. Moreover, gas temperature in the corona is strongly anti-correlated with gas density, which implies the corona formed by MRI turbulence is patchy. This mechanism to form an accretion disk corona may help explain the observed relation between the spectral index and luminosity from active galactic nucleus (AGNs), and the soft X-ray excess from some AGNs. It may also be relevant to spectral state changes in X-ray binaries.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Quasi-static model of collimated jets and radio lobes. I. Accretion disk and jets

This is the first of a series of papers showing that when an efficient dynamo can be maintained by accretion disks around supermassive black holes in active galactic nuclei, it can lead to the formation of a powerful, magnetic helix that could explain both the observed radio jet/lobe structures on very large scales and ultimately the enormous power inferred from the observed ultra-high-energy cosmic rays. In this work, we solve a set of one-dimensional equations similar to the steady-state standard accretion disk model, but now including the large-scale magnetic fields giving rises to jets. We find that the frequently made assumption that large-scale fields are frozen into the disk is fundamentally incorrect, due to the necessity for current and the accreting mass to flow perpendicular to magnetic flux surfaces. A correct treatment greatly simplifies the calculations, yielding fields that leave the disk nearly vertically with magnetic profiles uniquely determined by disk angular momentum conservation. Representative solutions of the magnetic fields in different radial regions of the disk surface are given, and they determine the overall key features in the jet structure and its dissipation, which will be the subjects of later papers.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Stellar parameters and accretion rate of the transition disk star HD 142527 from X-shooter

HD 142527 is a young pre-main-sequence star with properties indicative of the presence of a giant planet and/or a low-mass stellar companion. We have analyzed an X-Shooter/Very Large Telescope spectrum to provide accurate stellar parameters and accretion rate. The analysis of the spectrum, together with constraints provided by the spectral energy distribution fitting, the distance to the star (140 ± 20 pc), and the use of evolutionary tracks and isochrones, led to the following set of parameters: T{sub eff} = 6550 ± 100 K, log g = 3.75 ± 0.10, L{sub *}/L{sub ☉} = 16.3 ± 4.5, M{sub *}/M{sub ☉} = 2.0 ± 0.3, and an age of 5.0 ± 1.5 Myr. This stellar age provides further constraints to the mass of the possible companion estimated by Biller et al., being between 0.20 and 0.35 M{sub ☉}. Stellar accretion rates obtained from UV Balmer excess modeling and optical photospheric line veiling, and from the correlations with several emission lines spanning from the UV to the near-IR, are consistent with each other. The mean value from all previous tracers is 2 (±1) × 10{sup –7} M{sub ☉} yr{sup –1}, which is within the upper limit gas flow rate from the outer to the inner disk recently provided by Cassasus et al.. This suggests that almost all gas transferred between both components of the disk is not trapped by the possible planet(s) in between but fall onto the central star, although it is discussed how the gap flow rate could be larger than previously suggested. In addition, we provide evidence showing that the stellar accretion rate of HD 142527 has increased by a factor ∼7 on a timescale of 2 to 5 yr.

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DOE OSTI2014

Nonlinear dynamics of accretion disks with stochastic viscosity

We present a nonlinear numerical model for a geometrically thin accretion disk with the addition of stochastic nonlinear fluctuations in the viscous parameter. These numerical realizations attempt to study the stochastic effects on the disk angular momentum transport. We show that this simple model is capable of reproducing several observed phenomenologies of accretion-driven systems. The most notable of these is the observed linear rms-flux relationship in the disk luminosity. This feature is not formally captured by the linearized disk equations used in previous work. A Fourier analysis of the dissipation and mass accretion rates across disk radii show coherence for frequencies below the local viscous frequency. This is consistent with the coherence behavior observed in astrophysical sources such as Cygnus X-1.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Conservative GRMHD simulations of moderately thin, tilted accretion disks

