Hubble Space Telescope Goddard High Resolution Spectrograph Observations of the Be + sdO Binary φ Persei

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 φ Per. Here we present new Hubble Space Telescope Goddard High Resolution Spectrograph spectra of φ 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 T_eff = 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.

Based on observations 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 NAS5-26555.