HST/COS Detection of the Spectrum of the Subdwarf Companion of KOI-81
Abstract
KOI-81 is a totally eclipsing binary discovered by the Kepler mission that consists of a rapidly rotating B-type star and a small, hot companion. The system was forged through large-scale mass transfer that stripped the mass donor of its envelope and spun up the mass gainer star. We present an analysis of UV spectra of KOI-81 that were obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope that reveal for the first time the spectral features of the faint, hot companion. We present a double-lined spectroscopic orbit for the system that yields mass estimates of 2.92 {{M}⊙ } and 0.19 {{M}⊙ } for the B-star and hot subdwarf, respectively. We used a Doppler tomography algorithm to reconstruct the UV spectra of the components, and a comparison of the reconstructed and model spectra yields effective temperatures of 12 and 19-27 kK for the B-star and hot companion, respectively. The B-star is pulsating, and we identified a number of peaks in the Fourier transform of the light curve, including one that may indicate an equatorial rotation period of 11.5 hr. The B-star has an equatorial velocity that is 74% of the critical velocity where centrifugal and gravitational accelerations balance at the equator, and we fit the transit light curve by calculating a rotationally distorted model for the photosphere of the B-star.
Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program #12288.- Publication:
-
The Astrophysical Journal
- Pub Date:
- June 2015
- DOI:
- 10.1088/0004-637X/806/2/155
- arXiv:
- arXiv:1505.00817
- Bibcode:
- 2015ApJ...806..155M
- Keywords:
-
- binaries: spectroscopic;
- stars: evolution;
- stars: individual: KOI-81;
- subdwarfs;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 34 pages, 11 figures, accepted for ApJ