hst/cos observations of o(he) stars
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
HST/COS Observations of O(He) Stars
HST/COS Observations of O(He) Stars
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O(He) StarsO(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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O(He) Photospheric ParametersO(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
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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
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Evolution of O(He) StarsEvolution 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
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Miller Bertolami & Althaus, 2006, A&A, 454, 845
M = 0.512Mʘ
post early-AGB star
“numerical experiment”
increased mass-loss rates
hydrogen deficiency
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O(He) vs. RCrBO(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
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Evolution of O(He) StarsEvolution 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 ) ?
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Spectroscopy of O(He) StarsSpectroscopy 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 1150 - 3200Å R < 80 000
STIS @HST 1997 - 2004 1150 - 3175Å R < 114 000
FUSE 1999 - 2007 904 - 1190Å R 20 000
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UV ObservationsUV 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
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UV ObservationsUV Observations
FUSE– Cy03: accepted
( 4 stars, 25 ksec)– Cy08: accepted
(only 3 stars, 204 ksec)
observations scheduled for summer 2007FUSE failure July 12, 2007
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FUSE resolution reduced to 7Å
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hydrostatic models
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HotBlast “wind” modelsradiation-driven mass-loss rates (Pauldrach et al. 1988)
-7.6
-7.7
-9.1
-9.5
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Models with Fe group lines
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HS1522+6615
Elke Reiff
diploma thesis
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ConclusionsConclusions
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
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Conclusions IIConclusions IIlow-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
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KPD 0005+5106
is a successor of high-mass O(He) stars?
Poster #71 on KPD 0005+5106