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


OverviewOverview

O(He) stars

spectral analyses

evolutionary scenario


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


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


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


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


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


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

AGB

[WC] sdO(He)

PG 1159 O(He)

DA DO our picture 1998

?

?

?

??


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


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


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 ) ?


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


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


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

September 18, 2007 Hydrogen-Deficient Stars 17

Longmore 4


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.


FUSE resolution reduced to 7Å




static models


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

-7.6

-7.7

-9.1

-9.5


mass-loss rates from Pauldrach X 30


Models with Fe group lines

HS1522+6615


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


Miller Bertolami & Althaus, 2006, A&A, 454, 845

M = 0.512Mʘ

post early-AGB star

“numerical experiment”

increased mass-loss rates

hydrogen deficiency


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

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O(He) Stars