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Substellar companions and isolated planetary mass objects from protostellar disc fragmentation

W. K. M. Rice·P. J. Armitage·I. A. Bonnell·M. R. Bate·S. V. Jeffers·S. G. Vine
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Abstract

Self-gravitating protostellar discs are unstable to fragmentation if the gas can cool on a time scale that is short compared to the orbital period. We use a combination of hydrodynamic simulations and N-body orbit integrations to study the long term evolution of a fragmenting disc with an initial mass ratio to the star of M_disc/M_star = 0.1. For a disc which is initially unstable across a range of radii, a combination of collapse and subsequent accretion yields substellar objects with a spectrum of masses extending (for a Solar mass star) up to ~0.01 M_sun. Subsequent gravitational evolution ejects most of the lower mass objects within a few million years, leaving a small number of very massive planets or brown dwarfs in eccentric orbits at moderately small radii. Based on these results, systems such as HD 168443 -- in which the companions are close to or beyond the deuterium burning limit -- appear to be the best candidates to have formed via gravitational instability. If massive substellar companions originate from disc fragmentation, while lower-mass planetary companions originate from core accretion, the metallicity distribution of stars which host massive substellar companions at radii of ~1 au should differ from that of stars with lower mass planetary companions.

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W. K. M. Rice, P. J. Armitage, I. A. Bonnell, M. R. Bate, S. V. Jeffers, S. G. Vine. 2003-10-23. Substellar companions and isolated planetary mass objects from protostellar disc fragmentation. https://doi.org/10.1111/j.1365-2966.2003.07317.x

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