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Issue A&A Volume 410, Number 3, November II 2003 Page(s) 905 - 909 Section Formation, structure and evolution of stars DOI 10.1051/0004-6361:20031263

A&A 410, 905-909 (2003)
DOI: 10.1051/0004-6361:20031263

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

P. Patriarchi¹, L. Morbidelli¹ and M. Perinotto<sup>2

1</sup>  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

(Received 17 October 2002 / Accepted 24 July 2003 )

Abstract
We present new measurements of the interstellar reddening parameter R_V=A_V/E(B-V) 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  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  . All objects whose  R_V departs from this value by more than 2  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.

Key words: ISM: dust, extinction

Offprint request: P. Patriarchi, pat@arcetri.astro.it

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