2015ApJ...806..155M -
Astrophys. J., 806, 155 (2015/June-3)
HST/COS detection of the spectrum of the subdwarf companion of KOI-81.
MATSON R.A., GIES D.R., GUO Z., QUINN S.N., BUCHHAVE L.A., LATHAM D.W., HOWELL S.B. and ROWE J.F.
Abstract (from CDS):
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 2.92M☉ and 0.192M☉ 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.
Abstract Copyright:
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Journal keyword(s):
binaries: spectroscopic - stars: evolution - stars: individual: KOI-81 - subdwarfs
Simbad objects:
8
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