INTERSTELLAR REDDENING FROM THE HIPPARCOS AND TYCHO CATALOGS - I - DISTANCES TO NEARBY MOLECULAR CLOUDS AND STAR-FORMING REGIONS

The Hipparcos and Tycho Catalogues offer an interesting possibility to study the local distribution of interstellar reddening from the combi nation of data contained in the catalogues: Hipparcos parallaxes, Tych o B - V and spectral and luminosity classification compiled from the l iterature. Parallactic distances may be derived for known absorbing fe atures such as local molecular clouds and for large scale features suc h 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 interste llar 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 rho 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 EB-V approximate to 0.15 (or A(V) approximat e to 0.5) appears at 50 pc in this region. A distance of 170 pc is fou nd for CrA compared to the previous estimate of 129 pc, and finally 12 0 pc for rho Oph compared to the previous 160 pc, strictly speaking th e 120 pc are only measured for extinction values typical for the off c ore region in rho Ophiuchus. These distance changes are of some import ance since these four regions show different stages of the star formin g activity, as judged from the relative distribution of Class 0 - Clas s III YSOs (young stellar objects) in the L-bol - T-bol diagram. Preci se calibrations of the YSOs' bolometric luminosities, applied in the d efinition of the bsf parameter (bsf: bright star fraction), require ac curate distances of their parental clouds unless they are based on ind ividual distances of the pre main sequence stars/protostellar sources.