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Subluminous O stars
Origin and evolutionary links
Uli Heber
Hydrogen-Deficient Stars, Tübingen 20.9.2007
Outline
 Early results
 Atmospheric parameters
 Evolutionary scenarios
- Close binary evolution (RLOF, CEE & WD mergers)
vs
- Delayed core helium flashers
- (non-core helium-burning stars)
 Kinematics
 Summary & Outlook
sdO vs. sdB stars
sdO stars:
- H-deficient
- Hot > 40kK
- He-sdOs:
No hydrogen
sdO
sdB
sdB stars:
- helium-deficient
- „cool“: 20-40kK
Subluminous O and B stars
Greenstein & Sargent (1974)
sdB stars:
He-deficiency
from diffusion
Metal abundances
HST/STIS UV spectra
- Enrichment of heavy
elements (>100 times)
CPD-64 461
except Fe
207
207PB/208PB
- Radiative levitation
Fe
=solar
O´Toole & Heber 2006
Pb
Post-EHB vs post-AGB evolution
sdB = Extended Horizontal
Branch stars:
- He-burning core & inert
H-envelope (<0.01 Msun)
- How to loose the envelope?
- sdO stars=post-EHB?
Post-AGB Objects:
-rare
-linked to RCrB/EHe stars
sdB
Convective
transformation
(Wesemael et al. 1982)
Groth et al.
(1985):
Convection occurs
in He-rich atmospheres only
sdO
sdO
Convection
He/H=1
sdB
Hot subdwarfs from UVX Surveys
LTE- spectroscopic analyses of sdB stars:
- Palomar Green survey: Saffer et al. 1994, Maxted et al. 2000
- Hamburg Quasar Survey: Edelmann et al. 2003
- ESO-Supernova Progenitor Survey (SPY): Lisker et al. 2005
atmospheric parameters for >200 sdB
NLTE spectroscopic analyses of sdO-stars:
- SPY (Ströer et al. 2007)
- Sloan Digital Sky Survey (Hirsch et al. 2007)
atmospheric parameters for 130 sdO
Fits of UVES-spectra (SPY): sdO
high resolution spectra, [email protected],
sdO
TMAP NLTE models, H&He only
He sdO
Fits of SDSS-spectra: sdO
sdO
He sdO
- C&N strong:
diamonds
- C strong:
triangle
- N strong
- no C or N: open
solar
solar
SPY:
C & N lines
- All He-rich sdOs
have C and/or N
-None of the
He-poor have C/N
Carbon III/IV
SPY:
n(C)=0.13%
(Hirsch et al. 2007)
vrot sin i=0 km/s
Carbon III/IV
SPY:
n(C)=0.25%
(Hirsch et al. 2007)
vrot sin i=20 km/s
SPY
He-rich
He-poor
solar
The canonical picture
Smooth evolutionary
time scales:
- He-poor scattered in diagram
progeny of sdB stars
- Clumping of He-rich sdOs
can not be explained
He-ZAMS
SPY&SDSS: sdB, sdO & He-sdO
sdO stars:
He-sdO: clumping at
- Teff = 45000K
- log g = 5.8
He-sdO
clump
sdB
SPY-sds: without error bars
SPY&SDSS: sdB, sdO & He-sdO
Post EHB
He-ZAMS
EHB
SubHe-ZAMS
Hot He flashers
Core flash
Delayed He core flash
Core flash
Canonical evolution
Sweigart, 1987
Delayed helium shell flash
He sdO
Very late helium core flash
Could explain
He-sdOs below the
Helium ZAMS
He sdO
He/C
sds in binaries
SPY: fraction of close binaries: radial velocity variables with P<10d
sdBs; :40% (Napiwotzki et al.,2005)
sdOs: 4% RVV (from SPY)
 mostly single-lined:
RV curve: mass function
M vis (sin i )3
K 3P
f(M )

2 G ( M vis  M invis )2
Minimum mass of companion
Napiwotzki et al. 2007
Period distribution
Nature of companions: white dwarf or low mass m.s. stars
Morales-Rueda (2006)
MS
unknown
W
D
Binary Population Synthesis (BPS)
Han et al. (2003)
a: 1. CE ejection
b: 1. stable RLOF
c: 2. CE ejection
d: merger of two
helium white dwarfs
Comparison to Han et al. (HPMM)
sdBs:
best match: models with
correlated masses and low
CEE efficiency
Poor match: models with
100% CEE efficiency
O-types:
He-rich sdOs: stars clump at
45000K, too hot for any
HPMM simulation set
He-poor sdO: scattered in
(Teff, log g) diagram
Ströer et al. 2007
Non core helium-burning
evolution
M=0.8 Msun
η=0.75
Star leaves RGB
Before helium ignites
in the core
(e.g. by mass tranfer
to a companion)
Cooling tracks
to form
helium white dwarfs
Castellani, Castellani & Moroni (2006)
Non-core helium-burning sdB stars
HD 188112 (V=10.2)
(Heber et al., 2001)
- Hipparcos parallax
- distance = 80 pc
- mass = 0.22 Msun
No helium burning
Tracks: Driebe et al.
