The unusual pulsation spectrum of the cool ZZ Ceti star HS 0507+0434B

Abstract

We present the analysis of one week of single-site high-speed CCD photometric observations of the cool ZZ Ceti star HS 0507+0434B. Ten independent frequencies are detected in the star's light variations: one singlet and three nearly-equally spaced triplets. We argue that these triplets are due to rotationally split modes of spherical degree l=1. This is the first detection of consistent multiplet structure in the amplitude spectrum of a cool ZZ Ceti star and it allows us to determine the star's rotation period: 1.70 +/- 0.11 d. We report exactly equal frequency, not period, spacings between the detected mode groups. In addition, certain pairs of modes from the four principal groups have frequency ratios which are very close to 3:4 or 4:5; while these ratios are nearly exact (within one part in 10^4), they still lie outside the computed error bars. We speculate that these relationships between different frequencies could be caused by resonances. One of the three triplets may not be constant in amplitude and/or frequency. We compare our frequency solution for the combination frequencies (of which we detected 38) to Wu's (1998, 2001) model thereof. We obtain consistent results when trying to infer the star's convective thermal time and the inclination angle of its rotational axis. Theoretical combination-frequency amplitude spectra also resemble those of the observations well, and direct theoretical predictions of the observed second-order light-curve distortions were also reasonably successful assuming the three triplets are due to l=1 modes. Attempts to reproduce the observed combination frequencies adopting all possible l=2 identifications for the triplets did not provide similarly consistent results, supporting their identification with l=1.Comment: Accepted for publication in MNRAS; 12 pages, 8 figure

Observations of the Pulsating White Dwarf G 185–32

“…However, we do detect splittings around 141 s and 71 s. If we assume the observed frequencies are due to the rotational splitting of an = 2 mode, then P rot 0.7 h, which is fast compared to the P rot 1 day observed for other white dwarf stars, including the DAVs G 226-29, GD 385 (Kepler et al 1995), and HS 0507+434B, which has a period of 1.7 day (Handler et al 2002). In spite of the no detection by TC of any velocity variation at any frequency, indicating that the angle between the pulsations axis and the line-of-sight is 90 deg if the pulsations are m = 0, we detected the largest number of simultaneous pulsation of any ZZ Ceti star.…”

White dwarf spins from low-mass stellar evolution models

“…The dashed horizontal line indicates the spectroscopic upper limit on the white dwarf spins obtained by Berger et al (2005). Star symbols represent astroseismic measurements from ZZ Ceti stars (Bradley 1998(Bradley , 2001Dolez 2006;Handler 2001;Handler et al 2002;Kepler et al 1995;Kleinmann et al 1998;Winget et al 1994), where smaller symbols correspond to less certain measurements. The green hatched area is populated by magnetic white dwarfs (Ferrario & Wickramasinghe 2005;Brinkworth et al 2007).…”

Seismology of a Massive Pulsating Hydrogen Atmosphere White Dwarf

“…SDSS J132350.28+010304.22 has a mass higher than the average mass of DAs (Kepler et al 2007;Tremblay et al 2010), has an effective temperature close the blue edge of the ZZ Ceti instability strip, even though the blue edge is mass dependent, and has long pulsation periods. Kepler et al (1983), Handler et al (2002), and Dolez et al (2006), for example, used rotational splittings observed in some ZZ Ceti stars to estimate rotation periods of order of 1 day. We do not detect multiplets from rotational or magnetic splitting in this star, but such detections require longer and denser observations than reported here.…”