1,721,290 research outputs found
Classical disc physics
No full textI review the basic physical processes that determine the evolution of accretion discs. I first introduce the main properties of discs observed around young stars across the mass spectrum. I then turn to the analysis of the fundamental disc equations, highlighting several subtleties, in some cases rarely discussed in textbooks. I then discuss some classic accretion disc solutions, both steady state and time dependent. I emphasise the description of the outbursting FU Orionis objects, the class of protostellar objects best suited to investigating the accretion process. I discuss in some detail the possible physical mechanisms responsible for transport in accretion discs, with particular emphasis on gravitational instabilities, which are rarely discussed in this context
Self-gravitating accretion discs
No full textI review recent progress in the dynamics and the evolution of selfgravitating
accretion discs. Accretion discs are a fundamental component of several
astrophysical systems on very diverse systems, and can be found in external galaxies
around supermassive black holes in Active Galactic Nuclei (AGN), and also in our
Galaxy around stellar mass compact objects and around young stars. Notwithstanding
the specific differences arising from such diversity in physical extent, all these
systems share a common feature where a central object is fed from the accretion
disc, due to the effect of turbulence and disc instabilities, which are able to remove
the angular momentum from the gas and allow its accretion. In recent years, it
has become increasingly apparent that the gravitational field produced by the disc
itself (the disc’s self-gravity) is an important ingredient in the models, especially
in the context of protostellar discs and of AGN discs. Indeed, it appears that in
many cases (and especially in the colder outer parts of the disc) the development of
gravitational instabilities can be one of the main agents in the redistribution of angular
momentum. In some cases, the instability can be strong enough to lead to the
formation of gravitationally bound clumps within the disc, and thus to determine
the disc fragmentation. As a result, progress in our understanding of the dynamics
of self-gravitating discs is essential to identify the processes that lead to the feeding
of both young stars and of supermassive black holes in AGN. At the same time,
understanding the fragmentation conditions is important to determine under which
conditions AGN discs would fragment and form stars and whether, through disc
fragmentation, protostellar discs might form giant gaseous planets
Formation and evolution of massive black hole seeds at high redshift
No full textI describe a simple analytical model to predict the amount of baryonic matter
available in the center of dark matter halos at high redshift, to fuel the early formation and
growth of the seeds of supermassive black holes. In this model black hole seed formation
occurs in mini-halos with Tvir~104K before the Universe was significantly enriched by
metals. If H2 formation is inhibited, we find that such halos would naturally form central
mass concentrations of the order of 105M that we associate with the seed of supermassive
black holes. Our model produces low-redshift black hole densities in good agreement with
Soltan-type arguments, and predicts a flattening of the black hole density at redshift larger
than z~6. We also predict a relative dearth of SMBH in dwarf galaxies at z = 0
Evolution of supermassive black hole binaries in gaseous environments
No full textIn this contribution, I discuss some aspects of the dynamical evolution of supermassive black hole binaries and their accretion discs. Firstly, I discuss the issue of alignment of the spins of the two binary component, which has important implications for the shape of the gravitational wave emitted at merger and for the possibility of a strong recoil of the remnant black hole. Even under the favourable assumption that mass flow through the gap is not inhibited by tidal torque, we demonstrate that differential accretion onto the two components of the systems results in a very different spin evolution of the two black holes. Secondly, I revisit the issue of how much mass can flow within the cavity carved in the disc by an equal mass binary. Recent simulations have shown that the tidal torque of the binary is generally not sufficient to prevent accretion onto the binary component. Here, I demonstrate that such results are heavily dependent on the disc thickness. While for H/R ∼ 0.1 (the value adopted in most simulations to date), we reproduce the previous results, we show that as H/R is decreased to ∼ 0.01, mass flow through the gap is essentially shut off almost completely. Thirdly, I show numerical simulations of the process of gas squeezing during the merger proper, demonstrating that most of the disc mass is accreted producing a super-Eddington flare
The Role of Gravitational Instabilities in the Feeding of Supermassive Black Holes
