New Substellar Discoveries from Kepler and K2: Is There a Brown Dwarf Desert?

Abstract

We present the discoveries of a brown dwarf and a low mass star from the Kepler and K2 missions. The newly discovered brown dwarf is EPIC 212036875b and the low mass star is KOI-607b. EPIC 212036875b has a mass of M b = 52.3 ± 1.9 M J , a radius of R b = 0.874 ± 0.017 R J , and orbits its host star in P = 5.169885 ± 0.000027 days. Its host star is a late F-type star with M = 1.288 ± 0.065 M , R = 1.498 ± 0.025 R , and T eff = 6238 ± 60K. KOI-607b has a mass of M b = 95.1 ± 3.4 M J , a radius of R b = 1.089 ± 0.089 R J , and an orbital period of P = 5.89399148 ± 0.00000060 days. The primary star in the KOI-607 system is a G dwarf with M = 0.993 ± 0.052 M , R = 0.915 ± 0.031 R , and T eff = 5418±87K. We also revisit a brown dwarf, CWW 89Ab, that was previously published by Nowak et al. (2017) (under the designation EPIC 219388192b). CWW 89Ab is one of two known transiting brown dwarfs associated with a star cluster, which illustrates the need for more brown dwarfs with accurate masses and radii and reliable age determinations to test theoretical models. We find that the newly discovered brown dwarf, EPIC 212036875b, falls in the middle of the so-called "brown dwarf desert", indicating that EPIC 212036875b is either a particularly rare object, or the brown dwarf desert may not be so dry after all.

Greening of the brown-dwarf desert

“…Thanks to the evolutionary state of the host star, this is one of the few transiting brown dwarfs for which a relatively precise age can be estimated. Our results are in agreement with Carmichael et al (2019) who recently reported an independent discovery and characterisation of EPIC 212036875 b.…”

“…These values are longer than the system age, and hence the BD's orbit will not be currently tidally evolving. We note that the pre-print of Carmichael et al (2019) gives a slightly longer circularisation time of 47 Gyr. The difference is largely due to their considering only the tide raised on the brown dwarf.…”

“…The solid black line shows the theoretical relationship for H/He dominated giant objects with Z = 0.02, age = 5 Gyr, without irradiation (Baraffe et al 2003(Baraffe et al , 2008, and the dotted black line the same model including irradiation from a solar-type star at 0.045 AU (Baraffe et al 2008). Pont et al 2005Pont et al , 2006Demory et al 2009;Tal-Or et al 2013;Zhou et al 2014;Díaz et al 2014;Chaturvedi et al 2016 and references in Table 1; Gillen et al 2017;von Boetticher et al 2017;Shporer et al 2017;Chaturvedi et al 2018 and references in Table 4; Carmichael et al 2019) up to 450 M J (0.43 M ) mostly from ground-based discoveries. We only include objects with a precision in mass and density better than 20% (in total 253 GPs and BDs, and 43 low-mass stars).…”

“…If EPIC 212036875 had a typical radius of an F7 V star instead of being slightly evolved (with about 8% larger radius), the BD would be grazing. Carmichael et al (2019) used the TRES spectrograph at the 1.5 m Tillinghast telescope at Mt. Hopkins, Arizona, with a spectral resolution R = 44 000 covering 390-910 nm to measure 14 RVs with S /N ≈ 22-45 of EPIC 212036875.…”

“…EPIC 212036875 b was found in the K2 Campaign 16, and follow-up observations subsequently revealed that the object was the 18th transiting BD detected to date. We note that shortly before submitting this article, Carmichael et al (2019) publicly announced their discovery and RV observations of EPIC 212036875 b. We describe the K2 photometry in Sect.…”