Download The evolution of helium rich subdwarf B stars

Helium-Rich Subdwarf B Stars
Amir Ahmad
Armagh Observatory
C.S. Jeffery
and collaborators
20 September 2007
Tübingen
Outline
• A brief history of helium-rich subdwarf B stars
• PG1544+488
the He-sdB prototype
• JL87
a not so helium-rich He-sdB
• LSIV-14 116
a pulsating He-sdB
Introduction
• Subdwarf B stars form the dominant population of faint blue blue
stars (mB ~ 16) in our galaxy and giant elliptical galaxies
• They are 0.5 MSun core helium-burning stars
• Progenitors of White Dwarfs
• Evolution has been the subject of much debate although
evolution is though to play an important role
binary
• sdB stars have spectra dominated by Hydrogen Balmer lines
• A small subset (~ 70) of hot subdwarf stars includes the helium-rich
subdwarf B (or He-sdB) stars
H-R diagram
Dreizler 2000
“Cool” helium-rich subdwarf stars
• Early 1980’s some authors believed that helium-rich and helium-poor
subdwarf stars were separated by a temperature boundary at 40000 K
• PG survey reports a small fraction of subdwarf stars (classified as
sdOD) showing strong neutral Helium (HeI) lines
• Their evolution has since been the subject of much debate
• From optical and IUE spectra of He-sdB it is clear that there are two
distinct subclasses of He-sdB stars
› Carbon-rich, eg. PG1544+488
› Carbon-poor, eg. PG0914-037
He-sdB nomenclature
• Spectral classification criteria
› strong HeI lines plus weak HeII
› no detectable Balmer lines
• Helium-rich subdwarf B stars a.k.a.
› sdOD - Green et al. 1986
› He-sdB
– Moehler et al. 1990
– Beers et al. 1992
– Kilkenny et al. 1997
› sdB4 - Drilling 1996
› sdB:He4 - Jeffery et al. 1997
Optical spectrum of PG1544+488
More spectra
Teff - log g diagram
Position of He-sdB stars on the log g - Teff diagram with
other subluminous stars
Evolution
• It was realised early on that helium-rich subdwarfs do not evolve from
helium-poor sdB stars (Groth et al. 1985)
• sdB have nHe < 0.01 (Heber 1986)
• sdB evolution takes ~108 yr (Caloi 1989)
• Gravitational settling takes ~105 yr (Wesemael et al. 1982)
• Hence objects evolving from sdB stars should have depleted helium
abundance
Existing evolution models
• Single star evolution
›
Lanz et al. (2004) have argued
that stars evolving with high mass
loss on the red giant branch
undergo a late helium core flash
on the white dwarf cooling track
leading to convective “flash
mixing” of the envelope which
then forms a helium and carbon
rich hot subdwarf
Fig. from Brown et al. 2001
Evolutionary models
• Binary merger model
› Iben & Tutukov (1985, 1987) suggested that merger of two
degenerate white dwarf can produce hot subdwarf with depleted
hydrogen atmosphere
› Saio & Jeffery (2000, 2002) have
more recently modelled WD
mergers to explain the origin of EHe stars
Chemical abundances of He-sdB stars
• Single star (hot flasher) evolution model and white dwarf merger
models both can predict enriched carbon in the atmospheres of He-sdB
stars
• Carbon poor He-sdB stars must be products of He+He WD merge
while carbon-rich He-sdB stars can be products of a CO+He WD
merger
• PG1544+488 is the He-sdB proto-type hence the obvious target
• Bright (V~12 mag), hence possible to do high-resolution spectroscopy
• Observed in 2003 with the WHT in order to measure abundances
• Unexpected discovery!
PG1544+488 - the He-sdB prototype
Optical spectrum + model fit of PG 1544+488 (SFIT2)
Previous analyses
Teff (K)
logg
Reference
31 000
5.1
Heber et al. (1988)
optical / UV spectra
34 000
5.1
Ahmad & Jeffery (2003)
low resolution optical spectra
32 100
[5.0]
Ahmad & Jeffery (2004)
energy distribution
36 000
6.0
Lanz et al. (2004)
far ultraviolet spectra
He+He WD merger
flash-mixing model
Spectroscopic observations
April 2003
William Herschel
Telescope
ISIS dual beam
spectrograph
PG1544+488 – hydrogen deficient stars
New orbital solution
PG1544+488 in a nutshell
• PG1544+488 is a binary with a period of ~ 1/2 day
• It consists two low mass hot helium subdwarf stars of similar mass
• It is an excellent system for studying close binary evolution
• It opens the possibility of a third mechanism involving binary
evolution for the formation of He-sdB stars
• Is PG1544+488 a rare freak? We know another such system with two
He-sdO - HE 0301-3039 (Lisker et al. 2004)
JL87 - a not so helium-rich He-sdB
• JL87 - discovered by Jaidee & Lynga
(1974) as a possible “violet source”
• Classified a He-sdB by Schulz et al.
(1991)
• Subsequently called a “B type Herich” star by Kilkenny et al. (1995)
and classified B2He by Magee et al.
(1998)
• V ~ 12 mag hence possible to do high
resolution spectroscopy
• Echelle spectra of the star first
obtained in 1996 with UCLES on the
AAT subsequently followed up in
1999, 2002 and 2005
• Moderate resolution high S/N RGO
spectra also obtained
Chemical makeup of JL87
1
0.8
log (N/N solar)
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
H
He
C
N
O
Ne Mg Al
Element
Si
P
S
Ar
Fe
What is JL87?
• Chemically peculiar early-type B star
• CNO abundances and low vsini (unless inclination is close to zero)
rules out JL87 being a main sequence star
• Surface gravity lower than typical sdB stars and very helium-rich
• It is therefore a He-sdB star. Also does not appear to be a binary
• Overall metalicity [Fe/H] = -0.3 except C, N and O
• Atmospheric physical parameters and chemical parameters can be
explained by the late flash model however the mechanism for mass loss
is not known
LSIV-14 116 - pulsating He-sdB
• Pulsating star allow us to probe the interiors hence test stellar
evolution theories
• Pulsation in sdB stars first discovered by Kilkenny et al. 1996
• Two main types - short periods ~ 200s and long periods ~ 45 min
• Jeffery & Saio 1996 suggest a systematic study of variability in HesdB
• Systematic search for pulsation carried out for 18 stars at SAAO in
2003. However no conclusive evidence found for short period
pulsations
• 3 suspected pulsators were monitored in 2004 May at the SAAO 1.0m
• LSIV-14 116 not a suspect but more like a target of opportunity
Light Curve and Periodogram
To conclude
• He-sdB stars are a group of very in-homogenous group of stars
• Physical parameters indicate they are very distinct from normal
subdwarf B stars
• Evolution still under debate
• A lot more has to be learnt about these hydrogen deficient stars…