Emission lines in the atmosphere of the irradiated brown dwarf WD0137-349B

Emission lines in the atmosphere of the irradiated brown dwarf WD0137-349B

Authors:
Longstaff et al

Abstract:

We present new optical and near-infrared spectra of WD0137-349; a close white dwarf - brown dwarf non-interacting binary system with a period of ≈114 minutes. We have confirmed the presence of Hα emission and discovered He, Na, Mg, Si, K, Ca, Ti, and Fe emission lines originating from the brown dwarf atmosphere. This is the first brown dwarf atmosphere to have been observed to exhibit metal emission lines as a direct result of intense irradiation. The equivalent widths of many of these lines show a significant difference between the day and night sides of the brown dwarf. This is likely an indication that efficient heat redistribution may not be happening on this object, in agreement with models of hot Jupiter atmospheres. The Hα line strength variation shows a strong phase dependency as does the width. We have simulated the Ca II emission lines using a model that includes the brown dwarf Roche geometry and limb darkening and we estimate the mass ratio of the system to be 0.135±0.004. We also apply a gas-phase equilibrium code using a prescribed DRIFT-PHOENIX model to examine how the chemical composition of the brown dwarf upper atmosphere would change given an outward temperature increase, and discuss the possibility that this would induce a chromosphere above the brown dwarf atmosphere.

Two white dwarfs in ultrashort binaries with detached, eclipsing, likely substellar companions detected by K2

Two white dwarfs in ultrashort binaries with detached, eclipsing, likely substellar companions detected by K2

Authors:

Parsons et al

Abstract:
Using data from the extended Kepler mission in K2 Campaign 10 we identify two eclipsing binaries containing white dwarfs with cool companions that have extremely short orbital periods of only 71.2 min (SDSS J1205−0242, a.k.a. EPIC 201283111) and 72.5 min (SDSS J1231+0041, a.k.a. EPIC 248368963). Despite their short periods, both systems are detached with small, low-mass companions, in one case a brown dwarf, and the other case either a brown dwarf or a low-mass star. We present follow-up photometry and spectroscopy of both binaries, as well as phase-resolved spectroscopy of the brighter system, and use these data to place preliminary estimates on the physical and binary parameters. SDSS J1205−0242 is composed of a 0.39 ± 0.02M⊙ helium-core white dwarf which is totally eclipsed by a 0.049 ± 0.006M⊙ (51 ± 6MJ) brown dwarf companion, while SDSS J1231+0041 is composed of a 0.56 ± 0.07M⊙ white dwarf which is partially eclipsed by a companion of mass ≲ 0.095M⊙. In the case of SDSS J1205−0242 we look at the combined constraints from common-envelope evolution and brown dwarf models; the system is compatible with similar constraints from other post common-envelope binaries given the current parameter uncertainties, but has potential for future refinement.

"Hot" Brown Dwarfs may not be Possible Around Stars Larger Than M Dwarfs

Can brown dwarfs survive on close orbits around convective stars?

Authors:

Damiani et al

Abstract:

Brown dwarfs straddle the mass range transition from planetary to stellar objects. There is a relative paucity of brown dwarfs companions around FGKM stars compared to exoplanets for orbital periods less than a few years, but most of the short-period brown dwarf companions fully characterised by transits and radial velocities are found around F-type stars. We examine the hypothesis that brown dwarf companions could not survive on close orbit around stars with important convective envelopes because the tides and angular momentum loss through magnetic breaking should lead to a rapid orbital decay and quick engulfment of the companion. We use a classical Skumanich-type braking law, and constant time-lag tidal theory to assess the characteristic timescale for orbital decay for the brown dwarf mass range as a function of the host properties. We find that F-type stars may host massive companions for a significantly longer time than G-type stars for a given orbital period, which may explain the paucity of G-type hosts for brown dwarfs with orbital period less than 5 days. On the other hand, we show that the small radius of early M-type stars contributes to orbital decay timescales that are only half those of F-type stars, despite their more efficient tidal dissipation and magnetic braking. For fully convective later type M-dwarfs, orbital decay timescales could be orders of magnitude greater than for F-type stars. For orbital periods greater than 10 days, brown dwarf occurrence should largely be unaffected by tidal decay, whatever the mass of the host. On closer orbital periods, the rapid engulfment of massive companions could explain the lack of G and K-type hosts in the sample of known systems with transiting brown dwarfs. However, the paucity of M-type hosts can not be an effect of tidal decay alone, but may be the result of a selection effect in the sample and/or the formation mechanism.