Transcript ultracam observations of pulsating sdB stars

asteroseismology of
pulsating sdB stars
Simon Jeffery (Armagh Observatory)
Vik Dhillon (Sheffield University)
Tom Marsh (Warwick University)
Ramachandran (Armagh Observatory)
Conny Aerts, Paul Groot (Nijmegen)
MNRAS: July 2004
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subluminous B stars
origin of sdB stars
pulsations in sdB stars
ultracam
colorimetry
nrp mode evaluation
faint blue stars in the Galactic halo
Greenstein and Sargent 1974, ApJS 28, 157.
The nature of faint blue stars in the halo. II
Palomar Green survey of faint blue objects
Green Schmidt and Liebert 1986, ApJS 61, 305
Primarily a qso survey
UV excess in giant
elliptical galaxies
Excess flux observed in early UV
galaxy surveys. Seen as upturn in
flux shortward of c. 1300 A in
elliptical galaxies (Burstein et al.
1988 ApJ 328, 440)
Hypothesized to be due to postAGB and extreme horizontal branch
stars (Greggio & Renzini 1990 ApJ
364, 35)
Demonstrated by Brown et al. (1997
ApJ 482, 685) using HUT data for
M60 and other ellipticals.
NGC2808: Brown et al. 2001
evolution of sdBs
horizontal branch stars and
normal stellar evolution
Post-GB and He-flash
He-burning core 0.5 M
H-rich envelope
0.4 M/ metal-rich - red HB
0.2 M/ metal-poor - blue HB
0. - .05 M - EHB / sdB
Problems:
How does RGB star lose its
entire H envelope?
How does it still suffer Heflash?
stellar evolution
with mass loss on
the giant branch
Brown, Sweigart, Lanz,
Landsman & Hubeny
2001, ApJ 562, 368
If star reaches within
0.25mag of RG tip, a
helium flash will occur.
Final position on ZAHB
depends on Menv.
Mass loss could be RLOF as
binary on RGB.
origin of sdB stars
Binary evolution is important in at least 2/3 of sdBs (Green,
Liebert & Saffer, 2001, ASP 226). Key factor is Roche Lobe
Overflow in metal-rich low-mass giants near the Red Giant
Tip. Group III (composite) sdBs are the key:
i. low-mass binary with initial separation 415-520 R
ii. secondary has mass 0.7-0.9 M
iii. primary Roche lobe radius = 155-185 R < RGB tip radius
iv. at initial Roche lobe overflow, secondary accepts 0.3 M
dynamical mass transfer without overflowing its own Roche
lobe
v. mass ratio inverts, further mass transfer increases orbital
separation, no common envelope phase
vi. secondary now a blue straggler with mass 1.0-1.2 M
origin of sdB stars: II
Other binary outcomes depend on initial separation, masses and
mass ratio.
initial primary  sdB
sdB + dM (IIB)
sdB + BS (III)
initial primary  HeWD
HeWD + dM (pre-CV)
HeWD + BS
initial secondary  sdB
HeWD + sdB (IIA)
Evolution that produces single sdB stars (I) include:
enhanced mass loss from single stars (d’Cruz et al. 1996)
merger of two He WDs (Iben 1990, Saio & Jeffery 2000)
pulsations in sdBs
a comedy of errors...
