The Selection of Rr Lyrae Stars Using Single-epoch Data

We use a complete sample of RR Lyrae stars discovered by the Quasar Equatorial Survey Team survey using light curves to design selection criteria based on Sloan Digital Sky Survey (SDSS) colors. Thanks to the sensitivity of the u g color to surface gravity and of the g r color to effective temperature and to the small photometric errors ( 0.02 mag) delivered by SDSS, RR Lyrae stars can be efficiently and robustly recognized even with singleepoch data. In a 100% complete color-selected sample, the selection efficiency (the fraction of RR Lyrae stars in the candidate sample) is 6%, and, by adjusting color cuts, it can be increased to 10% with a completeness (the fraction of selected RR Lyrae stars) of 80% and to 60% with 28% completeness. Such color selection produces samples that are sufficiently clean for statistical studies of the Milky Way’s halo substructure, and we use it to select 3,643 candidate RR Lyrae stars from SDSS Data Release 1. We demonstrate that this sample recovers known clumps of RR Lyrae stars associated with the Sgr dwarf tidal tail and the Palomar 5 globular cluster and use it to constrain the halo substructure away from the Sgr dwarf tidal tail. These results suggest that it will be possible to study the halo substructure out to 70 kpc from the Galactic center in the entire area imaged by the SDSS, and not only in the multiply observed regions.