Work

Host-star metallicities and kinematics of directly imaged brown-dwarf companions

Year: 2026

Type: article

Abstract: General notes: (A) Per your request, I have edited to US English spelling and grammar conventions. (B) A&A uses the past tense to describe specific steps used in a study and the present tense to describe general methods as well as findings, including the findings of recent papers (within the past ten or so years). Please make sure my edits are accurate in this respect and kindly make any further necessary changes. (C) Acronyms should be spelled out upon first appearance. All subsequent appearances should be in the acronym form (unless at the beginning of a sentence). *** Brown dwarfs are common as free-floating objects but rare as close companions to Sun-like stars, a disparity known as the ``brown-dwarf desert.'' Host-star metallicity can constrain whether these companions form mainly through metal-sensitive core accretion or through less metal-dependent disk or cloud fragmentation. We extend our homogeneous spectroscopic analysis of directly imaged planet hosts into the brown-dwarf regime and compare their metallicities with those of planet hosts and close-in brown-dwarf hosts. We compiled 54 unique directly imaged brown-dwarf systems selected over an inclusive 13--80 M_ Jup interval and projected separations from about 5 au to several thousand au. Objects near the model-dependent 70--75 M_ Jup hydrogen-burning boundary may, however, be very low-mass stars. For 31 hosts with archival high-resolution spectra, we derived atmospheric parameters and metallicities using Bayesian spectral synthesis. Literature companion masses and projected separations are heterogeneous and often model-dependent, and are used only for demographic context. Galactic velocities were calculated for 46 hosts solely to characterize the youth-biased imaging sample. The host stars have a broadly solar metallicity distribution, with a median Fe/H of $+0.06$ dex and a median absolute deviation of 0.11 dex, and show no strong metal-rich bias. No statistically significant metallicity difference is detected between the directly imaged lower- and higher-mass subsamples at the current sample size. The hosts are kinematically cold, as expected from the youth-biased selection of direct-imaging surveys. The absence of a strong metal-rich bias suggests that classical core accretion does not dominate the wide-orbit brown-dwarf population. Disk instability and cloud fragmentation remain plausible, but the current sample and heterogeneous companion properties do not permit object-by-object discrimination between these channels. The comparisons involving the smallest samples remain exploratory.

Source: Astronomy and Astrophysics

DOI: 10.1051/0004-6361/202661179