Ensemble characteristics of the ZZ Ceti stars

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Mukadam, Anjum Shagufta

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Global 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.