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HST/COS Observations of O(He) Stars

HST/COS Observations of O(He) Stars. O(He) Stars. spectral sub-type O(He) by Méndez et al. (1986) spectra dominated by He II absorption lines CSPN K 1-27 CSPN LoTr 4 HS 1522+6615 HS 2209+8229 HS 0742+6520 preliminary analysis. NLTE analysis by Rauch et al. 1998.

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HST/COS Observations of O(He) Stars

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  1. HST/COS Observations of O(He) Stars

  2. O(He) Stars • spectral sub-type O(He) by Méndez et al. (1986) • spectra dominated by He II absorption lines • CSPN K 1-27 • CSPN LoTr 4 • HS 1522+6615 • HS 2209+8229 • HS 0742+6520 preliminary analysis NLTE analysis by Rauch et al. 1998 EUROWD10, August 17, 2010

  3. O(He) Photospheric Parameters • Teff / kK log g H/He C/He N/He O/He • CSPN K 1-27 105 6.5 < 0.2 < 0.005 0.005 • CSPN LoTr 4 120 5.5 0.5 < 0.004 0.001 < 0.008 • HS 1522+6615 140 5.5 0.1 0.003 • HS 2209+8229 100 6.0 < 0.2 • Rauch et al. 1998, A&A 338, 651 • based on optical, UV (IUE), and • X-ray (ROSAT) spectra EUROWD10, August 17, 2010

  4. O(He) stars found amongst PG 1159 stars • two pairs of spectroscopic twins • HS 1522+6615 + LoTr 4 • HS 2209+8829 + K 1-27 no PN PN

  5. Evolution of O(He) Stars • Evolutionary models (e.g. Herwig et al. 1999) • PG 1159 abundances (He:C:O=33:50:17 by mass) are result of late He-shell flash • O(He) cannot be explained EUROWD10, August 17, 2010

  6. Miller Bertolami & Althaus, 2006, A&A, 454, 845 M = 0.512Mʘ post early-AGB star “numerical experiment” increased mass-loss rates  hydrogen deficiency EUROWD10, August 17, 2010

  7. O(He) vs. RCrB • Teff / kK log g H/He C/He N/He O/He • K 1-27 105 6.5 < 0.2 < 0.005 0.005 • LoTr 4 120 5.5 0.5 < 0.004 0.001 < 0.008 • HS 1522+6615 140 5.5 0.1 0.003 • HS 2209+8229 100 6.0 < 0.2 • RCrB < 0.0001 0.010 0.004 0.005 V 854 Cen 0.5 0.030 0.0003 0.003 EUROWD10, August 17, 2010

  8. Evolution of O(He) Stars • evolutionary models (e.g. Herwig et al. 1999) • PG 1159 abundances (He:C:O=33:50:17 by mass) are result of late He-shell flash • O(He) cannot be explained • third post-AGB evolutionary sequence? • hydrogen-rich • hydrogen-deficient ( [WC] – PG 1159 – DO ) • hydrogen-deficient ( RCrB – O(He) – DO ) ? EUROWD10, August 17, 2010

  9. Spectroscopy of O(He) Stars • high Teff  flux maximum in the EUV • precise NLTE spectral analysis needs • metal lines (of highly ionized species) • ionization equilibria  Teff • abundances • high S/N, high-resolution UV spectra EUROWD10, August 17, 2010

  10. UV Observations • HST STIS • Cy13: accepted (starting from Cy06 …) • first observations scheduled for Aug 9, 2004 • STIS failure Aug 3, 2004 • HST COS • Cy17:accepted • to be performed late 2010 / 2011 • COS: deviation from nominal PSF • all four observations performed May – July 2010 EUROWD10, August 17, 2010

  11. UV Observations • FUSE • Cy03: accepted ( 4 stars, 25 ksec) • Cy08: accepted (only 3 stars, 204 ksec) • observations scheduled for summer 2007 • FUSE failure July 12, 2007 EUROWD10, August 17, 2010

  12. FUSE resolution reduced to 7Å EUROWD10, August 17, 2010

  13. EUROWD10, August 17, 2010

  14. EUROWD10, August 17, 2010

  15. hydrostatic models EUROWD10, August 17, 2010

  16. HotBlast “wind” models radiation-driven mass-loss rates (Pauldrach et al. 1988) -7.6 -7.7 -9.1 -9.5 EUROWD10, August 17, 2010

  17. Models with Fe group lines EUROWD10, August 17, 2010

  18. HS1522+6615 Elke Reiff diploma thesis

  19. Conclusions • mass-loss rates of O(He) stars are not higher than predicted by radiation-driven wind theory •  change of surface composition due to wind unlikely • FUSE spectra do not show isolated metal lines and thus, allow to give only upper limits for abundances • iron-group abundances are (probably) solar • UV spectroscopy with HST COS! • determination of C, N, O, and Si abundances to corroborate link to RCrBs EUROWD10, August 17, 2010

  20. EUROWD10, August 17, 2010

  21. Conclusions II • low-mass O(He) stars • post early-AGB stars • first thermal pulse (TP) after departure from AGB • higher mass-loss rates  hydrogen deficiency • high-mass O(He) stars • “normal” born-again scenario • (V)LTP  hydrogen deficiency • alternative O(He) scenario • double-degenerate merger • similar H/He surface composition suggests that the O(He) stars are the progeny of RCrB stars • RCrB  O(He)  non-DA WD EUROWD10, August 17, 2010

  22. KPD 0005+5106 • is a successor of • high-mass O(He) stars? • Poster #71 on KPD 0005+5106

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