A Smooth Transition from Giant Planets to Brown Dwarfs from the Radial Occurrence Distribution

TL;DR

This study combines radial velocity and astrometry data to analyze the true mass distribution of giant planets and brown dwarfs, revealing a smooth transition in their occurrence rates and challenging the idea of a sharp formation boundary.

Contribution

It introduces a method to accurately determine true companion masses and maps their occurrence distribution, showing a continuous transition from giant planets to brown dwarfs.

Findings

Brown dwarfs have a lower occurrence rate than Jupiter analogs.

The brown dwarf desert extends to 10 AU.

The distribution suggests overlapping formation mechanisms.

Abstract

Measuring the occurrence rates of celestial objects is a valuable way to study their origins and evolution. Giant planets and brown dwarfs produce large Doppler signatures that are easily detectable by modern instrumentation, and legacy radial velocity (RV) surveys have now achieved full orbital coverage for periods $\lesssim$30 years. However, the Doppler method's sensitivity to companion minimum mass $M_c \sin i$ -- as opposed to true mass $M_c$ -- prevents unambiguous characterization using RVs alone, as purported giant planets may be brown dwarfs or stars on inclined orbits. Here we combined legacy RVs with absolute astrometry to re-fit the orbits of 195 companions from the California Legacy Survey. Nearly 50% (8/18) of the ``brown dwarfs" ($M_c \sin i$=13--80 $M_{Jup}$) we refit had true masses above 80 $M_{Jup}$. We incorporated our orbital posteriors and target sensitivity maps into a Poisson likelihood model to calculate occurrence as a function of true companion mass $M_c$ (0.8--80 $M_{Jup}$) and separation $a$ (0.3--30 AU). The semi-major axis distributions of objects in this range vary smoothly with mass, with Jupiter analogs favoring an abrupt increase in occurrence near 1 AU and brown dwarfs exhibiting a gradual enhancement at wider separations. Marginalized companion occurrence between 1--10 AU decreases smoothly with mass, with brown dwarfs having the lowest occurrence rate: $0.9^{+0.5}_{-0.4}$%. Jupiter analogs are 10 times as common as brown dwarfs per mass interval in this range, demonstrating that the brown dwarf desert extends to 10 AU. The smooth variation in these distributions disfavors a sharp transition mass between ``bottom-up" core accretion and ``top-down" gravitational instability formation mechanisms, and rather suggests that these processes may produce companions in overlapping mass ranges.