Full text linkI review the recent progresses that have been obtained, especially through the use of high-resolution numerical simulations, on the dynamics of self-gravitating accretion discs. A coherent picture is emerging, where the disc dynamics is controlled by a small number of parameters that determine whether the disc is stable or unstable, whether the instability saturates in a self-regulated state or runs away into fragmentation, and whether the dynamics is local or global. I then apply these concepts to the case of AGN discs, discussing the implications of such evolution on the feeding of supermassive black holes. Nonfragmenting, self-gravitating discs appear to play a fundamental role in the process of formation of massive black hole seeds at high redshift (∼ 10–15) through direct gas collapse. On the other hand, the different cooling properties of the interstellar gas at low redshifts determine a radically different behaviour for the outskirts of the accretion discs feeding typical AGNs. Here the situation is much less clear from a theoretical point of view, and while several observational clues point to the important role of massive discs at a distance of roughly a parsec from their central black hole, their dynamics is still under debate
Two mechanisms for dust gap opening in protoplanetary discs
Full text linkWe acknowledge an ARC Future Fellowship and Discovery Project. GD and G. Lodato acknowledge funding via PRINMIUR prot. 2010LY5N2T. G. Laibe is funded by ERC FP7 grant ECOGAL.We identify two distinct physical mechanisms for dust gap opening by embedded planets in protoplanetary discs based on the symmetry of the drag-induced motion around the planet: (I) a mechanism where low-mass planets, that do not disturb the gas, open gaps in dust by tidal torques assisted by drag in the inner disc, but resisted by drag in the outer disc and (II) the usual, drag-assisted, mechanism where higher mass planets create pressure maxima in the gas disc, which the drag torque then acts to evacuate further in the dust. The first mechanism produces gaps in dust but not gas, while the second produces partial or total gas gaps which are deeper in the dust phase. Dust gaps do not necessarily indicate gas gaps.Peer reviewe
Challenges in the modeling of tidal disruption events lightcurves
Full text linkIn this contribution, I review the recent developments on the modeling of the lightcurve of tidal disruption events. Our understanding has evolved significantly from the earlier seminal results that imply a simple power-law decay of the bolometric light curve as t−5/3. We now know that the details of the rise to the peak of the lightcurve is determined mainly by the internal structure of the disrupted star. We also have improved models for the disc thermal emission, showing that in this case the decline of the luminosity with time should be much flatter than the standard t−5/3 law, especially in optical and UV wavelengths, while the X-ray lightcurve is generally best suited to track the bolometric one. Finally, we are just starting to explore the interesting general relativistic effects that might arise for such events, for which the tidal radius lies very close to the black hole event horizon
Supermassive black hole formation during the assembly of pre-galactic discs
No full textIn this paper, we discuss the evolution of gravitationally unstable pre-galactic discs that result from the collapse of haloes at high redshift z approximate to 10 or so, which have not yet been enriched by metals. In cases where molecular hydrogen formation is suppressed, the discs are maintained at a temperature of a few thousand Kelvin. However, when molecular hydrogen is present, cooling can proceed down to a few hundred Kelvin. Analogous to the case of the larger-scale protogalactic discs, we assume that the evolution of these discs is mainly driven by angular momentum redistribution induced by the development of gravitational instabilities in the disc. We also properly take into account the possibility of disc fragmentation. We thus show that this simple model naturally predicts the formation of supermassive black holes in the nuclei of such discs and provides a robust determination of their mass distribution as a function of halo properties. We estimate that roughly 5 per cent of discs resulting from the collapse of haloes with M approximate to 10(7) M-. should host a massive black hole with a mass M-BH approximate to 10(5) M-.. We confirm our arguments with time-dependent calculations of the evolution of the surface density and of the accretion rate in these primordial discs. The luminosity of the outer, colder disc is expected to be very low (in the range of a few thousand L-.), while the formation of the black hole is expected to produce a burst with a luminosity of a few times 10(9) L-.. This mechanism offers an efficient way to form seed black holes at high redshift. The predicted masses for our black hole seeds enable the comfortable assembly of 10(9)-M-. black holes powering the luminous quasars detected by the Sloan Digital Sky Survey at z = 6 for a concordance cosmology
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