1998A&A...338..897K

The Hipparcos and Tycho Catalogues offer an interesting possibility to study the local distribution of interstellar reddening from the combination of data contained in the catalogues: Hipparcos parallaxes, Tycho B - V and spectral and luminosity classification compiled from the literature. Parallactic distances may be derived for known absorbing features such as local molecular clouds and for large scale features such as sheets dividing bubbles. The stellar luminosity classes V and III offer more than 30.000 lines of sight for study, mostly for negative declinations where most classifications are available. We present some examples of this approach to derive information on the local interstellar medium. First we estimate the distance to the Southern Coalsack. Secondly, distances to the four nearby southern star forming clouds in the Chamaeleon region, the Lupus region, Corona Australis and finally the ρ Ophiuchi are estimated. We find that these clouds are at 150 (Cha), 100 pc (Lup), i.e. about 50 pc closer than previously estimated, and that a feature with E_B–V≃0.15 (or A_V≃0.5) appears at 50pc in this region. A distance of 170pc is found for CrA compared to the previous estimate of 129pc, and finally 120pc for ρ Oph compared to the previous 160pc, strictly speaking the 120pc are only measured for extinction values typical for the off core region in ρ Ophiuchus. These distance changes are of some importance since these four regions show different stages of the star forming activity, as judged from the relative distribution of Class 0 - Class III YSOs (young stellar objects) in the L_bol-T_bol diagram. Precise calibrations of the YSOs' bolometric luminosities, applied in the definition of the bsf parameter (bsf: bright star fraction), require accurate distances of their parental clouds unless they are based on individual distances of the pre main sequence stars/protostellar sources.