Hot subdwarf stars in binary systems

The Hot Stars of Old Open Clusters: M67, NGC 188, and NGC 6791

“…(The sdB stars are too faint for a radial velocity study, so their binary status is unknown.) The situation is reminiscent of that of the field sdB stars, which occupy a narrow region in the T ef f -log g diagram, consistent with single star evolution (Saffer et al 1994), but for which there is increasing evidence for a large binary population (Allard et al 1994, Theissen et al 1995, Green et al 1998b). One binary star mechanism consistent with a narrow range of gravity or luminosity is the scenario discussed by Mengel et al (1976, also see Bailyn et al 1992, in which an evolving red giant loses mass to a companion, so that it arrives on the horizontal branch with a small envelope mass.…”

“…Also shown are high-dispersion IUE spectra of the field sdO star Feige 67 (SWP 20488), and the field sdB star PG 0342+026 (SWP 27466), smoothed to the approximate GHRS resolution, and reddened to match the NGC 6791 reddening. From ultraviolet and optical spectrophotometry, Theissen et al (1995) found T ef f = 25,000 K, log g = 5.25 and E(B −V ) = 0.10, for PG 0342+026, while Saffer et al (1994) found T ef f = 26,200 K, log g = 5.63 from fitting the Balmer lines. The best-fit Kurucz model atmosphere (normalized to the V magnitude and reddened by E(B −V ) = 0.17) to the GHRS spectrum of B4 has T ef f = 26,800 K, log g = 5.0, and [Fe/H] = 0.2.…”

“…Spectroscopically, EHB and AGB-manqué stars are expected to appear, respectively, as sdB and sdO stars, and the measured effective temperatures and gravities of the field hot subdwarf stars is consistent with this scenario (Saffer et al 1994). However, the field hot subdwarfs apparently have a large binary fraction (Theissen et al 1995, Green et al 1998b, which instead suggests that binary interactions might be involved in their formation (Bailyn 1995).…”