Discovery of a helium-core white dwarf progenitor

Free access article

Issue		A&A Volume 411, Number 3, December I 2003


Page(s)		L477 - L480
Section		Letters
DOI		http://dx.doi.org/10.1051/0004-6361:20031553

A&A 411, L477-L480 (2003)
DOI: 10.1051/0004-6361:20031553

Letter

Discovery of a helium-core white dwarf progenitor

U. Heber, H. Edelmann, T. Lisker and R. Napiwotzki

Dr. Remeis-Sternwarte, Astronomisches Institut der Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
(Received 5 August 2003 / Accepted 5 October 2003 )

Abstract
We discovered that HD 188112, a bright ( V = $\rm 10 \stackrel{m}{.} 2$ ), nearby (80 pc) B-type star, is a unique subluminous B (sdB) star. SdB stars are usually identified with models of core helium burning Extreme Horizontal-Branch (EHB) stars of half a solar mass. A spectral analysis of the hydrogen and helium lines resulted in $T_{\rm eff} = 21\,500 \pm 500$ K, $\log g = 5.66 \pm 0.05$ placing the star below the EHB.

HD 188112 was found to be radial velocity (RV) variable and the RV curve has been measured to be perfectly sinusoidal with a period of 0.606585 days and a semi-amplitude of 188.3 km s ^-1 indicating that it is a close binary system. From the atmospheric parameters and the Hipparcos parallax we conclude that the sdB star is of low mass (0.24 $M_{\odot}$ ). The mass of the sdB is too low to sustain core helium burning and it is now evolving into a helium core white dwarf. A lower limit to the mass of its unseen companion of 0.73 $M_{\odot}$ is derived from the mass function. Because the companion does not contribute to the spectral energy distribution from the UV to the infrared it cannot be a main sequence star but must be a white dwarf (WD), a neutron star (NS) or a black hole. The system would qualify as pre-supernova Ia candidate (sdB+WD) if its total mass is above the Chandrasekhar limit (1.4 $M_{\odot}$ ), or as post-supernova (sdB+NS) if the companion mass is above that limit, requiring the inclination angle to be lower than 51 $^{\circ}$ or 48 $^{\circ}$ , respectively.

Key words: binaries: spectroscopic -- stars: early-type -- stars: fundamental parameters -- stars: individual: HD 188112

Offprint request: U. Heber, heber@sternwarte.uni-erlangen.de

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