- companion:
M>0.72Msun
A Hyper-velocity star (HVS)
amongst sdO stars from SDSS
Galactic
restframe
velocity
HVS
-500
0
+500 km/s
SMBH Slingshot
Hills (1988):
 Disruption of a
binary near a SuperMassive Black Hole
releases
companion at up to
1000 km/s
or more.
 Detection of a
single HVS:
evidence for a
SMBH
Gualandris et al. (2005)
Summary & Conclusion
 Origin of sdB/sdO stars?
(i) delayed core helium flash
(ii) close binary evolution (RLOF & CEE ejection),
mergers of He-WDs
 He-poor sdOs are the progeny of sdB stars
 He-rich sdO stars are hotter than predicted by (i) & (ii)
atmospheres: No metal line blanketing
metalicity effects evolution (Brown et al. 2007)
 Post-AGB-evolution & Non-core He-burning
evolution: rare due to short evolutionary time scales
Outlook: A pulsating sdO star
Strongest mode:
P=119.3 s
A=38.6 mmag
plus
- First Harmonic
plus
- 8 modes:
62 ... 118s
Woudt et al. (2001)
Stellar & Envelope Masses
sdB
Masses: 0.45 to 0.55 Msun
Envelope masses: 10-3.... 10-5 Msun
Thank You!
sdB Asteroseismology
Multi-periodic light variations (few mmag) at
periods from 2 to 10min.
Østensen et al. (2001)
Carbon and Nitrogen
SPY:
C and/or N lines
Detected
- in all helium-rich
- In none of the
helium-poor ones
(Ströer et al. 2007)
Carbon abundances
Challenges
 Observations: better statistics,
better data: the quest for high resolution.
metal abundances (see Poster 25)
 Evolution theory: Prediction of surface abundances for
late hot flasher (Cassisi et al. 2003) & He WD mergers
 Angular momentum and stellar rotation
 Stellar atmospheres & envelopes: diffusion (rad.
levitation) & metal line blanketing, see talk by G. Michaud
 Mass loss and diffusion
 The role of magnetic fields (O´Toole et al. 2005)
Grazie!
Blue Hook stars
HD128220B: Fe & Ni
Fe/H=1/100
solar
Ni/H=1/10
solar
US 708: Keck LRIS spectrum
•
•
•
•
•
Teff = 45500K,
log g = 5.23,
mass = 0.5 Mo
B=19.0 mag
Distance: 19 kpc
Run-away stars
 Ejection scenario:
born in the plane and ejected (Blaauw, 1961)
- binary supernova ejection
- 3 body interaction in an open cluster
 Calculate path and time of flight:
- radial velocities, distances & proper motion
- orbit integrator: Odenkirchen & Brosche (1992)
- Galactic potential: Allen & Santillan (1991)
BD+75 325 (Lanz et al. 1997)
- Slight enrichment of
Fe&Ni
- fully metal line
blanketed models:
Teff lower by 6000K
than metal free models
Metallicity effects on atmospheric
parameters for the sdB SB 707
Solar ([m/H]=0.0):
Teff = 33940K
log g= 5.82
log He/H=-2.95
10*solar ([m/H]=+1.0) :
Teff = 35380K
log g= 5.90
log He/H=-2.91
Metal line blanketed LTE
models
Summary II
Heavy metals in sdO and sdB stars:
 Non solar abundances of Fe & Ni in sdO stars
 Non solar Ni/Fe (>solar)
 Strong enrichment of many iron group elements in hot
sdB stars (except Fe), about solar in “cool” sdBs
(<30000K):
FUV flux suppression
UV upturn
 Teff scale significantly changed by supersolar metal
abundances (line blanketing)
Outlook: Radial velocities
Hypervelocity star
Vrad=700Km/s
Cosmic accelerator?
Ejection from a cluster by three body interaction?
SN II in a binary release companion at orbital
velocity?
Supermassive black hole in the Galactic center?
Better ideas??
HQS-sdB: comparison with Han et al.
Trends of helium abundance
He sdO
 sdB stars:
- 2 sequences
 sdO stars:
- Spread by
6 orders of
magnitude
- 1/3 heliumdeficient!
sdO
solar
sdB
sdB Helium abundances
Edelmann et al. 2003
Two sequences:
He/H vs. Teff
Hamburger Quasar Survey
sdB stars:
Edelmann et al.
