Estimating the Interstellar Extinction and the Contribution from an Accretion Shock to the Formation of Emission Continuum in DS Tau and DG Tau

Abstract

We analyze the UV spectra of the young stars DG Tau and DS Tau taken with the STIS spectrograph from the Hubble Space Telescope. For these stars, we found the upper limits of the interstellar extinction A_V. Their values proved to be lower than those obtained by other authors from optical observations. For DS Tau, DG Tau, and TW Hya, we also determined the ratio of the flux in the CIV 1550 doublet lines to the excess continuum flux. It proved to be an order of magnitude lower than its value predicted by the accretion-shock (AS) models of Lamzin (1998) and Calvet and Gullbring (1998). It thus follows that for these stars, the emission continuum originates mainly in the accretion disk and/or the boundary layer rather than in the AS, as has been thought previously. Since a similar conclusion has previously been reached for DR Tau, T Tau, and RY Tau, we may assume that the disks around most young stars reach the stellar surface and accretion mainly proceeds through the boundary layer.