Ensemble characteristics of the ZZ Ceti stars

textGlobal pulsations of stars can be used to probe their interiors, similar to the method of using earthquakes to explore the Earth’s interior. This technique, called asteroseismology, is the only systematic way to study stellar interiors. White dwarf stars represent a relatively simple stellar end state for most main sequence stars like the Sun. This is because they are not expected to have any central nuclear fusion and their evolution is dominated by cooling. These stars are scientifically interesting since they contain a fossil record of their previous evolution. Their high densities and temperatures make them good cosmic laboratories to study fundamental physics under extreme conditions. Besides, white dwarfs are not as centrally condensed as some other classes of variables, and hence the observed pulsations sample their interior better. Each pulsation mode is an independent constraint on the structure of the star. We can probe stellar structure and composition by finding a single star rich in pulsation modes, and/or by finding a large number of pulsators to use the method of ensemble asteroseismology. A fraction of white dwarf pulsators are observed to be extremely stable clocks; this property allows us to look for any orbiting planets. The drift rates of these stable clocks are expected to reveal the stellar cooling rate. Including this information in evolutionary white dwarf models allows us to determine the age of the star. Since most stars evolve into white dwarfs, we can use the distribution of white dwarf ages in different parts of the Galaxy to constrain the age of the Galaxy and its evolution. Variable white dwarfs can also be used as a means to measure Galactic distances. All these reasons motivate us to search for additional white dwarf pulsators. Four out of five white dwarfs show hydrogen in their outermost layers and are classified as DAs. These are observed to pulsate in a temperature range of 11000–12000 K. I decided to search specifically for DA white dwarf variables (DAVs), also known as ZZ Ceti stars. To substantially increase the sample of ZZ Ceti stars, I was forced to search at greater distances (or fainter magnitudes). This is because various research groups around the world have already examined the relatively nearby (or bright) candidates for variability. Hence, I helped Dr. R. E. Nather in building a high speed time-series CCD photometer for the prime focus of the 2.1m telescope at McDonald Observatory. This CCD instrument allows us to obtain usable time-series data on 19th magnitude objects, as opposed to a limiting magnitude of 17 with our previous instrument. The combination of an efficient new instrument and a large amount of telescope time (' 100 nights/yr) gave me a unique opportunity to search extensively for new ZZ Ceti stars. Other members of my research group also contributed towards the 15 month long observations at McDonald Observatory, and helped me in data analyses. We pre-selected candidates by using the photometric and spectroscopic observations of the Sloan Digital Sky Survey. I present 35 new pulsating DA (hydrogen atmosphere) white dwarf stars discovered from the Sloan Digital Sky Survey (SDSS) and the Hamburg Quasar Survey (HQS). This increases the sample of 39 known ZZ Ceti stars to 74; the first ZZ Ceti star was accidentally discovered in 1968. This is the first time in the history of white dwarf variables that we have a homogeneous set of spectra acquired using the same instrument on the same telescope, and with consistent data reductions, for a statistically significant sample of ZZ Ceti stars. The homogeneity of the spectra reduces the scatter in the spectroscopic temperatures; we have essentially re-defined the ZZ Ceti instability strip. We find a narrow ZZ Ceti strip of width ' 1000 K, as opposed to the previous determination of 1500 K. We question the purity of the DAV instability strip as we find several non-variables within. We present our best fit for the red (cool) edge and our constraint for the blue (hot) edge of the instability strip, determined using a statistical approach. I also present the observed pulsation spectra of 67 ZZ Ceti stars with reliable spectroscopic temperatures. I verify the well-established relation of the increase in observed pulsation periods and amplitudes for the new ZZ Ceti stars, traversing from the blue to the red edge of the instability strip. The data on the new ZZ Ceti stars suggests that pulsation amplitude declines prior to the red edge. This means that ZZ Ceti pulsations do not shut down abruptly at the red edge of the instability strip. This is the first possible detection of such an effect.Astronom

Ensemble characteristics of the ZZ Ceti stars

We present the observed pulsation spectra of all known noninteracting ZZ Ceti stars (hydrogen atmosphere white dwarf variables [DAVs]) and examine changes in their pulsation properties across the instability strip.We confirm the well-established trend of increasing pulsation period with decreasing effective temperature across the ZZ Ceti instability strip. We do not find a dramatic order-of-magnitude increase in the number of observed independent modes in ZZ Ceti stars, traversing from the hot to the cool edge of the instability strip; we find that the cool DAVs have one more mode on average than the hot DAVs. We confirm the initial increase in pulsation amplitude at the blue edge and find strong evidence of a decline in amplitude prior to the red edge.We present the first observational evidence that ZZ Ceti stars lose pulsation energy just before pulsations shut down at the empirical red edge of the instability strip

