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asteroseismology of pulsating sdB starsSimon 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
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faint blue stars in the Galactic haloGreenstein and Sargent 1974, ApJS 28, 157.The nature of faint blue stars in the halo. II
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Palomar Green survey of faint blue objects
Green Schmidt and Liebert 1986, ApJS 61, 305Primarily a qso survey
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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 post-AGB 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.
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NGC2808: Brown et al. 2001
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evolution of sdBs
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horizontal branch stars and normal stellar evolution
Post-GB and He-flashHe-burning core 0.5 MH-rich envelope 0.4 M/ metal-rich - red HB 0.2 M/ metal-poor - blue HB0. - .05 M - EHB / sdB
Problems: How does RGB star lose its entire H envelope?How does it still suffer He-flash?
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stellar evolution
with mass loss on the giant
branchBrown, 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.
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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
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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 + BSinitial 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)
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pulsations in sdBs
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a comedy of errors...
SAAO: high-speed photometry of pulsating white dwarf candidate EC14026-2647 (Kilkenny et al. 1997)
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Frequency Period Amplitude
(mHz) (s) (mmag)
6.930 144 10.2-12.5
7.490 133 3.5-4.5
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sdB stars and
pulsational instability
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Koen 1998, MNRAS
and also
Billères et al. 1998, ApJL
KPD2109+4401
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pulsators, non-pulsators and not yet observed sdBs vs. number of unstable l=0 models
from Charpinet et al. 2001, PASP
(now ~ 10 more pulsators)
sdB stars and
pulsational instability
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astero-seismology of
sdBs
comparison of number of excited frequencies and period ranges for observed and model sdB stars
Charpinet et al. 2001, PASP
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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
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nonradial oscillations
l=4, m=0 l=4, m=3
l=4, m=1l=20, m=10
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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 harmonicsrelated 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.
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Mode identification:l normally small
n from frequenciesm 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.
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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)
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coadded spectra
F star spectrum
H H
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radial velocities and frequency
analysisCross-correlate individual
spectra against template to obtain wavelength displacement .
Displacement corresponds to Doppler shift or radial velocity.
v/cPlot velocities as function of
time.Compute Fourier transform
of velocities to identify periods.
Compare peaks with periods identified from photometry.
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3 frame transfer CCDs with dual readout:windows optimized to required time resolution
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4 nights: 1 wiped out 1.2 cloudy
KPD2109+4401: mB~1310 hrs, 93000 CCD frames ~1s
HS0039+4302: mB~15 16 hrs, 43200 CCD frames ~4s
observations 2002
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ultracam light curve for KPD2109+4402
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ultracam light curve for HS0039+4302
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sampling window functions
KPD 2109+4401 HS 0039+4302
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KPD 2109+4401: light curve and fit, power spectrum + residual
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HS 0039+4302: light curve and fit, power spectrum + residual
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colour variations and the amplitude ratio diagram
ax’/au’
1
r’g’u’
l=2
l=1
l=0
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Amplitude Ratio Diagram
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Evolution tracks for extended horizontal branch stars (Charpinet et al. 2002)
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Linear pulsation models for EHB stars (Charpinet et al. 2002)
• Observations: – 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)
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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
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the future
2004 Autumn: WET campaign on PG0014+067+ WHT/ultracam
2005+ …
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quick look ultracam
light curve for
PG0014+067
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