HUBBLE-SPACE-TELESCOPE GODDARD HIGH-RESOLUTION SPECTROGRAPH OBSERVATIONS OF THE BE-PHI PERSEI(SDO BINARY)

Mass transfer during the evolution of intermediate-mass stars in a clo se binary system can result in a rejuvenated and spun-up secondary sta r (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 phi Per. Here we presen t new Hubble Space Telescope Goddard High Resolution Spectrograph spec tra of phi Per in several UV regions that show clearly for the first t ime the spectral signature of the faint remnant companion. We derive a double-lined solution for the radial velocity curve that yields masse s of 9.3 +/- 0.3 M-circle dot and 1.14 +/- 0.04 M-circle dot for the B e star and companion, respectively. A Doppler tomographic reconstructi on 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-e ff = 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 Angstrom regions, respectively. The spectrum of the Be p rimary appears normal for a very rapidly rotating early B-type star, b ut we argue that the star is overluminous for its mass (perhaps owing to accretion-induced mixing). Additional sharp lines of Fe IV appear w hen 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.