Tidally induced brown dwarf and planet formation in circumstellar discs

TL;DR

This study demonstrates that tidal interactions in massive, extended circumstellar discs can induce fragmentation leading to brown dwarf and massive planet formation, challenging previous assumptions about tidal effects inhibiting such processes.

Contribution

It reveals that even modest tidal encounters can trigger disc fragmentation in large, massive discs, providing a new formation pathway for brown dwarfs and massive planets.

Findings

Tidal perturbations can induce fragmentation at around 100 AU in massive discs.

Fragmented objects range from 0.01 to 0.1 solar masses, covering the brown dwarf range.

Formation of low-mass binary systems through disc interactions.

Abstract

Most stars are born in clusters and the resulting gravitational interactions between cluster members may significantly affect the evolution of circumstellar discs and therefore the formation of planets and brown dwarfs. Recent findings suggest that tidal perturbations of typical circumstellar discs due to close encounters may inhibit rather than trigger disc fragmentation and so would seem to rule out planet formation by external tidal stimuli. However, the disc models in these calculations were restricted to disc radii of 40 AU and disc masses below 0.1 M_sun. Here we show that even modest encounters can trigger fragmentation around 100 AU in the sorts of massive (~0.5 M_sun), extended (>=100 AU) discs that are observed around young stars. Tidal perturbation alone can do this, no disc-disc collision is required. We also show that very-low-mass binary systems can form through the interaction of objects in the disc. In our computations, otherwise non-fragmenting massive discs, once perturbed, fragment into several objects between about 0.01 and 0.1 M_sun, i.e., over the whole brown dwarf 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 brown dwarfs and very massive planets which, interestingly, leads to a mass distribution consistent with the canonical substellar IMF. As a minor outcome, a possible explanation for the origin of misaligned extrasolar planetary systems is discussed.