SAAO: high-speed
photometry of pulsating
white dwarf candidate
EC14026-2647
(Kilkenny et al. 1997)
Frequency
Period Amplitude
(mHz)
(s)
(mmag)
6.930
144
10.2-12.5
7.490
133
3.5-4.5
sdB stars and
pulsational
instability
KPD2109+4401
Koen 1998, MNRAS
and also
Billères et al. 1998, ApJL
sdB stars
and
pulsational
instability
pulsators, nonpulsators and not
yet observed
sdBs vs. number
of unstable l=0
models
from Charpinet et
al. 2001, PASP
(now ~ 10 more
pulsators)
asteroseismology of
sdBs
comparison of number
of excited frequencies
and period ranges for
observed and model sdB
stars
Charpinet et al. 2001,
PASP
asteroseismology
of PG1047+003
theoretical frequency
spectrum compared with an
observed power spectrum
adjust the stellar interior
model to match the observed
frequencies
Charpinet et al. 2001, PASP
nonradial oscillations
l=20, m=10
l=4, m=1
l=4, m=0
l=4, m=3
nonradial oscillations of stars
(simple version)
Nonradial oscillations (nro’s) are waves travelling through the
interior of a star. Surface displacement may be
characterized by spherical harmonic functions:
s = so Yl,m(,)
l: degree of the spherical harmonic
= number of lines of nodes on a spherical surface
m: azimuthal number
= number of lines of nodes passing through the polar axis
n: order of the spherical harmonics
related to number of nodes along the radial direction
In most non-radially oscillating stars (e.g. the Sun) many
modes are superposed. nro’s can affect total light, colour,
temperature, radial velocities and line profiles from a star.
Mode identification:
l normally small
n from frequencies
m from mode splitting ?
Normally difficult to disentangle n,l,m from light curve
alone…but:
If star is not rotating, m value does not alter frequency.
Ratio of photometric amplitude at different wavelengths
is independent of i, but sensitive to l.
Aim: Obtain additional information from multicolour light
curves to identify n and l, and compare with models.
observations 1998
WHT/ISIS
high speed spectroscopy
(drift mode)
PB8783:
5.3hrs, 1400 spectra,
(~8s)
KPD2109+4401:
5.8 hrs, 1200 spectra
(~10s)
Jeffery & Pollacco (2000)
coadded spectra
F star
spectrum
H
H
radial velocities
and frequency
analysis
Cross-correlate individual
spectra against template to
obtain wavelength
displacement .
Displacement corresponds to
Doppler shift or radial
velocity.
v/c
Plot velocities as function of
time.
Compute Fourier transform of
velocities to identify
periods.
Compare peaks with periods
identified from
photometry.
3 frame transfer CCDs with
dual readout:
windows optimized to
required time resolution
observations 2002
4 nights:
1 wiped out
1.2 cloudy
KPD2109+4401: mB~13
10 hrs,
93000 CCD frames ~1s
HS0039+4302: mB~15
16 hrs,
43200 CCD frames ~4s
ultracam light curve for
KPD2109+4402
ultracam light curve for
HS0039+4302
sampling window functions
KPD 2109+4401
HS 0039+4302
KPD 2109+4401: light curve and fit, power spectrum + residual
HS 0039+4302: light curve and fit, power spectrum + residual
colour variations
and the amplitude ratio diagram
1
l=2
ax’/au’
l=1
l=0
u’
g’
r’
Amplitude Ratio Diagram
Evolution tracks for extended horizontal branch stars
(Charpinet et al. 2002)
Linear pulsation models for EHB stars
(Charpinet et al. 2002)
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Observations:
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small v sin i  m splitting ~ 0
n,l pairs unique for each frequency
ax’/au’ l
given l, wave equations  simple
cadence in n
Theory:
– Linear analysis gives frequencies for
each mode in each model
– Plotted as l value versus frequency
(cf. chirp diagram for solar
oscillations)
– Lowest frequency is fn of stellar
radius
(cf models for KPD2109+4401)
– Frequency spacing is fn of envelope
structure
(cf models for HS0039+4302)
conclusions
• ultracam provides outstanding 3-channel light
curves for pulsating sdB stars down to 15th mag.
• amplitudes measurable to <0.5 mmag and new
frequencies identified
• g’/u’ persistently larger than r’/u’ - as expected
• l = 0,1,2 and 4 modes identified by ranking amp.
ratios
• n values assigned by demanding realistic cadence
from modes of same l
• Comparison with theoretical models - pointer to
powerful stellar structure diagnostics
the future
2004 Autumn: WET campaign on PG0014+067
+ WHT/ultracam
2005+ …
quick look
ultracam
light curve
for
PG0014+067