The binary fraction of extreme horizontal branch stars

By P. Maxted, U. Heber, Thomas R. Marsh, R. S. U. O. Southampton, Keele University, Astronomisches Institut der Universitat Erlangen-Nurnberg. 2001

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We have used precise radial velocity measurements of subdwarf-B stars from the Palomar–Green catalogue to look for binary extreme horizontal branch (EHB) stars. We have determined the effective temperature, surface gravity and surface helium abundance for 20 of the targets from new or existing blue spectra and have compiled published values for these quantities for all but one other. We identify 36 EHB stars in our sample and find that at least 21 of these stars are binaries. All but one or two of these are new identifications. The minimum binary fraction for EHB stars implied by our survey is 60±8 per cent. Our survey is sensitive to binaries with orbital periods P≲10 d. For reasonable assumptions concerning the period distribution and the mass ratio distribution of the binaries, we find that the mean detection efficiency of our survey over this range of orbital periods is 87 per cent. Allowing for this estimated detection efficiency, the fraction of EHB stars that are short-period binaries (0.03 d≲P≲10 d) is 69±9 per cent. The value is not strongly dependent on the period distribution below P≈10 d or the mean companion mass for these short-period binaries. The orbital separation of the stars in these binaries is much less than the size of the red giant from which the EHB star has formed. This is strong evidence that binary star evolution is fundamental to the formation of the majority of EHB stars. If there are also binary EHB stars, the orbital periods of which are ≳10 d, the fraction of EHB stars for which evolution was affected by the presence of a companion may be much higher, e.g. if one third of EHB stars are binaries with orbital periods 10 d≲P≲100 d, then our observations are consistent with all EHB stars being formed through some type of binary star evolution. We find that five of the other stars we observed are likely to be post-EHB stars, one of which is also a binary.

Subfields: Astronomy and Astrophysics, Education, Instrumentation

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In Astronomy and Astrophysics

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5. A. Milone
6. L. Bedin
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8. F. D’Antona

Citations in the corpus

• Hot Subdwarf Stars (2009, unknown): “While binary evolution scenarios for the formation of SDs have been developed early on (Mengel et al., 1976; Webbink, 1984), observational evidence for a large fraction of close binaries among SDs emerged only by the turn of the century (Sa ff er et al., 1998; Maxted et al., 2001), see Sect.”
• Flash Mixing on the White Dwarf Cooling Curve: Understanding Hot Horizontal Branch Anomalies in NGC 2808 (2001, background): “Also, the binary fraction in the Ðeld subdwarf population is unusually high (e.g., Maxted et al. 2001), which suggests that binarism might play a role in the formation of the Ðeld subdwarfs.”
• US 708 – an unbound hyper-velocity subluminous O star (2005, background): “(Maxted et al. 2001) found 2/3 of their sdB sample in be such binaries, whereas Napiwotzki et al. (2004) found a somewhat lower fraction of 40%.”
• The origin of subdwarf B stars – II (2003, methodology): “…1988; Soberman et al. 1997), even in a revised version taking non-conservative RLOF into account (Han et al. 2001), as such a qcrit would make a first phase of stable RLOF very unlikely and would not produce WD binaries with the parameters required to explain the sample of Maxted et al. (2001).”
• A Triple Main Sequence in the Globular Cluster NGC 2808 (2007, background): “After all, it has been demonstrated (Maxted et al. 2001) that a large fraction of hot field sdB stars formed in binaries that went through a masstransfer phase.”
• The origin of subdwarf B stars – I. The formation channels (2002, background): “The short ticks in these panels along the orbital period axis show the periods of sdB binaries with known orbital periods (Maxted et al. 2001).”
• A Driving Mechanism for the Newly Discovered Long-Period Pulsating Subdwarf B Stars (2003, methodology): “As a comparison, the independent analysis of Maxted et al. (2001) gives estimates of Teff ¼ 22; 800 K and log g ¼ 5:27 for PG 1627+017, while that of MoralesRueda et al. (2003) suggests Teff ¼ 21; 600 K and log g ¼ 5:12 for the same star.”
• A Binary Model for the UV-upturn of Elliptical Galaxies (2006, background): “…in binaries (Ferguson, Green & Liebert 1984; Allard et al. 1994; Thejll, Ulla & MacDonald 1995; Ulla & Thejll 1998; Aznar Cuadrado & Jeffery 2001; Maxted et al. 2001; Williams et al. 2001; Reed & Stiening 2004), and orbital parameters have been determined for a significant sample (Jeffery &…”
• The second and third parameters of the horizontal branch in globular clusters (2010, background): “…successful cases, they might explain some groups of stars with anomalous colours on the HB (e.g., most field O-B subdwarfs are binaries, see e.g., Maxted et al. 2001; Napiwotzki et al. 2004; Han et al. 2003; however most of these in GCs seem to be single stars, see e.g., Moni Bidin et al. 2006).”
• Dawes Review 6: The Impact of Companions on Stellar Evolution (2016, background): “Most are in binary systems (Jeffery & Pollacco, 1998; Maxted et al., 2001), some of which are eccentric, even though theory suggests they should be circular (Vos et al., 2012).”