RR Lyrae Variable Star Distribution in the Galactic Halo

Abstract

From the CCD/Transit Instrument (CTI) survey, we generated a list of 42 RR Lyrae variable stars, 34 of which were previously undiscovered (Wetterer et al. 1996, Al, 112, 742). The CTI survey is unique when compared to other surveys in that its survey area covers a large range of Galactic latitude and longitude in a single field. In this paper we use these RR Lyrae variable stars to examine their number density from 7 to 35 kpc from the Galactic center. The RR Lyrae number density was found to follow a R<SUP>-3.34±O.11</SUP> power-law function when modeled as a spherically symmetric distribution and an α<SUP>-3.76±0.13</SUP> power-law function when modeled as the ellipsoidal distribution of Preston et al. (1991, ApJ, 375, 121). We then combined the CTI results with that of other surveys to provide a more complete picture of the RR Lyrae number density throughout the Galactic halo (0.6-80 kpc). A spherically symmetric model for the RR Lyrae number density yields a R<SUP>-3.024±0.077</SUP> power-law function, although better agreement between surveys is reached if the number density is modeled as an ellipsoidal distribution, which yields an α-3.530±0.077 power-law function. The wide range of number densities measured at the Sun's distance from the Galactic center (&gt;10<SUP>-3</SUP> for both the local number density as calculated from bright RR Lyrae stars and the CTI survey, to &lt;1 kpc<SUP>-3</SUP> for a fields in the Lick RR Lyrae Survey and the Palomar-Groningen Variable Star Survey), as well as the possible underdensity of RR Lyraes beyond 25 kpc, are also discussed.