Massive discs around low-mass stars

Abstract

We use a suite of SPH simulations to investigate the susceptibility of protoplanetary discs to the effects of self-gravity as a function of star-disc properties. We also include passive irradiation from the host star using different models for the stellar luminosities. The critical disc-to-star mass ratio for axisymmetry (for which we produce criteria) increases significantly for low-mass stars. This could have important consequences for increasing the potential mass reservoir in a proto Trappist-1 system, since even the efficient Ormel et al. (2017) formation model will be influenced by processes like external photoevaporation, which can rapidly and dramatically deplete the dust reservoir. The aforementioned scaling of the critical M d /M * for axisymmetry occurs in part because the Toomre Q parameter has a linear dependence on surface density (which promotes instability) and only an M 1/2 * dependence on shear (which reduces instability), but also occurs because, for a given M d /M * , the thermal evolution depends on the host star mass. The early phase stellar irradiation of the disc (for which the luminosity is much higher than at the zero age main sequence, particularly at low stellar masses) can also play a key role in significantly reducing the role of self-gravity, meaning that even Solar mass stars could support axisymmetric discs a factor two higher in mass than usually considered possible. We apply our criteria to the DSHARP discs with spirals, finding that self-gravity can explain the observed spirals so long as the discs are optically thick to the host star irradiation.

The observational impact of dust trapping in self-gravitating discs

“…Common assumption is that GI requires q 0.5, therefore rendering these discs too low mass to generate prominent self-gravitating structure. However it may be possible for discs to display self-gravitating spirals for much lower mass ratios than previously thought, with the critical mass ratio having a strong dependence on the host star mass and disc opacity (Veronesi et al 2019;Cadman et al 2020;Haworth et al 2020). We therefore should caution against discarding GI as a plausible mechanism based off this simple mass criterion alone.…”

Disk Evolution Study Through Imaging of Nearby Young Stars (DESTINYS): A close low-mass companion to ET Cha

“…Most notably, Vorobyov (2013) performed simulations of disk gravitational fragmentation and found that they were unable to produce brown dwarf companions at small orbital separations. Furthermore, Kratter et al (2010), Offner et al (2010), andHaworth et al (2020) found that disk fragmentation is less likely around M-dwarf primary stars. We suggest, thus, that some circumstantial evidence exists to support the formation of ET Cha B via core fragmentation in the proto-stellar cloud.…”

Predicting the Kinematic Evidence of Gravitational Instability

“…If spirals are predominately produced by giant planets, then the detection rate of such spirals can directly inform us about the occurrence rate and properties of giant planets (Hall et al 2017;Forgan et al 2018a;Dong et al 2018a). If, on the other hand, most spirals are caused by GI, their existence and morphology can be used to infer fundamental disc properties, such as disc mass (Dong et al 2015a;Haworth et al 2020;Cadman et al 2020), and therefore constrain planetary mass budgets and formation timescales (Nayakshin et al 2020) on a comprehensive scale.…”