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Subluminous O stars Origin and evolutionary links Uli Heber Hydrogen-Deficient Stars, Tübingen 20.9.2007 Outline Early results Atmospheric parameters Evolutionary scenarios - Close binary evolution (RLOF, CEE & WD mergers) vs - Delayed core helium flashers - (non-core helium-burning stars) Kinematics Summary & Outlook sdO vs. sdB stars sdO stars: - H-deficient - Hot > 40kK - He-sdOs: No hydrogen sdO sdB sdB stars: - helium-deficient - „cool“: 20-40kK Subluminous O and B stars Greenstein & Sargent (1974) sdB stars: He-deficiency from diffusion Metal abundances HST/STIS UV spectra - Enrichment of heavy elements (>100 times) CPD-64 461 except Fe 207 207PB/208PB - Radiative levitation Fe =solar O´Toole & Heber 2006 Pb Post-EHB vs post-AGB evolution sdB = Extended Horizontal Branch stars: - He-burning core & inert H-envelope (<0.01 Msun) - How to loose the envelope? - sdO stars=post-EHB? Post-AGB Objects: -rare -linked to RCrB/EHe stars sdB Convective transformation (Wesemael et al. 1982) Groth et al. (1985): Convection occurs in He-rich atmospheres only sdO sdO Convection He/H=1 sdB Hot subdwarfs from UVX Surveys LTE- spectroscopic analyses of sdB stars: - Palomar Green survey: Saffer et al. 1994, Maxted et al. 2000 - Hamburg Quasar Survey: Edelmann et al. 2003 - ESO-Supernova Progenitor Survey (SPY): Lisker et al. 2005 atmospheric parameters for >200 sdB NLTE spectroscopic analyses of sdO-stars: - SPY (Ströer et al. 2007) - Sloan Digital Sky Survey (Hirsch et al. 2007) atmospheric parameters for 130 sdO Fits of UVES-spectra (SPY): sdO high resolution spectra, UVES@VLT, sdO TMAP NLTE models, H&He only He sdO Fits of SDSS-spectra: sdO sdO He sdO - C&N strong: diamonds - C strong: triangle - N strong - no C or N: open solar solar SPY: C & N lines - All He-rich sdOs have C and/or N -None of the He-poor have C/N Carbon III/IV SPY: n(C)=0.13% (Hirsch et al. 2007) vrot sin i=0 km/s Carbon III/IV SPY: n(C)=0.25% (Hirsch et al. 2007) vrot sin i=20 km/s SPY He-rich He-poor solar The canonical picture Smooth evolutionary time scales: - He-poor scattered in diagram progeny of sdB stars - Clumping of He-rich sdOs can not be explained He-ZAMS SPY&SDSS: sdB, sdO & He-sdO sdO stars: He-sdO: clumping at - Teff = 45000K - log g = 5.8 He-sdO clump sdB SPY-sds: without error bars SPY&SDSS: sdB, sdO & He-sdO Post EHB He-ZAMS EHB SubHe-ZAMS Hot He flashers Core flash Delayed He core flash Core flash Canonical evolution Sweigart, 1987 Delayed helium shell flash He sdO Very late helium core flash Could explain He-sdOs below the Helium ZAMS He sdO He/C sds in binaries SPY: fraction of close binaries: radial velocity variables with P<10d sdBs; :40% (Napiwotzki et al.,2005) sdOs: 4% RVV (from SPY) mostly single-lined: RV curve: mass function M vis (sin i )3 K 3P f(M ) 2 G ( M vis M invis )2 Minimum mass of companion Napiwotzki et al. 2007 Period distribution Nature of companions: white dwarf or low mass m.s. stars Morales-Rueda (2006) MS unknown W D Binary Population Synthesis (BPS) Han et al. (2003) a: 1. CE ejection b: 1. stable RLOF c: 2. CE ejection d: merger of two helium white dwarfs Comparison to Han et al. (HPMM) sdBs: best match: models with correlated masses and low CEE efficiency Poor match: models with 100% CEE efficiency O-types: He-rich sdOs: stars clump at 45000K, too hot for any HPMM simulation set He-poor sdO: scattered in (Teff, log g) diagram Ströer et al. 2007 Non core helium-burning evolution M=0.8 Msun η=0.75 Star leaves RGB Before helium ignites in the core (e.g. by mass tranfer to a companion) Cooling tracks to form helium white dwarfs Castellani, Castellani & Moroni (2006) Non-core helium-burning sdB stars HD 188112 (V=10.2) (Heber et al., 2001) - Hipparcos parallax - distance = 80 pc - mass = 0.22 Msun No helium burning Tracks: Driebe et al. - companion: M>0.72Msun A Hyper-velocity star (HVS) amongst sdO stars from SDSS Galactic restframe velocity HVS -500 0 +500 km/s SMBH Slingshot Hills (1988): Disruption of a binary near a SuperMassive Black Hole releases companion at up to 1000 km/s or more. Detection of a single HVS: evidence for a SMBH Gualandris et al. (2005) Summary & Conclusion Origin of sdB/sdO stars? (i) delayed core helium flash (ii) close binary evolution (RLOF & CEE ejection), mergers of He-WDs He-poor sdOs are the progeny of sdB stars He-rich sdO stars are hotter than predicted by (i) & (ii) atmospheres: No metal line blanketing metalicity effects evolution (Brown et al. 2007) Post-AGB-evolution & Non-core He-burning evolution: rare due to short evolutionary time scales Outlook: A pulsating sdO star Strongest mode: P=119.3 s A=38.6 mmag plus - First Harmonic plus - 8 modes: 62 ... 118s Woudt et al. (2001) Stellar & Envelope Masses sdB Masses: 0.45 to 0.55 Msun Envelope masses: 10-3.... 10-5 Msun Thank You! sdB Asteroseismology Multi-periodic light variations (few mmag) at periods from 2 to 10min. Østensen et al. (2001) Carbon and Nitrogen SPY: C and/or N lines Detected - in all helium-rich - In none of the helium-poor ones (Ströer et al. 2007) Carbon abundances Challenges Observations: better statistics, better data: the quest for high resolution. metal abundances (see Poster 25) Evolution theory: Prediction of surface abundances for late hot flasher (Cassisi et al. 2003) & He WD mergers Angular momentum and stellar rotation Stellar atmospheres & envelopes: diffusion (rad. levitation) & metal line blanketing, see talk by G. Michaud Mass loss and diffusion The role of magnetic fields (O´Toole et al. 2005) Grazie! Blue Hook stars HD128220B: Fe & Ni Fe/H=1/100 solar Ni/H=1/10 solar US 708: Keck LRIS spectrum • • • • • Teff = 45500K, log g = 5.23, mass = 0.5 Mo B=19.0 mag Distance: 19 kpc Run-away stars Ejection scenario: born in the plane and ejected (Blaauw, 1961) - binary supernova ejection - 3 body interaction in an open cluster Calculate path and time of flight: - radial velocities, distances & proper motion - orbit integrator: Odenkirchen & Brosche (1992) - Galactic potential: Allen & Santillan (1991) BD+75 325 (Lanz et al. 1997) - Slight enrichment of Fe&Ni - fully metal line blanketed models: Teff lower by 6000K than metal free models Metallicity effects on atmospheric parameters for the sdB SB 707 Solar ([m/H]=0.0): Teff = 33940K log g= 5.82 log He/H=-2.95 10*solar ([m/H]=+1.0) : Teff = 35380K log g= 5.90 log He/H=-2.91 Metal line blanketed LTE models Summary II Heavy metals in sdO and sdB stars: Non solar abundances of Fe & Ni in sdO stars Non solar Ni/Fe (>solar) Strong enrichment of many iron group elements in hot sdB stars (except Fe), about solar in “cool” sdBs (<30000K): FUV flux suppression UV upturn Teff scale significantly changed by supersolar metal abundances (line blanketing) Outlook: Radial velocities Hypervelocity star Vrad=700Km/s Cosmic accelerator? Ejection from a cluster by three body interaction? SN II in a binary release companion at orbital velocity? Supermassive black hole in the Galactic center? Better ideas?? HQS-sdB: comparison with Han et al. Trends of helium abundance He sdO sdB stars: - 2 sequences sdO stars: - Spread by 6 orders of magnitude - 1/3 heliumdeficient! sdO solar sdB sdB Helium abundances Edelmann et al. 2003 Two sequences: He/H vs. Teff Hamburger Quasar Survey sdB stars: Edelmann et al. 2003: 100 sdB stars sdB = Extreme HB stars Post-EHB EHB Saffer et al. 1994 The lower sequence Mcore Tracks from Driebe et al. (1998) sdB and sdO stars from SPY SPY: ESO-VLT+UVES: High-res. Spectra of >1000 Double degenerate candidates - sdB: 79 (Lisker et al. 2005) - He-sdO: 30 (Ströer et al. sdO: 28 2007) - fraction of RV variables (P<10d): sdB: 39% He-sdO: 4% (1 SB2 binary) sdB Trends and Sequences Combining all studies Neglecting selection bias SDSS sdBs: To be done SPY-sds: no error bars shown Gap? sdB BPS Han et al: Binary population synthesis a) Without GK selection b) With GK selection M 15 UV Post EHB & post-AGB evolution Post-AGB Post EHB UV spectroscopy of HB stars IUE Caloi, Castellani et al. 1986 Heber et al. 1986 SDSS-sdOs Atmospheric models: - NLTE: - H+He, no metals - PRO2 code (Dreizler &Werner) - improved He atomic models - temperature correction scheme (Dreizler, 2003) He sdO sdO Globular Cluster CMDs Blue hoo k Moehler (2000) NGC 2808 (Walker , 1999) NGC 6752: HB &EHB stars Moni-Bidin et al. (2007) LTE spectral analyses: Teff, logg g match (E)HB prediction Helium subsolar EHB Models Helium core mass: 0.47 Msun depending on He and metal abundance (fixed by onset of He core flash) Horizontal Branch= sequence of envelope mass Menv, EHB=very low Menv (0.01 Msun), inert H-rich envelope avoids AGB evolutions Castellani et al. 1994 Origin of EHB stars EHB-progenitor stars must loose almost their entire envelope by the time of the helium core flash strong RGB mass loss; Low mass stars (Pop. II, globular cluster): Very efficient RGB Reimers wind may be sufficient. Younger populations, i.e. more massive progenitors (field): ? M=0.8 Msun η=0.75 Castellani & Castellani 1993 KPD 1930+2752: sdB + massive WD Billeres et al, 2000 Maxted et al. 2000, Geier et al. 2007 Candidate SN Ia Progenitor KPD 1930+2752: Total mass=1.4 Msun (Chandrasekhar mass) -Double degenerate -System merges within 2 108 years -SN Ia explosion? (Geier et al. 2007) More on massive compaions: talk by Stephan Geier sdB Asteroseismology Non-radial p-mode Pulsationa driven by Iron opacity bump: Predicted instability Strip matches Observations Charpinet et al. (2001) sdB Asteroseismology Period matching technique: Linear theory: Amplitudes can not be predicted (PG1325+, Charpinet et al. 2006) Metal abundances: Fe & Ni Feige 34: He-poor sdO Teff=60kK Fe/H=10*solarN i/H=70*solar sdB Asteroseismology Model parameters: Teff, log g, Mtotal, Menv (PG1325, Charpinet et al. 2006) sdB Asteroseismology PG 1605+072: Time resolved spectroscopy (9000 spectra) Radial velocity variations (O´Toole et al. 2005): 20 periods (few km/s) Line profile variations (phase folded, Tillich et al. 2007): sdB Asteroseismology Dominant mode: Teff semi-amplitude: 800K Log gsemi-amplitude: 0.08 First harmonic detected Cleaning for dominant mode: 8 weaker modes detected sdO stars from SDSS candidates selected from all releases according to colour: u-g<0.2 (0.4) g-r<0.1 11000 spectra: 40 sdO + 43 He sdO (Hirsch, Dipl. Thesis) Fits with NLTE models He sdO The two sequences The two sequences Tracks from Dorman et al. (2003) with Z=0.02 The upper sequence Tracks from Dorman et al. (2003) The lower sequence Tracks from Dorman et al. (2003) Early NLTE Analyses : sdO Classification: He II > He I He-sdO: no Balmer detectable to the eye C and/or N strong sdO: otherwise Hunger et al. 1980 Heber (1987) Post-AGB Post-EHB Evolution of hot subluminous stars: the canonical picture SdB + sdO stars: Extreme Horizontal Branch stars sdO sdB HB EHB Dorman et al. (1993, ApJ 419, 596) He-rich sdOs: - diamonds: C&N strong - C strong triangles - N strong - (triangles) The lower sequence Mcore Tracks from Driebe et al. (1998)