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Scientific paper
Oct 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004phdt........17m&link_type=abstract
Thesis (PhD). THE UNIVERSITY OF TEXAS AT AUSTIN, Source DAI-B 65/08, p. 4067, Feb 2005, 147 pages.
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2
Scientific paper
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 2.1 m 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 ≃1000K, as opposed to the previous determination of 1500K. 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. (Abstract shortened by UMI.)
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