Mapping the Galactic halo. II. Photometric survey

摘要

We present imaging results from a high Galactic latitude survey designed to examine the structure of the Galactic halo. The objective of the survey is to identify candidate halo stars which can be observed spectroscopically to obtain radial velocities and confirm halo membership. The Washington filter system is used for its ability to distinguish between dwarfs and giants, as well as provide a metallicity indicator. Our most successful imaging run used the ETC camera on the CTIO 4 m telescope in 1999 April. Photometric conditions during these observations provided superb photometry, with average errors for a star at M = 18.5 of 0.009, 0.008, 0.011, and 0.009 for C, M, DDO51, and T-2, respectively. We use these data as a template to describe the details of our photometric reduction process. It is designed to perform CCD reductions and stellar photometry automatically during the observation run, without the aid of external packages, such as IRAF and IDL. We describe necessary deviations from this procedure for other instruments used in the survey up to 2000 June. Preliminary results from spectroscopic observations indicate a 97 efficiency in eliminating normal dwarfs from halo giant candidates for M < 18.5. Unfortunately, low-metallicity subdwarfs cannot be photometrically distinguished from giants using the Washington filters. These major contaminants unavoidably reduced the overall giant identification efficiency to 66 for M < 18.5. Our improved knowledge of these stars will increase this efficiency for future spectroscopic observations. References: 24