A quantitative in-depth analysis of the prototype sdB+BD system SDSS J08205+0008 revisited in the Gaia era

TL;DR

This paper provides a detailed re-analysis of the sdB+BD system SDSS J08205+0008 using high-quality data, revealing the system's properties and confirming the companion as a likely massive brown dwarf.

Contribution

It offers a comprehensive re-evaluation of the system with new data and methods, refining the parameters and confirming the substellar nature of the companion.

Findings

The hot subdwarf has a mass of 0.39-0.50 M_sun and a radius of 0.194±0.008 R_sun.

The companion's mass is 0.061-0.071 M_sun, below the hydrogen burning limit.

The system's period decrease indicates tidal spin-up and a synchronization timescale of 4 Myr.

Abstract

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 $-3.2(8)\cdot 10^{-12} \,\rm dd^{-1}$. 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 $0.39-0.50\,\rm M_\odot$ and a radius of $R_{\rm sdB}=0.194\pm0.008\,\rm R_\odot$, and the companion to $0.061-0.071\rm\,M_\odot$ with a radius of $R_{\rm comp}=0.092 \pm 0.005\,\rm R_\odot$, below the hydrogen burning limit. We therefore confirm that the companion is most likely a massive brown dwarf.