ABSTRACT
During the late stages of stellar evolution in massive stars (C fusion and later), the fusion luminosity in the core of the star exceeds the star's Eddington luminosity. This can drive ...vigorous convective motions which in turn excite internal gravity waves. The local wave energy flux excited by convection is itself well above Eddington during the last few years in the life of the star. We suggest that an interesting fraction of the energy in gravity waves can, in some cases, convert into sound waves as the gravity waves propagate (tunnel) towards the stellar surface. The subsequent dissipation of the sound waves can unbind up to several M⊙ of the stellar envelope. This wave-driven mass loss can explain the existence of extremely large stellar mass-loss rates just prior to core collapse, which are inferred via circumstellar interaction in some core-collapse supernovae (e.g. SNe 2006gy and PTF 09uj, and even Type IIn supernovae more generally). An outstanding question is understanding what stellar parameters (mass, rotation, metallicity and age) are the most susceptible to wave-driven mass loss. This depends on the precise internal structure of massive stars and the power spectrum of internal gravity waves excited by stellar convection.
ABSTRACT
Accreting black holes (BHs) launch relativistic collimated jets, across many decades in luminosity and mass, suggesting the jet launching mechanism is universal, robust, and scale-free. ...Theoretical models and general relativistic magnetohydrodynamic (GRMHD) simulations indicate that the key jet-making ingredient is large-scale poloidal magnetic flux. However, its origin is uncertain, and it is unknown if it can be generated in situ or dragged inward from the ambient medium. Here, we use the GPU-accelerated GRMHD code h-amr to study global 3D BH accretion at unusually high resolutions more typical of local shearing box simulations. We demonstrate that turbulence in a radially extended accretion disc can generate large-scale poloidal magnetic flux in situ, even when starting from a purely toroidal magnetic field. The flux accumulates around the BH till it becomes dynamically important, leads to a magnetically arrested disc (MAD), and launches relativistic jets that are more powerful than the accretion flow. The jet power exceeds that of previous GRMHD toroidal field simulations by a factor of 10 000. The jets do not show significant kink or pinch instabilities, accelerate to γ ∼ 10 over three decades in distance, and follow a collimation profile similar to the observed M87 jet.
ABSTRACT We show that for supergiants, net angular momentum is not a necessary condition for forming accretion discs during core collapse. Even absent net rotation, convective motions in the outer ...parts of supergiants generate mean horizontal flows at a given radius with velocities of ${\sim } 1 \, {\rm km \, s}^{-1}$; the direction of the mean flow will vary as a function of height through the convection zone. We confirm these analytic estimates using Cartesian Boussinesq convection simulations. These mean horizontal flows lead to a random angular momentum in supergiant convection zones that exceeds that of the last stable circular orbit of a black hole by a factor of ∼10. As a result, failed explosions of supergiants – in which the accretion shock on to the neutron star does not revive, leading to black hole formation – may often produce accretion discs that can power day–week (blue supergiants) or week–year (yellow and red supergiants) non-thermal and thermal transients through winds and jets. These transients will be especially time variable because the angular momentum of the accreting material will vary substantially in time. Observed sources such as Swift J1644+57, iPTF14hls, and SN 2018cow, as well as energetic Type II supernovae (OGLE-2014-SN-073), may be produced by this mechanism.
