Mira variables in the Galactic bulge with OGLE-II data

Abstract

We have extracted a total of 1968 Mira variables from the Optical Gravitational Lensing Experiment II (OGLE-II) data base in the Galactic bulge region. Among them, 1960 are associated with 2 Micron All-sky Survey (2MASS) sources, and 1541 are further identified with Midcourse Space Exploration (MSX) point sources. Their photometric properties are compared with those of Mira variables in the Large and Small Magellanic Clouds. We have found that mass-losing stars with circumstellar matter are reddened such that the colour dependence of the absorption coefficient is similar to that of interstellar matter. We also discuss the structure of the bulge. The surface number density of the bulge Mira variables is well correlated with the 2.2-μm surface brightness obtained by the Cosmic Background Explorer (COBE) satellite. Using this relation, the total number of Mira variables in the bulge is estimated to be about 6 × 105. The log P-K relation of the Mira variables gives their space distribution which supports the well-known asymmetry of the bar-like bulge.