Volume 525, January 2011
|Number of page(s)||7|
|Section||Stellar structure and evolution|
|Published online||01 December 2010|
Search for p-mode oscillations in DA white dwarfs with VLT-ULTRACAM
I. Upper limits to the p-modes ⋆
INAF–Osservatorio Astronomico di Torino, strada dell’Osservatorio
2 Département de Physique, Université de Montréal, CP 6128, Succ. Centre-Ville, Montréal, Québec H3C 3J7, Canada
3 Sternberg Astronomical Institute, Universitatskij Prospect 13, Moscow, Russia
4 Department of Physics, University of Warwick, Coventry CV4 7AL, UK
5 Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK
e-mail: email@example.com; firstname.lastname@example.org
6 INAF–Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
Accepted: 6 October 2010
Aims. The main goal of this project is to search for p-mode oscillations in a selected sample of DA white dwarfs near the blue edge of the DAV (g-mode) instability strip, where the p-modes should be excited following theoretical models.
Methods. A set of high quality time-series data on nine targets has been obtained in 3 photometric bands (Sloan u′, g′, r′) using ULTRACAM at the VLT with a typical time resolution of a few tens of ms. Such high resolution is required because theory predicts very short periods, of the order of a second, for the p-modes in white dwarfs. The data have been analyzed using Fourier transform and correlation analysis methods.
Results.P-modes have not been detected in any of our targets. The upper limits obtained for the pulsation amplitude, typically less than 0.1%, are the smallest limits reported in the literature. The Nyquist frequencies are large enough to fully cover the frequency range of interest for the p-modes. For the brightest target of our sample, G 185-32, a p-mode oscillation with a relative amplitude of 5 × 10-4 would have been easily detected, as shown by a simple simulation. For G 185-32 we note an excess of power below ~2 Hz in all the three nights of observation, which might be due in principle to tens of low-amplitude close modes. However, neither correlation analysis nor Fourier transform of the amplitude spectrum show significant results. We also checked the possibility that the p-modes have a very short lifetime, shorter than the observing runs, by dividing each run in several subsets and analyzing these subsets independently. The amplitude spectra show only a few peaks with S/N ratio higher than 4σ but the same peaks are not detected in different subsets, as we would expect, and we do not see any indication of frequency spacing.
As a secondary result of this project, the detection of a new g-mode DAV pulsator near the blue edge of the ZZ Ceti instability strip was claimed (Silvotti et al. 2006, MmSAI, 77, 486) and will be described in detail in a forthcoming paper (Silvotti et al., A&A, in prep. (Paper II)).
Key words: white dwarfs / stars: oscillations
© ESO, 2010
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