Unraveling the Brown Dwarf Desert: Four New Discoveries and a Unifying, Period-Coded Picture

TL;DR

This study reports four new transiting brown dwarfs from TESS data, with three having long periods over 100 days, expanding understanding of the brown dwarf desert and host star metallicity trends.

Contribution

It introduces four validated brown dwarfs, especially long-period ones, and analyzes their orbital and host star properties to better understand their formation and distribution.

Findings

Long-period brown dwarfs are often found around metal-poor stars.

Eccentricity distribution of brown dwarfs resembles low-mass stellar binaries.

Short-period brown dwarfs tend to orbit metal-rich stars.

Abstract

We present four newly validated transiting brown dwarfs identified through TESS photometry and confirmed with high-precision radial velocity measurements obtained from the FEROS and PLATOSpec spectrographs. Notably, three of these companions exhibit orbital periods exceeding 100 days, thereby expanding the sample of long-period transiting brown dwarfs from two to five systems. The host stars of long-period brown dwarfs show mild subsolar metallicity. These discoveries highlight the expansion of the metal-poor, long-period distribution and help us better understand the brown dwarf desert. In our comparative analysis of eccentricity and metallicity demographics, we utilize catalogues of long-period giant planets, brown dwarfs, and low-mass stellar companions. After accounting for tidal influences, the eccentricity distribution aligns with that of low-mass stellar binaries, presenting a different profile than that observed within the giant planet population. Additionally, the metallicity of the host stars reveals a noteworthy trend: short-period transiting brown dwarfs are predominantly associated with metal-rich stars, whereas long-period brown dwarfs are more often found around metal-poor stars, demonstrating statistical similarities to low-mass stellar hosts. This trend has also been previously observed in studies of hot and cold Jupiters and points to a period-coded mixture of channels. A natural explanation is that most brown dwarfs originate from fragmentation at wider separations, with long-period systems retaining this stellar-like imprint, while only those embedded in massive, long-lived, metal-rich protoplanetary discs are efficiently delivered and stabilised to short orbits.