A rigorous reanalysis of the IRAS variable population: scale lenghts, asymmetries, and microlensing

Abstract

Previous work reported a bar signature in color-selected IRAS variable stars. Here, we estimate the source density of these variables while consistently accounting for spatial incompleteness in data using a likelihood approach. The existence of the bar is confirmed with shoulder of 4 kpc and position angle of 19 +/- 1 degrees. The ratio of non-axisymmetric to axisymmetric components gives similar estimate for the bar size 3.3 +/- 0.1 kpc and position angle 24 +/- 2 degrees. We estimate a scale length 4.00 +/- 0.55 kpc for the IRAS variable population, suggesting that these stars represent the old disk population. We use this density reconstruction to estimate the optical depth to microlensing for the large-scale bar in the Galactic disk. We find an enhancement over an equivalent axisymmetric disk by 30% but still too small to account for the MACHO result. In addition, we predict a significant asymmetry at positive and negative longitudes along lines of sight through the end of the bar with optical depths comparable to that in Baade's window. An infrared microlensing survey may be a sensitive tool for detecting or constraining structural asymmetries. More generally, this is a pilot study for Bayesian star count analyses. Bayesian approach allows the assessment of prior probabilities to the unknown parameters of the model; the resulting likelihood function is straightforwardly modified to incorporate all available data.