Search for p-mode oscillations in DA white dwarfs with VLT-ULTRACAM. I. Upper limits to the p-modes

Abstract

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. 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 5x10**(-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 4 sigma 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, ... (see the paper)

7 pages, 6 figures, Astronomy & Astrophysics in press

References in corpus (5)

• Pulsating White Dwarf Stars and Precision Asteroseismology
• ULTRACAM: an ultra-fast, triple-beam CCD camera for high-speed astrophysics
• On the Purity of the ZZ Ceti Instability Strip: Discovery of More Pulsating DA White Dwarfs on the Basis of Optical Spectroscopy
• Towards an Empirical Determination of the ZZ Ceti Instability Strip
• Optical variability of the ultracool dwarf TVLM 513-46546: evidence for inhomogeneous dust clouds

Cited by in corpus (9)

• Discovery of pulsations, including possible pressure modes, in two new extremely low mass, He-core white dwarfs
• Ion-Acoustic Envelope Modes in a Degenerate Relativistic Electron-Ion Plasma
• Pulsating low-mass white dwarfs in the frame of new evolutionary sequences: I. Adiabatic properties
• Radial pulsation as a function of hydrogen abundance
• Quasi-periodic oscillations in accreting magnetic white dwarfs I. Observational constraints in X-ray and optical
• Puoko-nui: a flexible high-speed photometric system
• Stability of nucleus-acoustic waves in completely degenerate white dwarf cores and their nearly degenerate ambience
• Asteroseismology of the pulsating extremely low-mass white dwarf SDSS J111215.82+111745.0: a model with $p$-mode pulsations consistent with the observations
• On the possibility of using seismic probes to study the core composition in pulsating white dwarfs