Hubble Space TelescopeGoddard High Resolution Spectrograph Observations of the Be + sdO BinaryφPersei

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Abstract
Mass transfer during the evolution of intermediate-mass stars in a close binary system can result in a rejuvenated and spun-up secondary star (which may appear as a rapidly rotating Be star) orbiting an unseen, stripped-down, remnant companion. One of the best candidates for such a system is the long-period (127 days) binary Per. Here we present new Hubble Space Telescope Goddard High Resolution Spectrograph spectra of Per in several UV regions that show clearly for the first time the spectral signature of the faint remnant companion. We derive a double-lined solution for the radial velocity curve that yields masses of 9.3 ± 0.3 M☉ and 1.14 ± 0.04 M☉ for the Be star and companion, respectively. A Doppler tomographic reconstruction of the secondary spectrum shows a rich spectrum dominated by sharp Fe IV and Fe V lines, similar to those observed in hot sdO stars. Non-LTE spectrum synthesis indicates that the subdwarf has temperature Teff = 53 ± 3 kK and gravity log g = 4.2 ± 0.1 and that the subdwarf-to-Be star flux ratio is 0.165 ± 0.006 and 0.154 ± 0.009 for the 1374 and 1647 Å regions, respectively. The spectrum of the Be primary appears normal for a very rapidly rotating early B-type star, but we argue that the star is overluminous for its mass (perhaps owing to accretion-induced mixing). Additional sharp lines of Fe IV appear when the companion is in the foreground, and we show that these form in a heated region of the Be star's disk that faces the hot subdwarf.

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