Mira variables in the OGLE bulge fields

Abstract

The 222 000 I-band light curves of variable stars detected by the OGLE-II survey in the direction of the Galactic Bulge have been fitted and also correlated with the DENIS and 2MASS all-sky release databases and with lists of known objects. Lightcurves and the results of the lightcurve fitting (periods and amplitudes) and DENIS and 2MASS data are presented for 2691 objects with I-band semi-amplitude larger than 0.45 mag, corresponding to classical Mira variables. That the Mira period distribution of 6 fields at similar longitude but spanning latitudes from -1.2 to -5.8 are statistically indistinguishable indicates similar populations with initial masses of 1.5-2 M⊙, corresponding to ages of 1-3 Gyr. A field at similar longitude at b = -0.05 from Glass et al. (2001, MNRAS, 321, 77; erratum: 2002, MNRAS, 336, 1390) does show a significantly different period distribution, indicating the presence of a younger population of 2.5-3 M⊙ and ages below 1 Gyr. The K-band period-luminosity relation is presented for the whole sample and for sub-fields. The zero point depends on Galactic longitude. Simulations are carried out to show that the observed dependence of the zero point with l, and the number of stars per field are naturally explained using the model of disk and bulge stars of Binney et al. (1997, MNRAS, 288, 365), for a viewing angle (major-axis Bar - axis perpendicular to the line-of-sight to the Galactic Centre) of 43 ±17 degrees. The simulations also show that biases in the observed zero point are small, <0.02 mag. A comparison is made with similar objects in the Magellanic Clouds. The slope of the PL-relation in the Bulge and the MCs agree within the errorbars. Assuming the zero point does not depend on metallicity, a distance modulus difference of 3.72 between Bulge and LMC is derived. This implies a LMC DM of 18.21 for an assumed distance to the Galactic Centre (GC) of 7.9 kpc, or, assuming a LMC DM of 18.50, a distance to the GC of 9.0 kpc. From the results in Groenewegen (2004, A&A, 425, 595) it is found for carbon-rich Miras that the PL-relation implies a relative SMC-LMC DM of 0.38, assuming no metallicity dependence. This is somewhat smaller than the often quoted value near 0.50. Following theoretical work by Wood (1990, in From Miras to Planetary Nebulae, ed. M. O. Mennessier, & A. Omont (Gif-sur-Yvette: Éditions Frontières), 67) a metallicity term of the form MK ∼ β log Z is introduced. If a relative SMC-LMC DM of 0.50 is imposed, β= 0.4 is required, and for that value the distance to the GC becomes 8.6 ± 0.7 kpc (for a LMC DM of 18.50), within the errorbar of the geometric determination of 7.9 ± 0.4 kpc. An independent estimate leads to a distance estimate to the GC of 8.8 ± 0.4 kpc. © ESO 2005.