HST/COS Observations of O(He) Stars 18. September 2007 HST/COS Observations of O(He) Stars Hydrogen-Deficient Stars

O(He) Stars spectral sub-type O(He) by Méndez et al. (1986) 18. September 2007 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 Hydrogen-Deficient Stars

18. September 2007 Hydrogen-Deficient Stars

O(He) Photospheric Parameters 18. September 2007 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 Hydrogen-Deficient Stars

O(He) stars found amongst PG 1159 stars 18. September 2007 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 Hydrogen-Deficient Stars

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

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

O(He) vs. RCrB Teff / kK log g H/He C/He N/He O/He 18. September 2007 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 Hydrogen-Deficient Stars

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

Spectroscopy of O(He) Stars 18. September 2007 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 IUE 1978 - 1996 1150 - 3200Å R < 11 000 GHRS @HST 1990 - 1997 R < 80 000 STIS @HST 1997 - 2004 1150 - 3175Å R < 114 000 FUSE 1999 - 2007 904 - 1190Å R  20 000 EUROWD10, August 17, 2010 Hydrogen-Deficient Stars

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

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

FUSE resolution reduced to 7Å 18. September 2007 FUSE resolution reduced to 7Å EUROWD10, August 17, 2010 Hydrogen-Deficient Stars

18. September 2007 EUROWD10, August 17, 2010 Hydrogen-Deficient Stars

18. September 2007 EUROWD10, August 17, 2010 Hydrogen-Deficient Stars

hydrostatic models EUROWD10, August 17, 2010 18. September 2007 Hydrogen-Deficient Stars

HotBlast “wind” models 18. September 2007 HotBlast “wind” models radiation-driven mass-loss rates (Pauldrach et al. 1988) -7.6 -7.7 -9.1 -9.5 EUROWD10, August 17, 2010 Hydrogen-Deficient Stars

Models with Fe group lines 18. September 2007 Models with Fe group lines EUROWD10, August 17, 2010 Hydrogen-Deficient Stars

HS1522+6615 Elke Reiff diploma thesis

18. September 2007 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 Hydrogen-Deficient Stars

EUROWD10, August 17, 2010

Conclusions II low-mass O(He) stars post early-AGB stars 18. September 2007 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 Hydrogen-Deficient Stars

KPD 0005+5106 is a successor of high-mass O(He) stars? 18. September 2007 KPD 0005+5106 is a successor of high-mass O(He) stars? Poster #71 on KPD 0005+5106 Hydrogen-Deficient Stars