Very-low-mass stellar and substellar companions to solar-like stars from marvels. III. A short-period brown dwarf candidate around an active g0iv subgiant

We present an eccentric, short-period brown dwarf candidate orbiting the active, slightly evolved subgiant star TYC2087-00255-1, which has effective temperature T _eff = 5903 ± 42K, surface gravity log (g) = 4.07 ± 0.16(cgs), and metallicity [Fe/H] = -0.23 ± 0.07. This candidate was discovered using data from the first two years of the Multi-object APO Radial Velocity Exoplanets Large-area Survey, which is part of the third phase of Sloan Digital Sky Survey. From our 38 radial velocity measurements spread over a two-year time baseline, we derive a Keplerian orbital fit with semi-amplitude K = 3.571 ± 0.041kms^-1, period P = 9.0090 ± 0.0004days, and eccentricity e = 0.226 ± 0.011. Adopting a mass of 1.16 ± 0.11 M_· for the subgiant host star, we infer that the companion has a minimum mass of 40.0 ± 2.5 M _·Jup. Assuming an edge-on orbit, the semimajor axis is 0.090 ± 0.003AU. The host star is photometrically variable at the sim;1% level with a period of sim;13.16 ± 0.01 days, indicating that the host star spin and companion orbit are not synchronized. Through adaptive optics imaging we also found a point source 643 ± 10 mas away from TYC2087-00255-1, which would have a mass of 0.13 M_· if it is physically associated with TYC2087-00255-1 and has the same age. Future proper motion observation should be able to resolve if this tertiary object is physically associated with TYC2087-00255-1 and make TYC2087-00255-1 a triple body system. Core Ca II H and K line emission indicate that the host is chromospherically active, at a level that is consistent with the inferred spin period and measured v _rotsin i, but unusual for a subgiant of this T _eff. This activity could be explained by ongoing tidal spin-up of the host star by the companion.