Search for p-mode oscillations in DA white dwarfs with VLT-ULTRACAM

I. Upper limits to the p-modes ^⋆

¹ INAF–Osservatorio Astronomico di Torino, strada dell’Osservatorio 20, 10025 Pino Torinese, Italy
e-mail: silvotti@oato.inaf.it
² Département de Physique, Université de Montréal, CP 6128, Succ. Centre-Ville, Montréal, Québec H3C 3J7, Canada
e-mail: fontaine@astro.umontreal.ca
³ Sternberg Astronomical Institute, Universitatskij Prospect 13, Moscow, Russia
e-mail: pav@sai.msu.ru
⁴ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
e-mail: t.r.marsh@warwick.ac.uk
⁵ Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK
e-mail: vik.dhillonr@sheffield.ac.uk; s.littlefair@sheffield.ac.uk
⁶ INAF–Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
e-mail: tig@oacn.inaf.it

Received: 5 July 2010
Accepted: 6 October 2010

Abstract

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)).