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Helium-Rich Subdwarf B Stars

Helium-Rich Subdwarf B Stars. Amir Ahmad Armagh Observatory. C.S. Jeffery and collaborators. 20 September 2007. Tübingen. Outline. A brief history of helium-rich subdwarf B stars PG1544+488 the He-sdB prototype JL87 a not so helium-rich He-sdB LSIV-14 116 a pulsating He-sdB.

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Helium-Rich Subdwarf B Stars

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  1. Helium-Rich Subdwarf B Stars Amir Ahmad Armagh Observatory C.S. Jeffery and collaborators 20 September 2007 Tübingen

  2. Outline • A brief history of helium-rich subdwarf B stars • PG1544+488 the He-sdB prototype • JL87 a not so helium-rich He-sdB • LSIV-14 116 a pulsating He-sdB

  3. Introduction • Subdwarf B stars form the dominant population of faint blue blue stars (mB ~ 16) in our galaxy and giant elliptical galaxies • They are 0.5 MSun core helium-burning stars • Progenitors of White Dwarfs • Evolution has been the subject of much debate although binary evolution is though to play an important role • sdB stars have spectra dominated by Hydrogen Balmer lines • A small subset (~ 70) of hot subdwarf stars includes the helium-rich subdwarf B (or He-sdB) stars

  4. H-R diagram Dreizler 2000

  5. “Cool” helium-rich subdwarf stars • Early 1980’s some authors believed that helium-rich and helium-poor subdwarf stars were separated by a temperature boundary at 40000 K • PG survey reports a small fraction of subdwarf stars (classified as sdOD) showing strong neutral Helium (HeI) lines • Their evolution has since been the subject of much debate • From optical and IUE spectra of He-sdB it is clear that there are two distinct subclasses of He-sdB stars • Carbon-rich, eg. PG1544+488 • Carbon-poor, eg. PG0914-037

  6. He-sdB nomenclature • Spectral classification criteria • strong HeI lines plus weak HeII • no detectable Balmer lines • Helium-rich subdwarf B stars a.k.a. • sdOD - Green et al. 1986 • He-sdB • Moehler et al. 1990 • Beers et al. 1992 • Kilkenny et al. 1997 • sdB4 - Drilling 1996 • sdB:He4 - Jeffery et al. 1997

  7. Optical spectrum of PG1544+488

  8. More spectra

  9. Teff - log g diagram Position of He-sdB stars on the log g - Teff diagram with other subluminous stars

  10. Evolution • It was realised early on that helium-rich subdwarfs do not evolve from helium-poor sdB stars (Groth et al. 1985) • sdB have nHe< 0.01 (Heber 1986) • sdB evolution takes ~108 yr (Caloi 1989) • Gravitational settling takes ~105 yr (Wesemael et al. 1982) • Hence objects evolving from sdB stars should have depleted helium abundance

  11. Existing evolution models • Single star evolution • Lanz et al. (2004) have argued that stars evolving with high mass loss on the red giant branch undergo a late helium core flash on the white dwarf cooling track leading to convective “flash mixing” of the envelope which then forms a helium and carbon rich hot subdwarf Fig. from Brown et al. 2001

  12. Evolutionary models • Binary merger model • Iben & Tutukov (1985, 1987) suggested that merger of two degenerate white dwarf can produce hot subdwarf with depleted hydrogen atmosphere • Saio & Jeffery (2000, 2002) have more recently modelled WD mergers to explain the origin of EHe stars

  13. Chemical abundances of He-sdB stars • Single star (hot flasher) evolution model and white dwarf merger models both can predict enriched carbon in the atmospheres of He-sdB stars • Carbon poor He-sdB stars must be products of He+He WD merge while carbon-rich He-sdB stars can be products of a CO+He WD merger • PG1544+488 is the He-sdB proto-type hence the obvious target • Bright (V~12 mag), hence possible to do high-resolution spectroscopy • Observed in 2003 with the WHT in order to measure abundances • Unexpected discovery!

  14. PG1544+488 - the He-sdB prototype Optical spectrum + model fit of PG 1544+488 (SFIT2)

  15. Previous analyses Teff (K) logg Reference 31 000 5.1 Heber et al. (1988) optical / UV spectra 34 000 5.1 Ahmad & Jeffery (2003) low resolution optical spectra 32 100 [5.0] Ahmad & Jeffery (2004) energy distribution 36 000 6.0 Lanz et al. (2004) far ultraviolet spectra

  16. He+He WD merger flash-mixing model

  17. Spectroscopic observations April 2003 William Herschel Telescope ISIS dual beam spectrograph

  18. PG1544+488 – hydrogen deficient stars

  19. New orbital solution

  20. PG1544+488 in a nutshell • PG1544+488 is a binary with a period of ~ 1/2 day • It consists two low mass hot helium subdwarf stars of similar mass • It is an excellent system for studying close binary evolution • It opens the possibility of a third mechanism involving binary evolution for the formation of He-sdB stars • Is PG1544+488 a rare freak? We know another such system with two He-sdO - HE 0301-3039 (Lisker et al. 2004)

  21. JL87 - a not so helium-rich He-sdB • JL87 - discovered by Jaidee & Lynga (1974) as a possible “violet source” • Classified a He-sdB by Schulz et al. (1991) • Subsequently called a “B type He-rich” star by Kilkenny et al. (1995) and classified B2He by Magee et al. (1998) • V ~ 12 mag hence possible to do high resolution spectroscopy • Echelle spectra of the star first obtained in 1996 with UCLES on the AAT subsequently followed up in 1999, 2002 and 2005 • Moderate resolution high S/N RGO spectra also obtained

  22. Chemical makeup of JL87

  23. What is JL87? • Chemically peculiar early-type B star • CNO abundances and low vsini (unless inclination is close to zero) rules out JL87 being a main sequence star • Surface gravity lower than typical sdB stars and very helium-rich • It is therefore a He-sdB star. Also does not appear to be a binary • Overall metalicity [Fe/H] = -0.3 except C, N and O • Atmospheric physical parameters and chemical parameters can be explained by the late flash model however the mechanism for mass loss is not known

  24. LSIV-14 116 - pulsating He-sdB • Pulsating star allow us to probe the interiors hence test stellar evolution theories • Pulsation in sdB stars first discovered by Kilkenny et al. 1996 • Two main types - short periods ~ 200s and long periods ~ 45 min • Jeffery & Saio 1996 suggest a systematic study of variability in He-sdB • Systematic search for pulsation carried out for 18 stars at SAAO in 2003. However no conclusive evidence found for short period pulsations • 3 suspected pulsators were monitored in 2004 May at the SAAO 1.0m • LSIV-14 116 not a suspect but more like a target of opportunity

  25. Light Curve and Periodogram

  26. To conclude • He-sdB stars are a group of very in-homogenous group of stars • Physical parameters indicate they are very distinct from normal subdwarf B stars • Evolution still under debate • A lot more has to be learnt about these hydrogen deficient stars…

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