A class of self-gravitating accretion disks

We consider a class of steady-state self-gravitating accretion disks for which efficient cooling mechanisms are assumed to operate so that the disk is self-regulated at a condition of approximate marginal Jeans stability. In an earlier paper, this scenario had been shown to lead naturally, in the absence of a central point mass, to a self-similar solution characterized by a flat rotation curve. In this article we investigate the entire parameter space available for such self-regulated accretion disks and provide two non-trivial extensions of the model. The first extension is that of a bimodal disk, obtained by partially relaxing the self-regulation constraint, so that full matching with an inner "standard" Keplerian accretion disk takes place. The second extension is the construction of self-regulated accretion disks embedded in a diffuse spherical "halo". The analysis is further strengthened by a careful discussion of the vertical structure of the disk, in such a way that the transition from self-gravity dominated to non-gravitating disks is covered uniformly.Comment: To appear in A&

Supermassive black hole formation during the assembly of pre-galactic discs

In this paper we discuss the evolution of gravitationally unstable pre-galactic discs that result from the collapse of haloes at high redshift $z \approx 10$ 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 degrees Kelvin. However, when molecular hydrogen is present cooling can proceed down to a few hundred degrees Kelvin. Analogous to the case of the larger scale proto-galactic 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% of discs resulting from the collapse of haloes with $M\approx 10^7 M_{\odot}$ should host a massive black hole with a mass $M_{\rm BH}\approx 10^5 M_{\odot}$. We confirm our arguments with time-dependent calculations of the evolution of the surface density and of the accretion rate in these primordial discs. 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 $10^9 M_{\odot}$ black holes powering the luminous quasars detected by the Sloan Digital Sky Survey at $z = 6$ for a concordance cosmology. (abridged)Comment: 12 pages, 8 figures, submitted to MNRA

The response of self-graviting protostellar discs to slow reduction in cooling timescale: the fragmentation boundary revisited

A number of previous studies of the fragmentation of self-gravitating protostellar discs have modeled radiative cooling with a cooling timescale (t_{cool}) parameterised as a simple multiple (beta_{cool}) of the local dynamical timescale. Such studies have delineated the `fragmentation boundary' in terms of a critical value of beta_{cool} (beta_{crit}), where the disc fragments if beta_{cool} < beta_{crit}. Such an approach however begs the question of how in reality a disc could ever be assembled with beta_{cool} < beta_{crit}. Here we adopt the more realistic approach of gradually reducing beta_{cool}, as might correspond to changes in thermal regime due to secular changes in the disc density profile. We find that when beta_{cool} is gradually reduced (on a timescale longer than t_{cool}), the disc is stabilised against fragmentation, compared with models in which beta_{cool} is reduced rapidly. We therefore conclude that a disc's ability to remain in a self-regulated, self-gravitating state (without fragmentation) is partly dependent on its thermal history, as well as its current cooling rate. Nevertheless, a slow reduction in t_{cool} appears only to lower the fragmentation boundary by about a factor two in t_{cool} and thus only permits maximum alpha values (parameterising the efficiency of angular momentum transfer in the disc) that are about a factor two higher than determined hitherto. Our results therefore do not undermine the notion of a fundamental upper limit to the heating rate that can be delivered by gravitational instabilities before the disc is subject to fragmentation. An important implication of this work, therefore, is that self-gravitating discs can enter into the regime of fragmentation via secular evolution and it is not necessary to invoke rapid (impulsive) events to trigger fragmentation.Comment: accepted for publication in MNRA

Self-gravitating accretion discs

I review recent progresses in the dynamics and the evolution of self-gravitating accretion discs. Accretion discs are a fundamental component of several astrophysical systems on very diverse scales, and can be found 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 understand 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 protostellar discs might form giant gaseous planets through disc fragmentation.Comment: in press, La Rivista del Nuovo Cimento, 30, 293 (2007

Constraints on the formation mechanism of the planetary mass companion of 2MASS 1207334-393254

In this paper we discuss the nature and the possible formation scenarios of the companion of the brown dwarf 2MASS 1207334-393254. We initially discuss the basic physical properties of this object and conclude that, although from its absolute mass ($5M_{\rm Jup}$), it is a planetary object, in terms of its mass ratio $q$ and of its separation $a$ with respect to the primary brown dwarf, it is consistent with the statistical properties of binaries with higher primary mass. We then explore the possible formation mechanism for this object. We show that the standard planet formation mechanism of core accretion is far too slow to form this object within 10 Myr, the observed age of the system. On the other hand, the alternative mechanism of gravitational instability (proposed both in the context of planet and of binary formation) may, in principle, work and form a system with the observed properties.Comment: 5 pages, MNRAS in pres

Revised analysis of SPIRE observations for 2M1207

We have revised our analysis of the SPIRE observations of 2MASSW J1207334-393254 (2M1207). Recent PACS observations show a bright source located ~25" east of 2M1207. There are issues in terms of the detection/non-detection of the bright source when comparing the Spitzer, WISE, and PACS observations. It is apparently inconsistent, perhaps due to variability or low signal-to-noise of the data. We have looked into the possible misidentification of the target, and have revised the measured SPIRE fluxes and the disc parameters for 2M1207. We have also reviewed which among the various formation mechanisms of this system would still be valid.Comment: Revised SPIRE fluxe

