A Quantitative In-Depth Analysis of the Prototype Sdb+bd System SDSS J08205+0008 Revisited in the Gaia Era

Subdwarf B stars are core-helium burning stars located on the extreme horizontal branch. Extensive mass loss on the red giant branch is necessary to form them. It has been proposed that substellar companions could lead to the required mass-loss when they are engulfed in the envelope of the red giant star. J08205+0008 was the first example of a hot subdwarf star with a close, substellar companion candidate to be found. Here we perform an in-depth re-analysis of this important system with much higher quality data allowing additional analysis methods. From the higher resolution spectra obtained with ESO-VLT/XSHOOTER we derive the chemical abundances of the hot subdwarf as well as its rotational velocity. Using the it Gaia parallax and a fit to the spectral energy distribution in the secondary eclipse, tight constraints to the radius of the hot subdwarf are derived. From a long-term photometric campaign we detected a significant period decrease of -3.2(8)· 10^-12 dd^-1. This can be explained by the non-synchronised hot subdwarf star being spun up by tidal interactions forcing it to become synchronised. From the rate of period decrease we could derive the synchronisation timescale to be 4 Myr, much smaller than the lifetime on EHB. By combining all different methods we could constrain the hot subdwarf to a mass of 0.39-0.50 M_⊙ and a radius of R_ sdB=0.194±0.008 R_⊙, and the companion to 0.061-0.071 M_⊙ with a radius of R_ comp=0.092 ± 0.005 R_⊙, below the hydrogen burning limit. We therefore confirm that the companion is most likely a massive brown dwarf.