Constraints on the Spin Evolution of Young Planetary-Mass Companions

TL;DR

This study measures and compares the rotation rates of young planetary-mass companions and brown dwarfs, revealing similar spin distributions and suggesting rotation is influenced by late-stage accretion processes rather than formation mechanism.

Contribution

It provides the first comparative analysis of rotation rates between planetary-mass companions and brown dwarfs, indicating similar formation or regulation processes.

Findings

Rotation rates are below break-up velocities for both populations.

Rotation rates do not significantly evolve during the first few hundred million years.

Rotation may be regulated by interactions with circumplanetary disks during late accretion.

Abstract

Surveys of young star-forming regions have discovered a growing population of planetary-mass (<13 M_Jup) companions around young stars. There is an ongoing debate as to whether these companions formed like planets (that is, from the circumstellar disk), or if they represent the low-mass tail of the star formation process. In this study we utilize high-resolution spectroscopy to measure rotation rates of three young (2-300 Myr) planetary-mass companions and combine these measurements with published rotation rates for two additional companions to provide a look at the spin distribution of these objects. We compare this distribution to complementary rotation rate measurements for six brown dwarfs with masses <20 M_Jup, and show that these distributions are indistinguishable. This suggests that either that these two populations formed via the same mechanism, or that processes regulating rotation rates are independent of formation mechanism. We find that rotation rates for both populations are well below their break-up velocities and do not evolve significantly during the first few hundred million years after the end of accretion. This suggests that rotation rates are set during late stages of accretion, possibly by interactions with a circumplanetary disk. This result has important implications for our understanding of the processes regulating the angular momentum evolution of young planetary-mass objects, and of the physics of gas accretion and disk coupling in the planetary-mass regime.