• DOI:10.1088/0004-637X/806/2/155
• Corpus ID: 119111553

HST/COS DETECTION OF THE SPECTRUM OF THE SUBDWARF COMPANION OF KOI-81

@article{Matson2015HSTCOSDO,
  title={HST/COS DETECTION OF THE SPECTRUM OF THE SUBDWARF COMPANION OF KOI-81},
  author={Rachel A. Matson and Douglas R. Gies and Zhao Guo and Samuel N. Quinn and Lars A. Buchhave and David W. Latham and Steve B. Howell and Jason F. Rowe},
  journal={The Astrophysical Journal},
  year={2015},
  volume={806}
}

• R. Matson, D. Gies, +5 authors J. Rowe
• Published 4 May 2015
• Physics, Geology
• The Astrophysical Journal

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.