hst/cos observations of o(he) stars

HST/COS Observations of O(He) Stars

HST/COS Observations of O(He) Stars

EUROWD10, August 17, 2010

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

EUROWD10, August 17, 2010

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