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 starsA brief history of helium-rich subdwarf B stars PG the He-sdB prototypePG the He-sdB prototype JL87 a not so helium-rich He-sdBJL87 a not so helium-rich He-sdB LSIV a pulsating He-sdBLSIV a pulsating He-sdB

Introduction Subdwarf B stars form the dominant population of faint blue blue stars (m B ~ 16) in our galaxy and giant elliptical galaxiesSubdwarf B stars form the dominant population of faint blue blue stars (m B ~ 16) in our galaxy and giant elliptical galaxies They are 0.5 M Sun core helium-burning starsThey are 0.5 M Sun core helium-burning stars Progenitors of White DwarfsProgenitors of White Dwarfs Evolution has been the subject of much debate although binary evolution is though to play an important roleEvolution 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 linessdB 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) starsA small subset (~ 70) of hot subdwarf stars includes the helium-rich subdwarf B (or He-sdB) stars

H-R diagram Dreizler 2000

“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 KEarly 1980’s some authors believed that helium-rich and helium-poor subdwarf stars were separated by a temperature boundary at K PG survey reports a small fraction of subdwarf stars (classified as sdOD) showing strong neutral Helium (HeI) linesPG 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 debateTheir 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 starsFrom optical and IUE spectra of He-sdB it is clear that there are two distinct subclasses of He-sdB stars ›Carbon-rich, eg. PG ›Carbon-poor, eg. PG

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

Optical spectrum of PG

More spectra

T eff - log g diagram Position of He-sdB stars on the log g - T eff diagram with other subluminous stars

Evolution It was realised early on that helium-rich subdwarfs do not evolve from helium-poor sdB stars (Groth et al. 1985)It was realised early on that helium-rich subdwarfs do not evolve from helium-poor sdB stars (Groth et al. 1985) sdB have n He < 0.01 (Heber 1986)sdB have n He < 0.01 (Heber 1986) sdB evolution takes ~10 8 yr (Caloi 1989)sdB evolution takes ~10 8 yr (Caloi 1989) Gravitational settling takes ~10 5 yr (Wesemael et al. 1982)Gravitational settling takes ~10 5 yr (Wesemael et al. 1982) Hence objects evolving from sdB stars should have depleted helium abundanceHence objects evolving from sdB stars should have depleted helium abundance

Existing evolution models Single star evolutionSingle 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

Evolutionary models Binary merger modelBinary 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

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 starsSingle 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 mergerCarbon 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 PG is the He-sdB proto-type hence the obvious targetPG is the He-sdB proto-type hence the obvious target Bright (V~12 mag), hence possible to do high-resolution spectroscopyBright (V~12 mag), hence possible to do high-resolution spectroscopy Observed in 2003 with the WHT in order to measure abundancesObserved in 2003 with the WHT in order to measure abundances Unexpected discovery!Unexpected discovery!

PG the He-sdB prototype Optical spectrum + model fit of PG (SFIT2)

Previous analyses T eff (K)loggReference Heber et al. (1988) optical / UV spectra Ahmad & Jeffery (2003) low resolution optical spectra [5.0]Ahmad & Jeffery (2004) energy distribution Lanz et al. (2004) far ultraviolet spectra

flash-mixing model He+He WD merger

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

PG – hydrogen deficient stars

New orbital solution

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

JL87 - a not so helium-rich He-sdB JL87 - discovered by Jaidee & Lynga (1974) as a possible “violet source”JL87 - discovered by Jaidee & Lynga (1974) as a possible “violet source” Classified a He-sdB by Schulz et al. (1991)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)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 spectroscopyV ~ 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 2005Echelle 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 obtainedModerate resolution high S/N RGO spectra also obtained

Chemical makeup of JL87

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

LSIV pulsating He-sdB Pulsating star allow us to probe the interiors hence test stellar evolution theoriesPulsating star allow us to probe the interiors hence test stellar evolution theories Pulsation in sdB stars first discovered by Kilkenny et al. 1996Pulsation in sdB stars first discovered by Kilkenny et al Two main types - short periods ~ 200s and long periods ~ 45 minTwo main types - short periods ~ 200s and long periods ~ 45 min Jeffery & Saio 1996 suggest a systematic study of variability in He- sdBJeffery & Saio 1996 suggest a systematic study of variability in He- sdB Systematic search for pulsation carried out for 18 stars at SAAO in However no conclusive evidence found for short period pulsationsSystematic search for pulsation carried out for 18 stars at SAAO in However no conclusive evidence found for short period pulsations 3 suspected pulsators were monitored in 2004 May at the SAAO 1.0m3 suspected pulsators were monitored in 2004 May at the SAAO 1.0m LSIV not a suspect but more like a target of opportunityLSIV not a suspect but more like a target of opportunity

Light Curve and Periodogram

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