On the interstellar extinction law toward young stars

We have determined the atomic hydrogen column density N-HI toward all of the Young stars from the Taurus-Auriga-Perseus star-forming complex for which the corresponding spectra are available in the Hubble Space Telescope archive (nine stars) by analyzing the Ly alpha line profile. We show that the stars studied, except DR Tau, lie not far from the edge of the gaseous cloud of the star-forming region closest to LIS or, more precisely, inside the outer H I shell of the cloud. This shell with a column density of N-HI similar or equal to 6 x 10(20) cm(-2) surrounds the molecular gas of the Cloud composed of a diffuse component (the so-called diffuse screen) in which dense, compact TMC-1 cores are embedded. The properties of the dust grains toward the stars that lie at the front edge of the cloud most likely differ only slightly from those of the interstellar dust outside star-forming regions. This casts doubt on the validity of the hypothesis that the extinction curve toward Young stars has an anomalously low amplitude of the 2175 A bump-such an extinction curve is observed for the field stars HD 29647 and HD 283809 toward which the line of sight passes through the TMC-1 core.