Hot Subdwarf Stars in Close-up View: Orbits, Rotation, Abundances and Masses of their Unseen Companions Den Naturwissenschaftlichen Fakult¨aten der Friedrich-Alexander-Universit¨at Erlangen-N¨urnberg zur Erlangung des Doktorgrades vorgelegt von Stephan Geier aus Stadtsteinach Als Dissertation genehmigt von den Naturwissenschaftlichen Fakult¨aten der Universit¨at Erlangen-N¨urnberg Tag der m¨undlichen Pr¨ufung: 18. M¨arz 2009 Vositzender der Promotionskommission: Prof. Dr. E. B¨ansch Erstberichterstatter: Prof. Dr. U. Heber Zweitberichterstatter: Prof. Dr. P. Podsiadlowski, University of Oxford Drittberichterstatter: Prof. Dr. K. Werner, Universit¨at T¨ubingen Contents 1 Hot subdwarf stars: A review 9 1.1 Generalproperties ............................... .... 9 1.2 Single star formation and evolution scenarios . ............ 10 1.3 Hot subdwarf binaries: Observations, formation and evolution........... 12 1.4 Pulsating hot subdwarfs and asteroseismology . ............ 15 1.5 Hot subdwarf atmospheres and diffusion processes . .......... 20 1.6 Hot subdwarfs and extrasolar planets . ......... 21 1.7 Hotsubdwarfsashyper-velocitystars . ........ 23 1.8 Hot subdwarfs and globular clusters . ........ 24 1.9 Hot subdwarfs and the UV-upturn in early-type galaxies . ........... 25 1.10 Hot subdwarf stars, supernovae and cosmology . ........... 26 2 Hot subdwarf stars in close binary systems: Previous work and new discov- eries 30 2.1 Generalstatistics ............................... ..... 30 2.2 Determination of hot subdwarf and companion masses in close binaries . 33 2.3 Orbital parameters of new close binary subdwarfs . ........... 34 2.3.1 Target selection, observations and data reduction . ........... 35 2.3.2 Radial velocity measurements, power spectra and RV curves........ 35 2.3.3 Constraints on the nature of the unseen companions . ......... 36 2.3.4 Results ..................................... 38 2.3.5 Discussion.................................... 40 3 Constraining binary parameters 44 3.1 Analysismethod .................................. 44 3.2 Observations and data reduction . ....... 46 3.3 Measuring low projected rotational velocities from metallines .......... 47 3.4 Systematic errors in the determination of the projected rotational velocity from metallines........................................ 47 3.5 Measuring high projected rotational velocities from hydrogen and helium lines . 51 3.6 RotationofsdBsinclosebinaries . ....... 51 3.7 Determination of the surface gravity and systematic errors............. 52 3.8 Constraining masses and nature of the unseen companions ............. 57 3.8.1 Low mass companions - White dwarfs, M stars, brown dwarfs....... 57 3.8.2 High mass white dwarf companions - candidates for SN Ia progenitors . 63 3.8.3 Massive compact companions - Neutron stars or black holes........ 63 i 3.9 Distribution of companion masses and systematic effects . ............. 66 3.10 sdB+NS/BH systems as X-ray sources? . ...... 67 4 Orbital synchronisation of sdB binaries 71 4.1 Theoretical timescales for synchronisation . ............. 71 4.2 Empiricalevidence: Eclipsingsystems . .......... 75 4.3 Empirical evidence: Ellipsoidal deformation . ............. 75 4.4 Empiricalevidence: PG0101+039 . ...... 75 4.4.1 Radialvelocitycurve. 76 4.4.2 Gravity, projected rotational velocity and analysis ............. 77 4.4.3 Light curve and ellipsoidal variation . ........ 77 4.4.4 Implications for orbital synchronisation in sdB binaries .......... 78 4.5 Empiricalevidence: Asteroseismology . .......... 78 4.6 Post-RGB binaries and synchronisation . ........ 79 4.6.1 Analysisofpost-RGBbinaries . 79 4.6.2 Non-synchronous rotation in post-RGB binaries and the implications . 82 5 The formation of sdB+NS/BH binaries 87 6 The fraction of sdB+NS/BH systems and selection effects 90 6.1 Projected rotational velocity and quality of the data . .............. 90 6.2 Radialvelocityvariability . ........ 91 6.3 Photometricselection . ..... 94 6.4 The high fraction of sdB+NS/BH binaries and selection effects .......... 94 6.5 Low mass EHB stars and the fraction of sdB+NS/BH binaries .......... 95 7 Rotational properties of single sdBs 98 7.1 Previouswork.................................... 98 7.2 UniformrotationofsinglesdBs . ....... 99 7.3 Implications for sdB formation scenarios . ........... 100 7.4 Rotation along the Horizontal Branch . ........ 100 8 Metal Abundances of Hot Subdwarf Stars from the SPY Survey 103 8.1 Semi-automatic abundance analysis . ........ 103 8.2 Metalabundances ................................. 109 8.2.1 Generaltrends ................................. 110 8.2.2 Carbon,nitrogen,oxygen . 110 8.2.3 Neonandmagnesium ............................. 110 8.2.4 Aluminiumandsilicon. 111 8.2.5 Phosphorusandsulfur . 