Can brown dwarfs survive on close orbits around convective stars?

Abstract

Context. The mass range of brown dwarfs extends across the planetary domain to stellar objects. There is a relative paucity of brown dwarfs companions around FGKM-type stars compared to exoplanets for orbital periods of less than a few years, but most of the short-period brown dwarf companions that are fully characterised by transits and radial velocities are found around F-type stars. Aims. 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, the result of magnetic braking, would lead to a rapid orbital decay with the companion being quickly engulfed. Methods. 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. Results. 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 an orbital period less than five 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. Moreover, we find that, for a wide range of values of tidal dissipation efficiency and magnetic braking, it is safe to assume that orbital decay for massive companions can be neglected for orbital periods greater than ten days. Conclusions. For orbital periods greater than ten 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 cannot be an effect of tidal decay alone, but may be the result of a selection effect in the sample and/or the formation mechanism.

NGTS-7Ab: an ultrashort-period brown dwarf transiting a tidally locked and active M dwarf

“…As mentioned previously, one mechanism for migrating orbiting bodies inwards is through the combination of tidal forces and the magnetic wind of the host star (e.g. Damiani & Díaz 2016). These forces act in conjunction to migrate brown dwarfs inwards by transferring angular momentum from the orbit to the spin of the host star, which is then lost via magnetic breaking.…”

“…These interactions have been argued to be particularly efficient for G and K stars (Guillot et al 2014), due to their radiative interiors and moderate magnetic winds. F stars however have a much weaker wind, and the low masses and radii of M stars result in reduced tidal forces (Damiani & Díaz 2016). Both of these factors result in increased migration timescales for F and M stars.…”

“…where h is the orbital angular momentum of the system, α mb = 1.5×10 −14 is the magnetic braking parameter (Dobbs-Dixon et al 2004;Damiani & Díaz 2016), C * is the primary star moment of inertia and Ω is the angular velocity of the star in the synchronised state. For our two scenarios we estimate τ a as 5 and 10 Myr respectively, implying that NGTS-7Ab will not remain in the current state for long and is very close to the end of its lifetime.…”

WASP-128b: a transiting brown dwarf in the dynamical-tide regime

“…The orbital decay of the companion is significantly affected by magnetic braking as long as the total angular momentum of the system can remain above the critical limit (Damiani & Lanza 2015;Damiani & Díaz 2016),…”

“…Most massive substellar companions on close orbits have been found around F-type stars, and very few around G dwarfs, which has been interpreted as being due to rapid engulfment of massive planets and brown dwarfs around G dwarfs due to strong tidal coupling (Bouchy et al 2011;Guillot et al 2014;Damiani & Díaz 2016). Stars generally spin down as they age due to magnetic braking, where stellar winds carry highly ionised material that couples to the magnetic field lines and gets carried away from the star, leading to angular momentum loss.…”