This paper presents our latest numerical simulations of accretion disks that are misaligned with respect to the rotation axis of a Kerr black hole. In this work, we use a new, fully conservative version of the Cosmos++ general relativistic magnetohydrodynamics (GRMHD) code, coupled with an ad hoc cooling function designed to control the thickness of the disk. Together these allow us to simulate the thinnest tilted accretion disks ever using a GRMHD code. In this way, we are able to probe the regime where the dimensionless stress and scale height of the disk become comparable. We present results for both prograde and retrograde cases. The simulated prograde tilted disk shows no sign of Bardeen-Petterson alignment even in the innermost parts of the disk. The simulated retrograde tilted disk, however, does show modest alignment. The implication of these results is that the parameter space associated with Bardeen-Petterson alignment for prograde disks may be rather small, only including very thin disks. Unlike our previous work, we find no evidence for standing shocks in our simulated tilted disks. We ascribe this to the black hole spin, tilt angle, and disk scale height all being small in these simulations. We also add to the growing body of literature pointing out that the turbulence driven by the magnetorotational instability in global simulations of accretion disks is not isotropic. Finally, we provide a comparison between our moderately thin, untilted reference simulation and other numerical simulations of thin disks in the literature.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

A global three-dimensional radiation magneto-hydrodynamic simulation of super-eddington accretion disks

We study super-Eddington accretion flows onto black holes using a global three-dimensional radiation magneto-hydrodynamical simulation. We solve the time-dependent radiative transfer equation for the specific intensities to accurately calculate the angular distribution of the emitted radiation. Turbulence generated by the magneto-rotational instability provides self-consistent angular momentum transfer. The simulation reaches inflow equilibrium with an accretion rate ∼220 L {sub Edd}/c {sup 2} and forms a radiation-driven outflow along the rotation axis. The mechanical energy flux carried by the outflow is ∼20% of the radiative energy flux. The total mass flux lost in the outflow is about 29% of the net accretion rate. The radiative luminosity of this flow is ∼10 L {sub Edd}. This yields a radiative efficiency ∼4.5%, which is comparable to the value in a standard thin disk model. In our simulation, vertical advection of radiation caused by magnetic buoyancy transports energy faster than photon diffusion, allowing a significant fraction of the photons to escape from the surface of the disk before being advected into the black hole. We contrast our results with the lower radiative efficiencies inferred in most models, such as the slim disk model, which neglect vertical advection. Our inferred radiative efficiencies also exceed published results from previous global numerical simulations, which did not attribute a significant role to vertical advection. We briefly discuss the implications for the growth of supermassive black holes in the early universe and describe how these results provided a basis for explaining the spectrum and population statistics of ultraluminous X-ray sources.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

FORMATION OF BINARY MILLISECOND PULSARS BY ACCRETION-INDUCED COLLAPSE OF WHITE DWARFS UNDER WIND-DRIVEN EVOLUTION

Accretion-induced collapse (AIC) of massive white dwarfs (WDs) has been proposed to be an important channel to form binary millisecond pulsars (MSPs). Recent investigations on thermal timescale mass transfer in WD binaries demonstrate that the resultant MSPs are likely to have relatively wide orbit periods (≳ 10 days). Here we calculate the evolution of WD binaries taking into account the excited wind from the companion star induced by X-ray irradiation of the accreting WD, which may drive rapid mass transfer even when the companion star is less massive than the WD. This scenario can naturally explain the formation of the strong-field neutron star in the low-mass X-ray binary 4U 1822–37. After AIC the mass transfer resumes when the companion star refills its Roche lobe, and the neutron star is recycled owing to mass accretion. A large fraction of the binaries will evolve to become binary MSPs with an He WD companion, with the orbital periods distributed between ≳ 0.1 days and ≲ 30 days, while some of them may follow the cataclysmic variable-like evolution toward very short orbits. If we instead assume that the newborn neutron star appears as an MSP and that part of its rotational energy is used to ablate its companion star, the binaries may also evolve to be the redback-like systems.

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