The role of the energy equation in the fragmentation of protostellar discs during stellar encounters

In this paper, we use high-resolution smoothed particle hydrodynamics (SPH) simulations to investigate the response of a marginally stable self-gravitating protostellar disc to a close parabolic encounter with a companion discless star. Our main aim is to test whether close brown dwarfs or massive planets can form out of the fragmentation of such discs. We follow the thermal evolution of the disc by including the effects of heating due to compression and shocks and a simple prescription for cooling and find results that contrast with previous isothermal simulations. In the present case we find that fragmentation is inhibited by the interaction, due to the strong effect of tidal heating, which results in a strong stabilization of the disc. A similar behaviour was also previously observed in other simulations involving discs in binary systems. As in the case of isolated discs, it appears that the condition for fragmentation ultimately depends on the cooling rate.Comment: 9 pages, 10 figures, accepted in MNRA

Attualità e future prospettive terapeutiche

La ricerca di base e clinica sul glaucoma costituisce oggi uno dei principali campi di impegno scientifico del mondo oftalmologico. E questo dipende da svariati fattori, quali la cronicità di una patologia seconda causa di cecità nel mondo, con il conseguente onere sociale che ne deriva; il progressivo invecchiamento anagrafico delle aree occidentali con incremento di prevalenza del glaucoma primario ad angolo aperto (POAG) o chiuso (PACG); i recenti successi terapeutici in campo chirurgico e soprattutto medico, che negli ultimi 25 anni hanno creato un circolo virtuoso in termini di impiego di uomini e di risorse. C’è da aggiungere che man mano che la generazione del “baby boom” post-bellico invecchia, si incrementerà la richiesta di un trattamento che preservi la visione, senza intaccare il godimento di attività sociali e ricreative che è oggi tipico anche dell’età avanzata. I tentativi di ridurre in modo sostanziale il deficit visivo e la cecità da glaucoma necessiteranno di una diagnostica sempre più aggressiva, in modo da rendere coscienti della propria condizione un maggior numero di pazienti. Inoltre è necessario un più attento trattamento di POAG e PACG che possa includere le acquisizioni derivanti dalla ricerca corrente. A causa delle conseguenze della cecità sulla salute e sull’economia, il fardello di POAG e POCG ricade non solo sui pazienti e i loro medici ma anche, come detto, sulla società. Pertanto le strategie di controllo della malattia devono andare oltre il semplice input al medico di base per uno screening sistematico, avvalendosi di più ampie campagne di informazione pubblica che possano sottolineare la necessità di un periodico esame dell’occhio, specialmente tra coloro con storia familiare o altri fattori di rischio per il glaucoma. Le metodiche per la diagnosi precoce e per il followup della malattia sono sempre più raffinate e promettenti per il futuro, come le sezioni precedenti di questo volume hanno evidenziato. Una loro trattazione prospettica renderebbe troppo ampio questo capitolo, che pertanto si incentrerà sulle problematiche future terapeutiche, mediche e chirurgiche

Efficient dust ring formation in misaligned circumbinary discs

Binary systems exert a gravitational torque on misaligned discs orbiting them, causing differential precession which may produce disc warping and tearing. While this is well understood for gas-only discs, misaligned cirumbinary discs of gas and dust have not been thoroughly investigated. We perform SKI simulations of misaligned gas and dust discs around binaries to investigate the different evolution of these two components. We choose two different disc aspect ratios: A thin case for which the gas disc always breaks, and a thick one where a smooth warp develops throughout the disc. For each case, we run simulations of five different dust species with different degrees of coupling with the gas component, varying in Stokes number from. 0.002 (strongly coupled dust) to 1000 (effectively decoupled dust). We report two new phenomena: First, large dust grains in thick discs pile up at the warp location, forming narrow dust rings, due to a difference in precession between the gas and dust components. These pile ups do not form at gas pressure maxima, and hence are different.from conventional dust traps. This effect is most evident for St &lt;^&gt; 10-100. Secondly, thin discs tear and break only in the gas, while dust particles with St &gt; 10 form a dense dust trap due to the steep pressure gradient caused by the break in the gas. We find that dust with St &lt; 0.02 closely follow the gas particles, for both thin and thick discs, with radial drift becoming noticeable only for the largest grains in this range

Failed tidal disruption events and X-ray flares from the Galactic Centre

The process of tidal disruption of stars by a supermassive black hole provides luminous UV and soft X-ray flares with peak luminosities of 481046 erg\u2009s 121 and duration of a few months. As part of a wider exploration of the effects of stellar rotation on the outcome of a TDE, we have performed hydrodynamical simulations of the disruption of a rotating star whose spin axis is opposite to the orbital axis. Such a retrograde rotation makes the star more resilient to tidal disruption, so that, even if its orbit reaches the formal tidal radius, it actually stays intact after the tidal encounter. However, the outer layers of the star are initially stripped away from the core, but then fall back on to the star itself, producing a newly formed accretion disc around the star. We estimate that the accretion rate on to the star would be strongly super-Eddington (for the star) and would result in an X-ray flare with luminosity of the order of 481040 erg\u2009s 121 and duration of a few months. We speculate that such events might be responsible for the known X-ray flares from Sgr A* in the recent past