The Transition from Giant Planets to Brown Dwarfs beyond 1 au from the Stellar Metallicity Distribution

TL;DR

This study identifies a transition in host star metallicity at around 27 Jupiter masses for companions between 1-50 au, indicating different formation mechanisms for giant planets and brown dwarfs.

Contribution

It provides the first evidence of a metallicity transition at a higher mass (~27 M_Jup) for companions at wider orbital separations, challenging previous lower-mass transition estimates.

Findings

Companions below 27 M_Jup orbit metal-rich stars.

Companions above 27 M_Jup orbit stars with near-solar or lower metallicity.

The transition mass is significantly higher than previous estimates of ≤10 M_Jup.

Abstract

Giant planets and brown dwarfs are thought to form via a combination of pathways, including bottom-up mechanisms in which gas is accreted onto a solid core and top-down mechanisms in which gas collapses directly into a gravitationally-bound object. One can distinguish the prevalence of these mechanisms using host star metallicities. Bottom-up formation thrives in metal-rich environments, whereas top-down formation is weakly dependent on ambient metal content. Using a hierarchical Bayesian model and the results of the California Legacy Survey (CLS), a low-bias and homogeneously analyzed radial velocity survey, we find evidence for a transition in the stellar metallicity distribution at a companion mass of $\gamma = 27_{-8}^{+12} \, M_{\rm Jup}$ for companions with orbital separations between $1-50$ au. Companions below and above this threshold tend to orbit stars with higher ($\rm{[Fe/H]} = 0.17 \pm 0.12$ dex) and lower ($\rm{[Fe/H]} = -0.03 \pm 0.10$ dex) metallicities, respectively. Previous studies of relatively close-in companions reported evidence of a lower transition mass of $\leq 10 \, {\rm M_{\rm Jup}}$. When applied to the CLS sample, our model predicts the probability of a transition in the stellar metallicity distribution at or below $10 \, { M_{\rm Jup}}$ to be $< 1 \%$. We compare our results to estimates of $\gamma$ gleaned from other observational metrics and discuss implications for planet formation theory.