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 non-uniform interstellar extinction toward the observed fields. We 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. Similar test is performed for the most reddened region observed in the LMC. We find that while the slope of the location of red clump stars in the color-magnitude diagram (CMD) in the LMC is very similar to the slope of the reddening line following the standard extinction law (R_V=3.1), they significantly differ in the Galactic bulge CMDs. We discuss possible systematic effects that could be responsible for the observed discrepancy. Anomalous extinction toward the Galactic bulge seems to be the most natural explanation. Our data indicate that, generally, the ratio of the total to selective absorption, R_VI, is much smaller toward the Galactic bulge than the standard value of 2.5. However, it varies from one line-of-sight to another. Our results nicely explain too red colors of red clump and RR Lyr stars in the Baade's window compared to the local population when dereddened with standard value of R_VI