| Summary: | 博士 === 國立中央大學 === 天文研究所 === 99 === The work contains two parts about using stellar photometric variation and astrometric variation to probe the stellar evolution and the Galactic structure. One is to search for variable stars in the Galactic Open Cluster NGC 7039 by the photometric variation. The time-series observations provide information of the transient photometry for population I in the Galactic disk. The other part in this work is to use the astrometric variation to find high proper motion white dwarfs. High proper motion objects can be used to investigate the stellar populations in, and the formation of, the Galaxy. The different moving orbits of the halo and the disk give us a way to find the population II stars such as ultracool and old white dwarfs in the halo.
We report variable stars identified in the field of the Galactic open cluster, NGC 7039. In the fall/winter of 2009, imaging photometry has been acquired, mainly in the R band, for 8 open clusters using the 81-cm Tenagra telescope in Arizona. We present the results for our first target, NGC 7039. We have found 42 new variable stars and 6 suspected variables stars. Among the variable stars, there are 14 eclipsing binaries including 1 RS Canum Venaticorum type system, 1 RR Lyrae, 2 Cepheid, 1 beta-Cephei, 3 delta-Scuti, 8 pulsating-like stars, 5 variable stars with periods longer than the observation window, 41 days, and 8 irregular variable stars. Two contact binary systems could be members of NGC 7039 because of their loci in the color-magnitude diagram and their distance moduli close to that of the open cluster. One of the contact binary systems shows infrared excess in the (J-H) and (H-K) diagram. It could has circumstellar dust. None of the pulsating stars belong to the open cluster.
For high proper motion white dwarfs, we present the results of a search
in the deep survey of the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS). The CFHTLS Deep Survey covers 4 square degrees in 5 filters (u*,g'',r'',i'',and z''). For the first and the fourth fields, D1 and D4, we use data for a five-year baseline from 2004 to 2009. For the second and the third fields, D2 and D3, we have a four-year baseline from 2004 to 2008. Proper motion selection is used to distinguish cool high velocity white dwarfs from distant objects with similar blue colors such as compact faint galaxies and quasars.
We discovered twenty white dwarf candidates brighter than g''=24 on the basis of their spectral energy distribution and reduced proper motions. We found four white dwarf candidates with effective temperatures less than to 4000 K. From their estimated velocities and distances they all appear to be located in the thin or thick disk of the Galaxy. One object with the effective temperatures 4500K has a high proper morion of 140 mas/yr. The object could be a halo object because of its high tangential velocity.
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