A study of R_V in Galactic O stars from the 2MASS catalogue

Abstract

We present new measurements of the interstellar reddening parameter R_V=A_V/E(B-V) towards 185 O stars, using J, H, K_s photometry from the 2MASS project. The results are combined with data from the literature of 95 stars where R_V has been derived with the same technique, 22 of which in common with our present sample from the 2MASS project catalogue. The average R_V from these 258 O stars is of 3.19 +/- 0.50. All objects whose R_V departs from this value by more than 2 sigma have been recognized. Ten objects have R_V higher than this value and two lower. It is found that anomalous R_V can scarcely be associated with anomalies in the general interstellar medium, e.g. with different behaviour in different spiral arms. They are clearly linked to local cloud effect. In the Cygnus region R_V values follow the behaviour of the general interstellar medium, while in the Carina arm, in spite of the relatively larger distance, local cloud effects prevail. An explanation for this is suggested. The relatively few stars of our sample whose Hipparcos parallaxes are reliable, are found to have distances systematically smaller than the distances derived by the spectroscopic parallaxes. We argue that this effect is consistent with the recently claimed discovery of grey extinction towards OB stars.

This publication makes use of data products from the Two Micron All Sky Survey (2MASS), which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.

Tables 1 and 2 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/410/905