IRA/CNR, Largo E. Fermi 5, 50125 Firenze, Italy
2 Dipartimento di Astronomia e Scienza dello Spazio, Universitá di Firenze, Largo E. Fermi 2, 50125 Firenze, Italy
Corresponding author: P. Patriarchi, email@example.com
Accepted: 24 July 2003
We present new measurements of the interstellar reddening parameter towards 185 O stars, using J, H, photometry from the 2MASS project. The results are combined with data from the literature of 95 stars where RV has been derived with the same technique, 22 of which in common with our present sample from the 2MASS project catalogue. The average RV from these 258 O stars is of . All objects whose RV departs from this value by more than 2 σ have been recognized. Ten objects have RV higher than this value and two lower. It is found that anomalous RV 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 RV 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.
Key words: ISM: dust, extinction
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.
© ESO, 2003