ABSTRACT
We study the flow structure in 3D magnetohydrodynamic (MHD) simulations of accretion on to Sagittarius A* via the magnetized winds of the orbiting Wolf–Rayet stars. These simulations cover ...over 3 orders of magnitude in radius to reach ≈300 gravitational radii, with only one poorly constrained parameter (the magnetic field in the stellar winds). Even for winds with relatively weak magnetic fields (e.g. plasma β ∼ 106), flux freezing/compression in the inflowing gas amplifies the field to β ∼ few well before it reaches the event horizon. Overall, the dynamics, accretion rate, and spherically averaged flow profiles (e.g. density, velocity) in our MHD simulations are remarkably similar to analogous hydrodynamic simulations. We attribute this to the broad distribution of angular momentum provided by the stellar winds, which sources accretion even absent much angular momentum transport. We find that the magneto-rotational instability is not important because of (i) strong magnetic fields that are amplified by flux freezing/compression, and (ii) the rapid inflow/outflow times of the gas and inefficient radiative cooling preclude circularization. The primary effect of magnetic fields is that they drive a polar outflow that is absent in hydrodynamics. The dynamical state of the accretion flow found in our simulations is unlike the rotationally supported tori used as initial conditions in horizon scale simulations, which could have implications for models being used to interpret Event Horizon Telescope and GRAVITY observations of Sgr A*.
Simple assumptions made regarding electron thermodynamics often limit the extent to which general relativistic magnetohydrodynamic (GRMHD) simulations can be applied to observations of low-luminosity ...accreting black holes. We present, implement, and test a model that self-consistently evolves an entropy equation for the electrons and takes into account the effects of spatially varying electron heating and relativistic anisotropic thermal conduction along magnetic field lines. We neglect the backreaction of electron pressure on the dynamics of the accretion flow. Our model is appropriate for systems accreting at ...10... of the Eddington accretion rate, so radiative cooling by electrons can be neglected. It can be extended to higher accretion rates in the future by including electron cooling and proton-electron Coulomb collisions. We present a suite of tests showing that our method recovers the correct solution for electron heating under a range of circumstances, including strong shocks and driven turbulence. Our initial applications to axisymmetric simulations of accreting black holes show that (1) physically motivated electron heating rates that depend on the local magnetic field strength yield electron temperature distributions significantly different from the constant electron-to-proton temperature ratios assumed in previous work, with higher electron temperatures concentrated in the coronal region between the disc and the jet; (2) electron thermal conduction significantly modifies the electron temperature in the inner regions of black hole accretion flows if the effective electron mean free path is larger than the local scaleheight of the disc (at least for the initial conditions and magnetic field configurations we study). The methods developed in this work are important for producing more realistic predictions for the emission from accreting black holes such as Sagittarius A* and M87; these applications will be explored in future work. (ProQuest: ... denotes formulae/symbols omitted.)
Abstract
We calculate the radiative properties of Sagittarius A* – spectral energy distribution, variability and radio-infrared images – using the first 3D, physically motivated black hole accretion ...models that directly evolve the electron thermodynamics in general relativistic MHD simulations. These models reproduce the coupled disc-jet structure for the emission favoured by previous phenomenological analytic and numerical works. More specifically, we find that the low frequency radio emission is dominated by emission from a polar outflow while the emission above 100 GHz is dominated by the inner region of the accretion disc. The latter produces time variable near-infrared (NIR) and X-ray emission, with frequent flaring events (including IR flares without corresponding X-ray flares and IR flares with weak X-ray flares). The photon ring is clearly visible at 230 GHz and 2 μm, which is encouraging for future horizon-scale observations. We also show that anisotropic electron thermal conduction along magnetic field lines has a negligible effect on the radiative properties of our model. We conclude by noting limitations of our current generation of first-principles models, particularly that the outflow is closer to adiabatic than isothermal and thus underpredicts the low frequency radio emission.
We calculate the flux of internal gravity waves (IGWs) generated by turbulent convection in stars. We solve for the IGW eigenfunctions analytically near the radiative-convective interface in a local, ...Boussinesq and Cartesian domain. We consider both discontinuous and smooth transitions between the radiative and convective regions and derive Green's functions to solve for the IGWs in the radiative region. We find that if the radiative-convective transition is smooth, the IGW flux depends on the exact form of the buoyancy frequency near the interface. IGW excitation is most efficient for very smooth interfaces, which gives an upper bound on the IGW flux of ∼F
conv(d/H), where F
conv is the flux carried by the convective motions, d is the width of the transition region and H is the pressure scale height. This can be much larger than the standard result in the literature for a discontinuous radiative-convective transition, which gives a wave flux
, where
is the convective Mach number. However, in the smooth transition case, the most efficiently excited perturbations will break in the radiative zone. The flux of IGWs which do not break and are able to propagate in the radiative region is at most
, larger than the discontinuous transition result by
. The transition region in the Sun is smooth for the energy-bearing waves; as a result, we predict that the IGW flux is a few to five times larger than previous estimates. We discuss the implications of our results for several astrophysical applications, including IGW-driven mass loss and the detectability of convectively excited IGWs in main-sequence stars.
