Unraveling the Brown Dwarf Desert: Four New Discoveries and a Unifying, Period-Coded Picture
Ján Šubjak, Rafael Brahm, Jozef Lipták, Jan Eberhardt, Marcelo Tala Pinto, Sarah L. Casewell, Thomas Henning, Katharine Hesse, Trifon Trifonov, Andrés Jordán, Felipe I. Rojas, Michaela Vítková, Helem Salinas, Gavin Boyle, Vincent Suc, Luca Antonucci, Krzysztof Bernacki, César Briceño, Karen A. Collins, Jorge Fernández Fernández, Samuel Gill, Jan Janík, Nicholas Law, Andrew W. Mann, James McCormac, Adam Popowicz, Daniel Sebastian, Marek Skarka, Ján Václavík, Leonardo Vanzi, Richard G. West, Francis P. Wilkin, Carl Ziegler
TL;DR
Four transiting brown dwarfs discovered by TESS and confirmed with high-precision RVs extend the long-period BD tail, with three systems having periods above $100$ days and subsolar host metallicities. A population-level comparison of BD eccentricities and host metallicities with long-period giant planets and low-mass stellar companions shows long-period BDs ($P>16$ d) resemble LMS binaries, while short-period BDs favor metal-rich hosts, implying a period-coded mixture of formation channels. Excluding six nearly circular objects in the $10< P le 16$ d range, the BD eccentricity signal is shown to be driven by a small subset, and full CTL tidal-evolution modeling demonstrates that modest initial eccentricities and plausible stellar dissipation can reproduce observed eccentricities within ~10 Gyr. The authors propose a framework where fragmentation dominates wide-separation BD formation, while metal-rich discs enable inward delivery and survival of short-period BDs; expanding the BD census and homogenizing host metallicities are crucial for testing this scenario.
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.
