THE ASTROPHYSICAL JOURNAL, 493 : 440È450, 1998 January 20 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A. ( HUBBL E SPACE TEL ESCOPE GODDARD HIGH RESOLUTION SPECTROGRAPH OBSERVATIONS OF THE Be ] sdO BINARY / PERSEI1 DOUGLAS R. WILLIAM G. ELIZABETH C. GIES, BAGNUOLO,JR., FERRARA, ANTHONY B. AND MICHELLE L. KAYE, THALLER Center for High Angular Resolution Astronomy and Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30303 ; gies=chara.gsu.edu, bagnuolo=chara.gsu.edu, ferrara=chara.gsu.edu, kaye=chara.gsu.edu, thaller=chara.gsu.edu LAURA R. PENNY Department of Physics, University of Montreal, C.P. 6128, Succ. Centre Ville, Montreal, PQ H3C 3J7, Canada ; penny=astro.umontreal.ca AND GERALDINE J. PETERS Space Sciences Center, University of Southern California, University Park, Los Angeles, CA 90089-1341 ; gjpeters=mucen.usc.edu Received 1997 April 23 ; accepted 1997 September 2 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 Ðrst 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 and 1.14 ^ 0.04 for the Be star and companion, respectively. A M _ M _ 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 kK and gravity log g \ 4.2 ^ 0.1 and that the T eff \ 53 ^ 3 subdwarfÈtoÈBe star Ñux ratio is 0.165 ^ 0.006 and 0.154 ^ 0.009 for the 1374 and 1647 regions, A  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 starÏs disk that faces the hot subdwarf. Subject headings : stars : binaries : spectroscopic È stars : emission-line, Be È stars : evolution È stars : individual (/ Persei, HD 10516) È stars : subdwarfs 1. INTRODUCTION Mass transfer in close binary systems is a potential means to spin up the mass gainer, and a series of investigators have suggested that the rapidly rotating Be stars may result from the mass transfer process & Harmanec (Kr— iz— 1975 ; & Kr—iz— & van den Heuvel Harmanec 1976 ; Rappaport der Linden et al. et al. 1982 ; van 1987 ; Waters 1989 ; Pols Zwart According to these models, the 1991 ; Portegies 1995). Be star should have an evolved companion, either a helium star, a white dwarf, or a neutron star. The term ““ helium star ÏÏ in this context refers to a stripped-down, bare stellar core ; the actual surface abundance will depend on how much, if any, of the envelope was retained after rapid mass transfer. A signiÐcant number of Be stars could be formed in this way ; estimates for the fraction of Be stars with evolved companions range from 50% et al. to 5%È20% (Pols 1991) Bever & Vanbeveren However, despite the large (Van 1997). estimated number of Be binaries, only Be plus neutron star 1 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. (Be/X-ray) systems have been found den Heuvel & (van Rappaport Systems with helium star or white dwarf 1987). companions have yet to be detected et al. (Meurs 1992), although the presence of such hot companions has been advocated in the cases of HR 2142 Cote, & Pols (Waters, and / Per 1991) (Poeckert 1981). presented the Ðrst evidence of a helium Poeckert (1981) star companion in the bright Be binary / Persei (HR 496, HD 10516). Based on the antiphase velocity curve of the He II 4686 emission line, Poeckert argued that emission A  originates in hot gas surrounding the companion. The high temperature required to ionize helium led Poeckert to suggest that the companion must be a very hot star, poss- ibly the stripped-down remnant from prior mass transfer. Poeckert used the velocity curves to determine the masses, and (Throughout this paper, M p \ 21 M _ M s \ 3.4 M _ . we will refer to the Be star as the ““ primary ÏÏ and to the hot companion as the ““ secondary ÏÏ based on their current masses.) Poeckert estimated that the companion has an e†ective temperature of 50,000 K, and, because of this high temperature, the companion contributes more to the com- bined Ñux at shorter wavelengths (a magnitude brighter in the UV and dominant in the EUV region). Subsequently, et al. and et al. observed an anti- Gies (1993) Boz— ic (1995) 440