O(He) Stars

Tübingen, 18.9.2007 Hydrogen-Deficient Stars 1

O(He) StarsO(He) Stars

Thomas RauchElke Reiff

Klaus WernerJeffrey W. Kruk

Institute for Astronomy and Astrophysics Kepler Center for Astro

and Particle PhysicsEberhard-Karls University

TübingenGermany

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OverviewOverview

O(He) stars

spectral analyses

evolutionary scenario

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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

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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O(He) CSPNO(He) CSPN

construction of consistent models CS + PN– NLTE model-atmosphere fluxes used as ionizing

spectra in photoionization models

H [O III]

K 1-27

LoTr 4

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K 1-27 (PN G286.9-29.5)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 possible born again star!

• M = 0.55 M

• d = 1.3 kpc– PN

• solar abundances

• M = 0.018 M possible born again PN?

• texp << tevol

• N54eV much too low

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LoTr 4 (PN G274.3+09.1)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 possible born again star!

• M = 0.65 M

• d = 6 kpc– PN

• Solar abundances

• M = 0.29 M normal PN

• texp >> tevol

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Evolutionary Status of O(He) StarsEvolutionary Status of O(He) Stars

AGB

[WC] sdO(He)

PG 1159 O(He)

DA DO our picture 1998

?

?

?

??

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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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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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HST + FUSE SpectroscopyHST + 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

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UV ObservationsUV 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, 2004STIS failure Aug 3, 2004

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Longmore 4

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UV ObservationsUV 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 2007FUSE failure July 12, 2007

Rauch

Thomas, heard about the new wheel failure of FUSE today?

They have to terminate the mission.

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FUSE resolution reduced to 7Å

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static models

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“wind” modelsradiation-driven mass-loss rates (Pauldrach et al. 1988)

-7.6

-7.7

-9.1

-9.5

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mass-loss rates from Pauldrach X 30

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Models with Fe group lines

HS1522+6615

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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 will be performed with COS / STIS?– determination of C, N, O, and Si abundances to

corroborate link to RCrBs

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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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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

KPD 0005+5106

is a successor of high-mass O(He) stars?

“Truth suffers from too many analysis.”

Ancient Fremen Saying, Dune Messiah