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

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 longperiod (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.

Spectral and spatial imaging of the Be+sdO binaryϕPersei

“…We began making circular fits with the period derived by Božić et al (1995), P = 126.6731±0.0071 d, and we derived an orbital semiamplitude of K a = 11.4 ± 1.0 km s −1 . This agrees within the uncertainties with the value reported by Gies et al (1998), K a = 10.0 ± 0.8 km s −1 .…”

“…They classified the primary star as B0.5e. The companion was finally confirmed by Gies et al (1998) thanks to high-resolution UV spectroscopy with the Hubble Space Telescope (HST). The subdwarf-to-Be flux ratio is 0.165±0.006 (resp.…”

“…In this case, the companion would lose most of its envelope and would appear as a hot, stripped-down He-star remnant. The Be binary star ϕ Persei represents the first detection of a Be star with a hot, faint companion (Poeckert 1981;Thaller et al 1995;Gies et al 1998), and it is one of three such systems known at present (Peters et al 2013). Clearly it is important to study such Be binaries to determine the physical properties of the stars at the conclusion of their transformative interaction.…”

Properties and nature of Be stars

“…If the secondary would be a normal late M dwarf corresponding to its mass, it would be fainter in the visual region by more than 10 mag and the only chance to search for it would be in the far IR region, where, however, the IR excess from the Be envelope can complicate the detection. However -if it were a hot subdwarf, similar to the one found for another Be binary ϕ Per by Gies et al (1998) -it might be observable in the optical region since the absolute visual magnitude of BU Tau is fainter for some 2 mag than for the ϕ Per B0.5e primary. Finally, a cool Rochelobe filling secondary seems improbable since it would probably produce binary eclipses.…”

On the role of duplicity in the Be phenomenon

“…Already Plavec (1976) pointed out that if all Be stars were binaries with Roche-lobe filling secondaries, one should observe more eclipsing binaries among them than what is actually observed. While Harmanec (1987) slightly weakened this objection, there is a stronger one: detailed studies of several known Be binaries (ϕ Per = HD 10516: Poeckert 1981, Gies et al 1998; V839 Her = 4 Her: Koubský et al 1997, for instance) clearly demonstrated that the secondaries in those binaries are not Roche-lobe filling objects but very small stars. The same is also true of binaries composed of a Be star and a compact, X-ray companion.…”