Evolutionary timescale of the pulsating white dwarf G117-B15A : the most stable optical clock known

We observe G117-B15A, the most precise optical clock known, to measure the rate of change of the main pulsation period of this blue-edge DAV white dwarf. Even though the obtained value is only within 1 σ, P = (2.3±1.4) x 10 -15 s s-ˡ, it is already constraining the evolutionary timescale of this cooling white dwarf star

Evidence for temperature change and oblique pulsation from light curve fits of the pulsating white dwarf GD 358

Convective driving, the mechanism originally proposed by Brickhill for pulsating white dwarf stars, has gained general acceptance as the generic linear instability mechanism in DAV and DBV white dwarfs. This physical mechanism naturally leads to a nonlinear formulation, reproducing the observed light curves of many pulsating white dwarfs. This numerical model can also provide information on the average depth of a star’s convection zone and the inclination angle of its pulsation axis. In this paper, we give two sets of results of nonlinear light curve fits to data on the DBV GD 358. Our first fit is based on data gathered in 2006 by the Whole Earth Telescope; this data set was multiperiodic containing at least 12 individual modes. Our second fit utilizes data obtained in 1996, when GD 358 underwent a dramatic change in excited frequencies accompanied by a rapid increase in fractional amplitude; during this event it was essentially monoperiodic. We argue that GD 358’s convection zone was much thinner in 1996 than in 2006, and we interpret this as a result of a short-lived increase in its surface temperature. In addition, we find strong evidence of oblique pulsation using two sets of evenly split triplets in the 2006 data. This marks the first time that oblique pulsation has been identified in a variable white dwarf star

Evidence for temperature change and oblique pulsation from light curve fits of the pulsating white dwarf GD 358

Convective driving, the mechanism originally proposed by Brickhill for pulsating white dwarf stars, has gained general acceptance as the generic linear instability mechanism in DAV and DBV white dwarfs. This physical mechanism naturally leads to a nonlinear formulation, reproducing the observed light curves of many pulsating white dwarfs. This numerical model can also provide information on the average depth of a star’s convection zone and the inclination angle of its pulsation axis. In this paper, we give two sets of results of nonlinear light curve fits to data on the DBV GD 358. Our first fit is based on data gathered in 2006 by the Whole Earth Telescope; this data set was multiperiodic containing at least 12 individual modes. Our second fit utilizes data obtained in 1996, when GD 358 underwent a dramatic change in excited frequencies accompanied by a rapid increase in fractional amplitude; during this event it was essentially monoperiodic. We argue that GD 358’s convection zone was much thinner in 1996 than in 2006, and we interpret this as a result of a short-lived increase in its surface temperature. In addition, we find strong evidence of oblique pulsation using two sets of evenly split triplets in the 2006 data. This marks the first time that oblique pulsation has been identified in a variable white dwarf star

New pulsating db white dwarf stars from the Sloan Digital Sky Survey

We are searching for new He atmosphere white dwarf pulsators (DBVs) based on the newly found white dwarf stars from the spectra obtained by the Sloan Digital Sky Survey. DBVs pulsate at hotter temperature ranges than their better known cousins, the H atmosphere white dwarf pulsators (DAVs or ZZ Ceti stars). Since the evolution of white dwarf stars is characterized by cooling, asteroseismological studies of DBVs give us opportunities to study white dwarf structure at a different evolutionary stage than the DAVs. The hottest DBVs are thought to have neutrino luminosities exceeding their photon luminosities, a quantity measurable through asteroseismology. Therefore, they can also be used to study neutrino physics in the stellar interior. So far we have discovered nine new DBVs, doubling the number of previously known DBVs. Here we report the new pulsators’ light curves and power spectra

Measuring the evolution of the most stable optical clock G 117-B15A

We report our measurement of the rate of change of period with time ( ˙ P) for the 215s periodicity in the pulsating white dwarf G 117–B15A, the most stable optical clock known. After 31 years of observations, we have finally obtained a 4σ measurement ˙ Pobserved = (4.27 ± 0.80) × 10−15 s/s. Taking into account the proper-motion effect of ˙ Pproper = (7.0±2.0)×10 −16 s/s, we obtain a rate of change of period with time of P ˙ −15 = (3.57 ± 0.82) × 10 s/s. This value is consistent with the cooling rate in our white dwarf models only for cores of C or C/O. With the refinement of the models, the observed rate of period change can be used to accurately measure the ratio of C/O in the core of the white dwarf