Tidally Induced Brown Dwarf and Planet Formation in Circumstellar Disks

Abstract

Most stars are born in clusters and the resulting gravitational interactions between cluster members may significantly affect the evolution of circumstellar disks and therefore the formation of planets and brown dwarfs (BDs). Recent findings suggest that tidal perturbations of typical circumstellar disks due to close encounters may inhibit rather than trigger disk fragmentation and so would seem to rule out planet formation by external tidal stimuli. However, the disk models in these calculations were restricted to disk radii of 40 AU and disk masses below 0.1 M . Here, we show that even modest encounters can trigger fragmentation around 100 AU in the sorts of massive (∼0.5 M ), extended ( 100 AU) disks that are observed around young stars. Tidal perturbation alone can do this; no disk-disk collision is required. We also show that very low mass binary systems can form through the interaction of objects in the disk. In our computations, otherwise non-fragmenting massive disks, once perturbed, fragment into several objects between about 0.01 and 0.1 M , i.e., over the whole BD mass range. Typically, these orbit on highly eccentric orbits or are even ejected. While probably not suitable for the formation of Jupiter-or Neptune-type planets, our scenario provides a possible formation mechanism for BDs and very massive planets which, interestingly, leads to a mass distribution consistent with the canonical substellar initial mass function. As a minor outcome, a possible explanation for the origin of misaligned extrasolar planetary systems is discussed.

SOPHIE velocimetry ofKeplertransit candidates

“…Numerical simulations (e.g. Thies et al 2010) have shown that numerous BDs can be formed in the outer regions of massive circumstellar disks. The mass spectrum of objects formed around a 0.7 M star has a maximum around the mass of KOI-205 b (Fig.…”

Using binary statistics in Taurus-Auriga to distinguish between brown dwarf formation processes

“…However, if BDs (or isolated planets) form in the disks of proto-stars through either encounter-triggered perturbations (Thies et al 2010) or gravitational instabilities (Stamatellos & Whitworth 2009;Basu & Vorobyov 2012;Forgan & Rice 2013) and are nudged out or ejected, the resultant free-floating objects will carry with them their own mass reservoir. Using their SPH computations, this has explicitly been shown by Stamatellos & Whitworth (2009) and Thies et al (2015).…”

Stellar orbit evolution in close circumstellar disc encounters

“…This idea has been revisited both with pure N -body approaches (Pfalzner 2003;Pfalzner, Umbreit & Henning 2005;Thies, Kroupa & Theis 2005;Pfalzner & Olczak 2007) and with gas dynamics (Forgan & Rice 2009;Sheppard & Trujillo 2006;Shen et al 2010;Thies et al 2010). All the hydrodynamical studies included self-gravity, although some of the N -body ones used test particles (e.g.…”

“…Pfalzner 2003); on disc fragmentation and formation of substellar mass objects (e.g. Lin et al 1998;Sheppard & Trujillo 2006;Shen et al 2010;Thies et al 2010) and disc truncation (e.g. Forgan & Rice 2009) and, more recently, on the stability of disc-embedded planets (Picogna & Marzari 2014).…”

“…The detailed effects of stellar flybys on single gas discs have also been explored through E-mail: dmunoz@astro.cornell.edu isolated simulations (e.g. Forgan & Rice 2009;Sheppard & Trujillo 2006;Moeckel & Bally 2006;Shen et al 2010;Thies et al 2010). Close passages can truncate discs, which affects planet formation, and also trigger gravitational instabilities in discs (e.g Shen et al 2010;Thies et al 2010).…”