111 8.2.6 Argon,potassium,calcium . 111 8.2.7 Scandium, titanium, vanadium, chromium . ....... 111 8.2.8 Iron,cobalt,zinc .............................. 112 8.3 DiffusioninsdBatmospheres . 112 8.4 Beyond the EHB - diffusion on the Horizontal Branch . ........ 113 8.5 TwopopulationsofhotsdBs . 117 8.6 Carbon rich sdBs – a missing link in sdB formation? . ......... 118 ii 8.7 Metal line blanketing and improved model atmospheres . ............ 122 9 Outlook: The (HYPER-)MUCHFUSS survey 124 A Radial velocities of programme stars 127 B Radial velocity curves and power spectra 133 C Metal abundances 139 iii List of Figures 1.1 Schematic Hertzsprung-Russell-Diagram. .......... 10 1.2 Single and binary channels to produce hot subdwarfs. ............ 13 1.3 Artist’s conception of a WD merger process.. .......... 16 1.4 Artist’s conception of stable mass transfer to a compact companion. 17 1.5 Artist’s conception of common envelope ejection. ............. 17 1.6 Example of a very high quality light curve for the short period sdB pulsator PG1219+534....................................... 18 1.7 Mass distribution of 12 short period pulsating sdBs. ............. 19 1.8 Colour-coded concentrations of elements in a 0.61 M⊙ BHBstar. ......... 20 1.9 O-C-diagram of the pulsating sdB V 391 Pegasi. ........ 22 1.10 Schematic view of the evolution of V 391 Pegasi. ........... 22 1.11 Spectrum of the HVS He-sdO US 708. 23 1.12 Color magnitude diagram and DSS image of the peculiar globular cluster NGC 2808. 25 1.13 Composite UV-optical energy distribution for the center of the Sb galaxy M 31. 27 1.14 Supernova type Ia SN1994d in the galaxy NGC 4526. .......... 28 1.15 Hubble diagram from the Supernova Cosmology Project. ............. 29 2.1 Radial velocity curves of the single-lined close binary sdB+WD system KPD 1930+2752 and the double-lined close binary He-sdB+He-sdB system PG 1544+488 . 31 2.2 Orbital period distribution of close binary sdBs. .............. 32 2.3 Light curve of the sdB+M binary PG 1017+086 showing a reflectioneffect . 32 2.4 LightcurvesofeclipsingsdBbinaries. .......... 42 2.5 Radial velocity curve and power spectrum of ASAS 102322. ............ 43 2.6 Histogramoforbitalperiods. ....... 43 3.1 Schematic view of a single-lined binary system. ........... 45 3.2 Schematic view of a single-lined binary system with synchronised rotation. 46 3.3 Rotational broadening fit result for a standard sdB model. ............. 48 3.4 Rotational broadening fit result for HE 1047 0436.................. 49 − 3.5 Rotational broadening fit result for HE 0532 4503.................. 50 − 3.6 Measured vrot sin i-distribution of 51 RV variable sdBs. 55 3.7 Example fit of LTE model spectra to Balmer and helium lines of KPD 1930+2752. 56 3.8 Mass of the sdB primary plotted against the mass of the unseen companion for HE 2135 0238. ..................................... 60 − 3.9 Mass of the sdB primary plotted against the mass of the unseen companion for HE 0532 4503. ..................................... 61 − iv 3.10 Mass of the sdB primary plotted against the mass of the unseen companion for PG 1232 136....................................... 62 − 3.11 Total mass plotted against logarithmic period of double degenerate systems from theSPYsurvey...................................... 64 3.12 Mass distribution of the unseen companion stars. ............. 67 3.13 Detail of the mass distribution shown in Fig. 3.12. ............. 68 4.1 Logarithm of the orbital period plotted against the synchronisation times. 73 4.2 Radial velocity curve and power spectrum of PG 0101+039 (see Fig. 2.5). 76 4.3 Lightcurve data of PG 0101+039 with superimposed models............. 77 4.4 Logarithm of the orbital period plotted against the synchronisation times. 83 4.5 T log g-diagram with post-RGB evolutionary tracks. 84 eff − 4.6 T log g-diagram with EHB evolutionary tracks. 85 eff − 5.1 Schematic diagram of formation scenarios leading to hot subdwarf binaries with neutron-star or black-hole companions. ...... 89 6.1 Schematic view of binary inclination. .......... 91 6.2 sin i-distribution of binaries under the assumption of canonical sdB mass 0.47 M⊙. 92 6.3 sin i-distribution of binaries under the assumption of canonical sdB mass 0.47 M⊙ andselectioneffects. ................................. 93 6.4 Simulated distribution of sdB masses for binaries formed via CE ejection. 95 6.5 Mass distribution of the unseen companion stars under the assumption of the lowest possible sdB mass 0.30 M⊙. .......................... 97 7.1 Measured vrot sin i distibution of 49 single sdBs. 99 8.1 Teff –log g-diagram. ................................... 104 8.2 Example fits of common oxygen and nitrogen lines for two spectra with different quality. ........................................