Swift 1644+57: the longest gamma‐ray burst? Quataert, E.; Kasen, D.
Monthly notices of the Royal Astronomical Society. Letters,
January 2012, 2012-01-01, 20120101, Letnik:
419, Številka:
1
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
Swift recently discovered an unusual gamma‐ray and X‐ray transient (Swift 1644+57) that was initially identified as a long‐duration gamma‐ray burst (GRB). However, the ∼10 keV X‐ray emission ...has persisted for over approximately a month with a luminosity comparable to its peak value. The astrometric coincidence of the source with the centre of its host galaxy, together with other considerations, motivated the interpretation that Swift 1644+57 was produced by an outburst from a ∼106–107 M⊙ black hole at the centre of the galaxy. Here we consider the alternate possibility that Swift 1644+57 is indeed a long‐duration GRB, albeit a particularly long one! We discuss the general properties of very long‐duration, low‐power GRB‐like transients associated with the core‐collapse of a massive star. Both neutron star (magnetar) spin‐down and black hole accretion can power such events. The requirements for producing low‐power, very long duration GRBs by magnetar spin‐down are similar to those for powering extremely luminous supernovae by magnetar spin‐down, suggesting a possible connection between these two unusual types of transients. Alternatively, Swift 1644+57 could be associated with the faintest core‐collapse explosions: the collapse of a rotating red supergiant in a nominally failed supernova can power accretion on to a solar‐mass black hole for up to ∼100 d; the jet produced by black hole accretion inevitably unbinds the outer envelope of the progenitor, leading to a weak ∼1049 erg explosion. In both neutron‐star and black hole models, a jet can burrow through the host star in a few days, with a kinetic luminosity ∼1045–1046 erg s−1, sufficient to power the observed emission of Swift 1644+57.
We study the distribution of cold dark matter (CDM) in cosmological simulations from the FIRE (Feedback In Realistic Environments) project, for M
* ∼ 104–11 M⊙ galaxies in M
h ∼ 109–12 M⊙ haloes. ...FIRE incorporates explicit stellar feedback in the multiphase interstellar medium, with energetics from stellar population models. We find that stellar feedback, without ‘fine-tuned’ parameters, greatly alleviates small-scale problems in CDM. Feedback causes bursts of star formation and outflows, altering the DM distribution. As a result, the inner slope of the DM halo profile (α) shows a strong mass dependence: profiles are shallow at M
h ∼ 1010–1011 M⊙ and steepen at higher/lower masses. The resulting core sizes and slopes are consistent with observations. This is broadly consistent with previous work using simpler feedback schemes, but we find steeper mass dependence of α, and relatively late growth of cores. Because the star formation efficiency M
*/M
h is strongly halo mass dependent, a rapid change in α occurs around M
h ∼ 1010 M⊙ (M
* ∼ 106–107 M⊙), as sufficient feedback energy becomes available to perturb the DM. Large cores are not established during the period of rapid growth of haloes because of ongoing DM mass accumulation. Instead, cores require several bursts of star formation after the rapid build-up has completed. Stellar feedback dramatically reduces circular velocities in the inner kpc of massive dwarfs; this could be sufficient to explain the ‘Too Big To Fail’ problem without invoking non-standard DM. Finally, feedback and baryonic contraction in Milky Way-mass haloes produce DM profiles slightly shallower than the Navarro–Frenk–White profile, consistent with the normalization of the observed Tully–Fisher relation.
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