The optical gravitational lensing experiment: Is interstellar extinction toward the galactic center anomalous?

Photometry of the Galactic bulge collected during the OGLE II microlensing search indicates high and nonuniform interstellar extinction toward the observed fields. I use the mean I-band magnitude and V I color of red clump stars as a tracer of interstellar extinction toward four small regions of the Galactic bulge with highly variable reddening. A similar test is performed for the most reddened region observed in the LMC. I find that the slope of the location of red clump stars in the color-magnitude diagrams (CMDs) in the Galactic bulge is significantly smaller than the slope of the reddening line following the standard extinction law (R-V = 3.1) for approximations of the extinction curve by both Cardelli, Clayton, & Mathis and Fitzpatrick. The differences are much larger for the Cardelli, Clayton, & Mathis approximation which, on the other hand, indicates the same slopes for the control field in the LMC, contrary to the Fitzpatrick approximation. I discuss possible systematic effects that could cause the observed discrepancy. Anomalous extinction toward the Galactic bulge seems to be the most natural explanation. My data indicate that, generally, the ratio of the total to selective absorption, R-VI, is much smaller toward the Galactic bulge than the value corresponding to the standard extinction curve (R-V = 3.1). However, R-VI varies from one line of sight to another. My results explain why the red clump and RR Lyr stars in the Baade's window dereddened with standard value of R-VI are redder compared to those of the local population.