2009A&A...498..455O

The distance to an isolated dark globule is often unknown and yet crucial for understanding its properties, in particular its mass. A new approach to this problem is discussed The purpose of the present paper is to investigate how well the distances of more or less reddened field stars can be determined by using multi-colour imaging. We observed a test globule, B335 in U, B, g, r, and I, and together with the 2MASS survey, this data set gives a well-defined spectral energy distribution (SED) of a large number of stars. The SED of each star depends on the interstellar extinction, the distance to the star, and its intrinsic SED. As we had good reasons to suspect that the wavelength dependence of the extinction (the reddening) changes from the outskirts of the globule to the central parts, we did not assume any specific reddening law. Instead, we use a scheme that allows independent determination of the extinction in each line of sight as determined by groups of adjacent stars. The method is based on the use of stellar atmospheric models to represent the intrinsic SEDs of the stars. Formally, it is then possible to determine the spectral class of each star and thereby its distance. For some of the stars we have optical spectra, allowing us to compare the photometric classification to the spectrometric. As expected, the main problem is that there are few stars found within each distance bin for the small field size defining a typical dark globule. However, the characterisation of the extinction and photometric classification give consistent results and we can identify one star at the front side of the globule. It has a photometric distance of 90pc. The closest star behind the B335 globule has a distance of only ≃120pc and we therefore determine the distance to B335 as 90-120pc. Our deep U image shows a relatively bright south-western rim of the globule, and we investigate whether it might be due to a local enhancement of the radiation field. A candidate source, located 1.5 arcmin outside our field, would be the field star, HD184982. This star has an entry in the Hipparcos Catalogue and its distance is 140-200pc. However, we come to the conclusion that the bright SW rim is more likely due to the wing of the point-spread-function (PSF) of this star.