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

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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>γ</mml:mi> <mml:mo>=</mml:mo> <mml:mn>2</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>7</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>12</mml:mn> </mml:mrow> </mml:msubsup> <mml:mspace width="0.25em"/> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">Jup</mml:mi> </mml:mrow> </mml:msub> </mml:math> for companions with orbital separations between 1 and 50 au. Companions below and above this threshold tend to orbit stars with higher ([Fe/H] = 0.17 ± 0.12 dex) and lower ([Fe/H] = −0.03 ± 0.10 dex) metallicities, respectively. Previous studies of relatively close-in companions reported evidence of a lower transition mass of ≤10 M 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 Jup to be &lt;1%. We compare our results to estimates of γ gleaned from other observational metrics and discuss implications for planet-formation theory.