Multicolour high-speed photometry of pulsating subdwarf B stars with ULTRACAM

ABSTRACT High-speed multicolour photometry is presented for two pulsating subdwarf B stars, KPD 2109+4401 and HS 0039+4302. The observations were obtained using the high-speed multichannel photometer ULTRACAM on the 4.2-m William Herschel Telescope with a sampling interval of 1 and 4 s, respectively...

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Bibliographic Details
Published in:Monthly notices of the Royal Astronomical Society 2004-08, Vol.352 (2), p.699-707
Main Authors: Jeffery, C. S., Dhillon, V. S., Marsh, T. R., Ramachandran, B.
Format: Article
Language:English
Subjects:
stars: individual: HS 0039+4302
stars: individual: KPD 2109+4401
stars: interiors
stars: oscillations
stars: variables: other
subdwarfs
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Summary:ABSTRACT High-speed multicolour photometry is presented for two pulsating subdwarf B stars, KPD 2109+4401 and HS 0039+4302. The observations were obtained using the high-speed multichannel photometer ULTRACAM on the 4.2-m William Herschel Telescope with a sampling interval of 1 and 4 s, respectively, for the two targets. Both targets show multiperiodic oscillations, generally considered to be due to p-mode pulsations. The frequency spectra are similar to those observed previously, although the amplitudes of some modes have altered. The observations are compared with theoretical multicolour light curves for non-radially oscillating extreme horizontal branch stars using the amplitude ratio method. The radial and spherical degrees n and l of all unambiguously identified frequencies have been determined. In general, n≤ 3 and l≤ 2, but both stars show one l= 4 mode. The spectra of frequency versus spherical degree are compared with models for evolved extended horizontal branch stars. These confirm that KPD 2109+4401 has a post-zero-age horizontal branch age of approximately 47 Myr and an envelope mass ∼0.0002 M⊙. HS 0039+4302 lies on the upper edge of the horizontal branch and hence its envelope mass and age are less well determined spectroscopically. The pulsation properties suggest a more massive envelope and evolved structure. In both stars, the frequencies of the radial (l= 0) modes, if correctly identified, do not match the selected models well, suggesting that the density structure or opacity in the stellar envelopes may be incorrect.
ISSN:0035-8711
1365-2966
DOI:10.1111/j.1365-2966.2004.07960.x