Classification and properties of UV extinction curves

¹ Dipartimento di Astronomia, Vicolo Osservatorio 3, 35122 Padova, Italy
² Osservatorio Astronomico, Vicolo Osservatorio 5, 35122 Padova, Italy
³ CAISMI-CNR, Firenze, Italy
⁴ Dipartimento di Astronomia, Firenze, Italy

G. Barbaro

Received: 15 February 2000
Accepted: 2 October 2000

Abstract

The catalog of Savage et al. ([CITE]) reporting colour excesses of 1415 stars from ANS photometry offers the opportunity to deeply investigate the characteristics of UV extinction curves which differ from the standard extinction of the diffuse interstellar medium. To this aim we have selected a sample of 252 curves, which have been compared with the relations derived by Cardelli et al. ([CITE]; CCM in the following) for a variety of R_V values in the range 2.4-5 and have been classified as normal if they fit at least one of the CCM curves or anomalous otherwise. We find that normal curves with small R_V are just as numerous as those with large R_V. The anomalous objects are arranged into two groups according to the strength of the bump at . For a given value of c₂ this increases along the sequence: type A anomalous, normals and type B anomalous, suggesting that this sequence should correspond to an increase of the amount of small grains along the sightline. Considerations concerning the environmental characteristics indicate that the anomalous behaviour is not necessarily tied to the existence of dense gas clouds along the line of sight.