A twostep gravitational cascade for the fragmentation of selfgravitating discs
Abstract
Selfgravitating discs are believed to play an important role in astrophysics in particular regarding the star and planet formation process. In this context, discs subject to an idealized cooling process, characterized by a cooling timescale β expressed in unit of orbital timescale, have been extensively studied. We take advantage of the Riemann solver and the 3D Godunov scheme implemented in the code Ramses to perform highresolution simulations, complementing previous studies that have used smoothed particle hydrodynamics (SPH) or 2D grid codes. We observe that the critical value of β for which the disc fragments is consistent with most previous results, and is not well converged with resolution. By studying the probability density function of the fluctuations of the column density (ΣPDF), we argue that there is no strict separation between the fragmented and the unfragmented regimes but rather a smooth transition with the probability of apparition of fragments steadily diminishing as the cooling becomes less effective. We find that the high column density part of the ΣPDF follows a simple power law whose slope turns out to be proportional to β and we propose an explanation based on the balance between cooling and heating through gravitational stress. Our explanation suggests that a more efficient cooling requires more heating implying a larger fraction of dense material which, in the absence of characteristic scales, results in a shallower scalefree power law. We propose that the gravitational cascade proceeds in two steps, first the formation of a dense filamentary spiral pattern through a sequence of quasistatic equilibrium triggered by the viscous transport of angular momentum, and second the collapse alongside these filaments that eventually results in the formation of bounded fragments.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 May 2021
 DOI:
 10.1093/mnras/stab738
 arXiv:
 arXiv:2103.05508
 Bibcode:
 2021MNRAS.503.4192B
 Keywords:

 accretion;
 accretion discs;
 gravitation;
 hydrodynamics;
 instabilities;
 methods: numerical;
 protoplanetary discs;
 Astrophysics  Astrophysics of Galaxies;
 Astrophysics  Earth and Planetary Astrophysics;
 Astrophysics  Solar and Stellar Astrophysics
 EPrint:
 17 pages, 17 figures. Accepted for publication in MNRAS