Accretion Rates as a Diagnostic Tool for the Origin of Planetary-mass Companions

Direct imaging surveys have revealed a new population of substellar companions with masses around the deuterium burning limit and orbits often farther than 100 AU. The formation of these planetary-mass companions (PMCs) is not fully understood. While core/pebble accretion and dynamical scattering seem unlikely, disk fragmentation and prestellar core collapse remain to be tested. Simulations have shown that disk fragmentation generally leads to higher mass accretion rates than prestellar core collapse. Therefore, PMCs are expected to have higher accretion rates than free-floating planets/brown dwarfs if disk fragmentation is the dominant formation channel. Alternatively, if both populations have indistinguishable accretion rates, that would suggest a common origin. To test this prediction, we propose to observe the UV continuum excess and H-alpha emission for PMCs and free-floating objects with WFC3/UVIS multi-band imaging. Our targets are of comparable mass and age, and have evidence indicative of disks. We will carry out shock modeling and search for any population-level accretion rate differences between the two samples. We will also investigate the empirical relationship between H-alpha line luminosities and accretion luminosities from the stellar-mass toward the planetary-mass regime, in order to establish whether the mass assembly process for stars, brown dwarfs, and planetary-mass objects are analogous and continuous.