asteroseismology of pulsating sdb stars simon jeffery (armagh observatory) vik dhillon (sheffield...

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

• subluminous B stars• origin of sdB stars• pulsations in sdB stars• ultracam • colorimetry• nrp mode evaluation

faint blue stars in the Galactic haloGreenstein 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, 305Primarily 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 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.

NGC2808: Brown et al. 2001

evolution of sdBs

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?

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.

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 + 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)

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

Koen 1998, MNRAS

and also

Billères et al. 1998, ApJL

KPD2109+4401

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

astero-seismology 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=4, m=0 l=4, m=3

l=4, m=1l=20, m=10

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.

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.

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

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.

3 frame transfer CCDs with dual readout:windows optimized to required time resolution

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

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

ax’/au’

1

r’g’u’

l=2

l=1

l=0

Amplitude Ratio Diagram

Evolution tracks for extended horizontal branch stars (Charpinet et al. 2002)

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)

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