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O(He) Stars

O(He) Stars. Thomas Rauch Elke Reiff Klaus Werner Jeffrey W. Kruk Institute for Astronomy and Astrophysics Kepler Center for Astro and Particle Physics Eberhard-Karls University Tübingen Germany. Overview. O(He) stars spectral analyses evolutionary scenario. O(He) Stars.

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O(He) Stars

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  1. O(He) Stars Thomas Rauch Elke Reiff Klaus Werner Jeffrey W. Kruk Institute for Astronomy and Astrophysics Kepler Center for Astro and Particle Physics Eberhard-Karls University Tübingen Germany Hydrogen-Deficient Stars

  2. Overview • O(He) stars • spectral analyses • evolutionary scenario Hydrogen-Deficient Stars

  3. 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 Hydrogen-Deficient Stars

  4. 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 Hydrogen-Deficient Stars

  5. 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

  6. O(He) CSPN • construction of consistent models CS + PN • NLTE model-atmosphere fluxes used as ionizing spectra in photoionization models LoTr 4 K 1-27 H [O III] Hydrogen-Deficient Stars

  7. K 1-27 (PN G286.9-29.5) • Rauch, Köppen, Werner 1994, A&A 286, 543 • O(He) CSPN • Teff = 105 kK • log g = 6.5 (cgs) • H/He < 0.2 possibleborn again star! • M = 0.55 M • d = 1.3 kpc • PN • solar abundances • M = 0.018 Mpossibleborn again PN? • texp<<tevol • N54eV much too low Hydrogen-Deficient Stars

  8. LoTr 4 (PN G274.3+09.1) • Rauch, Köppen, Werner 1996, A&A 310, 613 • O(He) CSPN • Teff= 120 kK • log g = 5.5 (cgs) • H/He = 0.5 possibleborn again star! • M = 0.65 M • d = 6 kpc • PN • Solar abundances • M = 0.29 Mnormal PN • texp>>tevol Hydrogen-Deficient Stars

  9. Evolutionary Status of O(He) Stars • AGB • [WC] sdO(He) • PG 1159 O(He) • DA DO our picture 1998 ? ? ? ? ? Hydrogen-Deficient Stars

  10. 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 Hydrogen-Deficient Stars

  11. 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 Hydrogen-Deficient Stars

  12. 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 ) ? Hydrogen-Deficient Stars

  13. 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 Hydrogen-Deficient Stars

  14. HST + FUSE Spectroscopy • photospheric spectra characterized by a few, broad and shallow, absorption lines from highly ionized species • e.g. He II, C IV, O VI, Si IV Hydrogen-Deficient Stars

  15. UV Observations • HST GHRS (Cy06) + STIS • Cy06: if C and N deficient  lines not visible • Cy07: optical analyses will answer questions • Cy08: line profiles mainly sensitive to velocity field • Cy09: data analysis not well described • Cy10: not as compelling as other proposals • Cy11: unclear how precise the abundances have to be (changed PI: Werner) • Cy12: these objects are only a small group in WDs – general interest not clear • Cy13: accepted (added “successors of RCrB stars?” to title) • first observations scheduled for Aug 9, 2004 • STIS failure Aug 3, 2004 Hydrogen-Deficient Stars

  16. Longmore 4 Hydrogen-Deficient Stars

  17. UV Observations • FUSE • Cy03: accepted ( 25 ksec) • Cy06: abundances of 4 stars will not fit a clear pattern (204 ksec) • Cy07: no good justification to repeat for higher S/N (204 ksec) • Cy08: accepted (only 3 stars, 204 ksec) • observations scheduled for summer 2007 • FUSE failure July 12, 2007 Hydrogen-Deficient Stars

  18. Thomas, heard about the new wheel failure of FUSE today? They have to terminate the mission. Rauch

  19. FUSE resolution reduced to 7Å Hydrogen-Deficient Stars

  20. Hydrogen-Deficient Stars

  21. Hydrogen-Deficient Stars

  22. static models Hydrogen-Deficient Stars

  23. “wind” models radiation-driven mass-loss rates (Pauldrach et al. 1988) -7.6 -7.7 -9.1 -9.5 Hydrogen-Deficient Stars

  24. mass-loss rates from Pauldrach X 30 Hydrogen-Deficient Stars

  25. Models with Fe group lines Hydrogen-Deficient Stars

  26. HS1522+6615

  27. 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 will be performed with COS / STIS? • determination of C, N, O, and Si abundances to corroborate link to RCrBs Hydrogen-Deficient Stars

  28. Miller Bertolami & Althaus, 2006, A&A, 454, 845 M = 0.512Mʘ post early-AGB star “numerical experiment” increased mass-loss rates  hydrogen deficiency Hydrogen-Deficient Stars

  29. 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 Hydrogen-Deficient Stars

  30. KPD 0005+5106 • is a successor of • high-mass O(He) stars? “Truth suffers from too many analysis.” Ancient Fremen Saying, Dune Messiah

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