Greening of the brown-dwarf desert

Abstract

Context. Although more than 2000 brown dwarfs have been detected to date, mainly from direct imaging, their characterisation is difficult due to their faintness and model-dependent results. In the case of transiting brown dwarfs, however, it is possible to make direct high-precision observations. Aims. Our aim is to investigate the nature and formation of brown dwarfs by adding a new well-characterised object, in terms of its mass, radius and bulk density, to the currently small sample of less than 20 transiting brown dwarfs. Methods. One brown dwarf candidate was found by the KESPRINT consortium when searching for exoplanets in the K2 space mission Campaign 16 field. We combined the K2 photometric data with a series of multicolour photometric observations, imaging, and radial velocity measurements to rule out false positive scenarios and to determine the fundamental properties of the system. Results. We report the discovery and characterisation of a transiting brown dwarf in a 5.17-day eccentric orbit around the slightly evolved F7 V star EPIC 212036875. We find a stellar mass of 1.15 ± 0.08 M⊙, a stellar radius of 1.41 ± 0.05 R⊙, and an age of 5.1 ± 0.9 Gyr. The mass and radius of the companion brown dwarf are 51 ± 2 MJ and 0.83 ± 0.03 RJ, respectively, corresponding to a mean density of 108−13+15 g cm−3. Conclusions. EPIC 212036875 b is a rare object that resides in the brown-dwarf desert. In the mass-density diagram for planets, brown dwarfs, and stars, we find that all giant planets and brown dwarfs follow the same trend from ~0.3 MJ to the turn-over to hydrogen burning stars at ~ 73 MJ. EPIC 212036875 b falls close to the theoretical model for mature H/He dominated objects in this diagram as determined by interior structure models. We argue that EPIC 212036875 b formed via gravitational disc instabilities in the outer part of the disc, followed by a quick migration. Orbital tidal circularisation may have started early in its history for a brief period when the brown dwarf’s radius was larger. The lack of spin–orbit synchronisation points to a weak stellar dissipation parameter (Q⋆′ ≳ 108), which implies a circularisation timescale of ≳23 Gyr, or suggests an interaction between the magnetic and tidal forces of the star and the brown dwarf.

MuSCAT2 multicolour validation of TESS candidates: an ultra-short-period substellar object around an M dwarf

“…Baraffe et al 2003), there is a non-negligible probability that our target is high-mass planet or a brown dwarf with a mass in the interval 13M Jup < M < 75M Jup . This would make TOI 263 an even more exciting system representing a unique and extreme case in the (disputed) "brown dwarf desert" (Marcy & Butler 2000, but see also Carmichael et al 2019 andPersson et al 2019), with the period of the transiting body falling between the 10.6 h orbital period of CVSO 30b (van Eyken et al 2012, labelled in Fig. 8) and the 16.2 h orbital period of the recently discovered NGTS-7Ab (Jackman et al 2019, labelled in Fig.…”

“…Article number, page 11 of 12 Schneider et al 2011Schneider et al , accessed 4.5.2019) and brown dwarfs (collected from Carmichael et al 2019;Persson et al 2019;Jackman et al 2019) to date with periods between 0.3 and 200 days and radii between 0 and 2 R Jup . TOI 263.01 is marked with a black-edged white star with 68% and 99% central posterior intervals for its true radius marked with dark and light orange shading, respectively.…”

TOI-503: The First Known Brown-dwarf Am-star Binary from the TESS Mission*

“…from association with a stellar cluster). EPIC 212036875b does have an age estimated with PARAM 1.3 of 5.1 ± 0.9 Gyr from Persson et al (2019). The only other known BD around an A-type star is HATS-70b (Zhou et al 2019) and this very low-mass (12.9 M J ) BD actually appears to be inflated.…”

“…For the transiting BD population, this gap appears at a mass centered around 42.5 M J (Ma & Ge 2014a). Different studies suggest suppressing the distinction between BDs and the coolest M stars given their similarities (Whitworth 2018), while others suggest that BDs and giant planets form a continuum based on their mass-density relation (Hatzes & Rauer 2015;Persson et al 2019), which, in turn, implies that the range of giant planets spans 0.3-60 M J or 0.3-73 M J , respectively. Given this variety of interpretations of what separates giant planets, BDs, and low-mass stars, each new, well-characterized BD system, especially the ones that reside in the driest part of the brown dwarf desert, will be important to understanding this population as a whole.…”

“…Based on the age of 180 Myr that we derive from stellar isochrones and the circular orbital solution for the companion BD, we argue that this particular BD formed in-situ. This work is the result of a collaboration between the KESPRINT consortium (e.g., Hjorth et al 2019;Korth et al 2019;Livingston et al 2019;Palle et al 2019;Gandolfi et al 2019;Persson et al 2019), PARAS-PRL India (Chakraborty et al 2014), and the Harvard-Smithsonian Center for Astrophysics. We describe the observations in Section 2, the data analysis in Section 3 and provide a final discussion in Section 4.…”