Distances to nearby molecular clouds and star forming regions.III. Localizing extinction jumps with a Hipparcos calibration of 2mass photometry

We want to estimate the distance to molecular clouds in the solar vicinity in a statistically precise way. Clouds are recognized as extinction discontinuities. The extinction is estimated from the $(H-K) \ vs. \ (J-H)$ diagram and distances from a $(J-K)_0 \ vs. \ M_J$ relation based on Hipparcos. The stellar sample of relevance for the cloud distance is confined by the FWHM of the $A_V / D_{\star}(pc)$ or of its derivative. The cloud distance is estimated from fitting a function to the $(A_V, 1/ \pi_{JHK})$ pairs in this sample with a function like $arctanh^p (D_\star /D_{cloud})$ where the power $p$ and $D_{cloud}$ both are estimated. The fit follows the $(A_V, 1/\pi_{JHK})_{cloud}$ data rather well. Formal standard deviations less than a few times 10 pc seem obtainable implying that cloud distances are estimated on the $\lesssim$10$\%$ level. Such a precision allows estimates of the depths of cloud complexes in some cases. As examples of our results we present distances for $\sim$25 molecular clouds in Table ~\ref{t2}. $Keywords$: interstellar medium: molecular cloud distances