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

2. 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 HS
HS preliminary analysis
NLTE analysis by Rauch et al. 1998
EUROWD10, August 17, 2010
Hydrogen-Deficient Stars

3. 18. September 2007
Hydrogen-Deficient Stars

4. 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 < <
CSPN LoTr < < 0.008 HS
HS < 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

5. 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 LoTr 4
HS K 1-27
no PN PN
Hydrogen-Deficient Stars

6. 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

7. 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

8. 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 < <
LoTr < < 0.008 HS
HS < 0.2
RCrB <
V 854 Cen
EUROWD10, August 17, 2010
Hydrogen-Deficient Stars

9. 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

10. 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 Å
R <
GHRS @HST
R <
STIS @HST Å
R <
FUSE Å
R 
EUROWD10, August 17, 2010
Hydrogen-Deficient Stars

11. 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

12. 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

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

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

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

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

17. 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

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

19. HS
Elke Reiff
diploma thesis

20. 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

23. EUROWD10, August 17, 2010

24. 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

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