O(He) Stars
Thomas Rauch
Elke Reiff
Klaus Werner
Jeffrey W. Kruk
Institute for Astronomy and Astrophysics
Kepler Center for Astro and Particle Physics
Eberhard-Karls University
Tübingen
Germany
Tübingen, 18.9.2007
Hydrogen-Deficient Stars
1
Overview
O(He) stars
spectral analyses
evolutionary scenario
Tübingen, 18.9.2007
Hydrogen-Deficient Stars
2
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
NLTE analysis
by Rauch et al. 1998
• HS 0742+6520
preliminary analysis
Tübingen, 18.9.2007
Hydrogen-Deficient Stars
3
O(He) Photospheric Parameters
Teff / kK log g
H/He
C/He
N/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
HS 1522+6615
140
5.5
0.1
0.003
HS 2209+8229
100
6.0
< 0.2
O/He
< 0.008
Rauch et al. 1998, A&A 338, 651
based on optical, UV (IUE), and
X-ray (ROSAT) spectra
Tübingen, 18.9.2007
Hydrogen-Deficient Stars
5
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) CSPN
construction of consistent models CS + PN
– NLTE model-atmosphere fluxes used as
ionizing spectra in photoionization models
LoTr 4
K 1-27
H
Tübingen, 18.9.2007
[O III]
Hydrogen-Deficient Stars
7
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
Tübingen, 18.9.2007
Hydrogen-Deficient Stars
8
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
Tübingen, 18.9.2007
Hydrogen-Deficient Stars
9
Evolutionary Status of O(He) Stars
AGB
[WC]
sdO(He)
?
PG 1159
?
?
O(He)
?
?
DA
Tübingen, 18.9.2007
DO
our picture 1998
Hydrogen-Deficient Stars
10
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
Tübingen, 18.9.2007
Hydrogen-Deficient Stars
11
O(He) vs. RCrB
Teff / kK log g
H/He
C/He
N/He
K 1-27
105
6.5
< 0.2
< 0.005
0.005
LoTr 4
120
5.5
0.5
< 0.004
0.001
HS 1522+6615
140
5.5
0.1
0.003
HS 2209+8229
100
6.0
< 0.2
RCrB
V 854 Cen
Tübingen, 18.9.2007
< 0.0001 0.010 0.004
0.5
0.030 0.0003
Hydrogen-Deficient Stars
O/He
< 0.008
0.005
0.003
12
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 ) ?
Tübingen, 18.9.2007
Hydrogen-Deficient Stars
13
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
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
1999 - 2007
904 - 1190Å
R  20 000
FUSE
Tübingen, 18.9.2007
Hydrogen-Deficient Stars
14
HST + 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
Tübingen, 18.9.2007
Hydrogen-Deficient Stars
15
UV 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, 2004
STIS failure Aug 3, 2004
Tübingen, 18.9.2007
Hydrogen-Deficient Stars
16
Longmore 4
September 18, 2007
Hydrogen-Deficient Stars
17
UV 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 2007
FUSE failure July 12, 2007
Tübingen, 18.9.2007
Hydrogen-Deficient Stars
18
Thomas, heard about the new wheel failure of FUSE today?
They have to terminate the mission.
Rauch
FUSE resolution reduced to 7Å
Tübingen, 18.9.2007
Hydrogen-Deficient Stars
20
Tübingen, 18.9.2007
Hydrogen-Deficient Stars
21
Tübingen, 18.9.2007
Hydrogen-Deficient Stars
22
static models
Tübingen, 18.9.2007
Hydrogen-Deficient Stars
23
“wind” models
radiation-driven mass-loss rates
(Pauldrach et al. 1988)
-7.6
-7.7
-9.1
-9.5
Tübingen, 18.9.2007
Hydrogen-Deficient Stars
24
mass-loss rates from Pauldrach X 30
Tübingen, 18.9.2007
Hydrogen-Deficient Stars
26
Models with Fe group lines
Tübingen, 18.9.2007
Hydrogen-Deficient Stars
29
HS1522+6615
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 will be performed with COS / STIS?
– determination of C, N, O, and Si abundances to
corroborate link to RCrBs
Tübingen, 18.9.2007
Hydrogen-Deficient Stars
32
Miller Bertolami &
Althaus, 2006, A&A,
454, 845
M = 0.512Mʘ
post early-AGB star
“numerical experiment”
increased
mass-loss rates 
hydrogen deficiency
Tübingen, 18.9.2007
Hydrogen-Deficient Stars
33
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
Tübingen, 18.9.2007
Hydrogen-Deficient Stars
34
KPD 0005+5106
is a successor of
high-mass O(He) stars?
“Truth suffers from too many analysis.”
Ancient Fremen Saying, Dune Messiah