A quantitative in-depth analysis of the prototype sdB+BD system SDSS J08205+0008 revisited in the Gaia era. (arXiv:2011.10013v1 [astro-ph.SR])

A quantitative in-depth analysis of the prototype sdB+BD system SDSS J08205+0008 revisited in the Gaia era. (arXiv:2011.10013v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Schaffenroth_V/0/1/0/all/0/1">V. Schaffenroth</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Casewell_S/0/1/0/all/0/1">S. L. Casewell</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Schneider_D/0/1/0/all/0/1">D. Schneider</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kilkenny_D/0/1/0/all/0/1">D. Kilkenny</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Geier_S/0/1/0/all/0/1">S. Geier</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Heber_U/0/1/0/all/0/1">U. Heber</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Irrgang_A/0/1/0/all/0/1">A. Irrgang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Przybilla_N/0/1/0/all/0/1">N. Przybilla</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Marsh_T/0/1/0/all/0/1">T. R. Marsh</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Littlefair_S/0/1/0/all/0/1">S. P. Littlefair</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dhillon_V/0/1/0/all/0/1">V. S. Dhillon</a>

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 $R_{rm sdB}=0.194pm0.008,rm R_odot$, and the companion to
$0.061-0.071rm,M_odot$ 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.

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 $R_{rm sdB}=0.194pm0.008,rm R_odot$, and the companion to
$0.061-0.071rm,M_odot$ 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.

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