2003:
100 sdB stars
sdB = Extreme HB stars
Post-EHB
EHB
Saffer et al. 1994
The lower sequence
Mcore
Tracks from Driebe et al. (1998)
sdB and sdO stars from SPY
SPY: ESO-VLT+UVES:
High-res. Spectra of
>1000 Double degenerate
candidates
- sdB: 79 (Lisker et al. 2005)
- He-sdO: 30 (Ströer et al.
sdO: 28
2007)
- fraction of RV variables
(P<10d):
sdB: 39%
He-sdO: 4% (1 SB2 binary)
sdB
Trends and Sequences
Combining all studies
Neglecting
selection bias
SDSS sdBs:
To be done
SPY-sds: no error
bars shown
Gap?
sdB
BPS
Han et al:
Binary population
synthesis
a) Without GK
selection
b) With GK
selection
M 15
UV
Post EHB & post-AGB evolution
Post-AGB
Post EHB
UV spectroscopy of HB stars
IUE
Caloi, Castellani et al. 1986
Heber et al. 1986
SDSS-sdOs
Atmospheric models:
- NLTE:
- H+He, no metals
- PRO2 code
(Dreizler &Werner)
- improved He atomic
models
- temperature correction
scheme (Dreizler,
2003)
He sdO
sdO
Globular Cluster CMDs
Blue
hoo
k
Moehler (2000)
NGC 2808 (Walker , 1999)
NGC 6752: HB &EHB stars
Moni-Bidin et al. (2007)
LTE spectral analyses:
Teff, logg g match
(E)HB prediction
Helium subsolar
EHB Models
Helium core mass: 0.47 Msun depending on He and metal abundance
(fixed by onset of He core flash)
Horizontal Branch=
sequence of envelope mass Menv,
EHB=very low Menv (0.01 Msun),
inert H-rich envelope
avoids AGB evolutions
Castellani et al. 1994
Origin of EHB stars
EHB-progenitor stars must loose
almost their entire envelope by
the time of the helium core flash
strong RGB mass loss;
Low mass stars (Pop. II, globular
cluster):
Very efficient RGB Reimers wind
may be sufficient.
Younger populations, i.e.
more massive progenitors (field): ?
M=0.8 Msun
η=0.75
Castellani & Castellani 1993
KPD 1930+2752: sdB + massive WD
Billeres et al, 2000
Maxted et al. 2000, Geier et al. 2007
Candidate SN Ia Progenitor
KPD 1930+2752:
Total mass=1.4 Msun
(Chandrasekhar mass)
-Double degenerate
-System merges within
2 108 years
-SN Ia explosion?
(Geier et al. 2007)
More on massive
compaions: talk by
Stephan Geier
sdB Asteroseismology
Non-radial p-mode
Pulsationa driven by
Iron opacity bump:
Predicted instability
Strip matches
Observations
Charpinet et al. (2001)
sdB
Asteroseismology
Period matching technique:
Linear theory:
Amplitudes can not be
predicted
(PG1325+, Charpinet et al. 2006)
Metal abundances: Fe & Ni
Feige 34:
He-poor sdO
Teff=60kK
Fe/H=10*solarN
i/H=70*solar
sdB Asteroseismology
Model parameters: Teff, log g, Mtotal, Menv
(PG1325, Charpinet et al. 2006)
sdB Asteroseismology
PG 1605+072: Time resolved spectroscopy (9000 spectra)
Radial velocity variations (O´Toole et al. 2005): 20 periods (few km/s)
Line profile variations (phase folded, Tillich et al. 2007):
sdB
Asteroseismology
Dominant mode:
Teff semi-amplitude: 800K
Log gsemi-amplitude: 0.08
First harmonic detected
Cleaning for
dominant mode:
8 weaker modes
detected
sdO stars from SDSS
candidates selected from
all releases according to
colour: u-g<0.2 (0.4)
g-r<0.1
11000 spectra:
 40 sdO + 43 He sdO
(Hirsch, Dipl. Thesis)
Fits with NLTE models
He sdO
The two sequences
The two sequences
Tracks from Dorman et al. (2003) with Z=0.02
The upper sequence
Tracks from Dorman et al. (2003)
The lower sequence
Tracks from Dorman et al. (2003)
Early NLTE Analyses : sdO
Classification:
He II > He I
He-sdO: no Balmer
detectable to the eye
C and/or N strong
sdO: otherwise
Hunger et al. 1980
Heber (1987)
Post-AGB
Post-EHB
Evolution of hot subluminous stars:
the canonical picture
SdB + sdO stars:
Extreme
Horizontal
Branch
stars
sdO
sdB
HB
EHB
Dorman et al. (1993, ApJ 419, 596)
He-rich sdOs:
- diamonds:
C&N strong
- C strong
triangles
- N strong
- (triangles)
The lower sequence
Mcore
Tracks from Driebe et al. (1998)
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