Small dense molecular clouds which envelope groups of T-Tauri stars

Abstract

Dense molecular clouds within the Taurus and NGC 2264 regions have undergone gravitational collapse and fragmentation to form groups of low mass (∼1M⊙) T-Tauri stars which are still embedded within the clouds and which are kinematically associated with them. Molecular column densities on the order of 1014 cm−2 are inferred from the emission lines of OH and NH3. Emission line widths are ≲2 km s−1 and the antenna beamwidths include linear extents of order 0.1 pc. The OH emission appears to be in a condition of local thermodynamic equilibrium, and it cannot arise from circumstellar sheils similar to those surrounding the masing infrared stars. The OH and NH3 emission occurs in clouds of ∼1 pc in extent with optical depths of 0.1 to 1.0 and excitation temperatures of the order of 10 K. The molecular clouds have radii of ∼0.5 pc, molecular hydrogen densities of ∼4000 cm−3, masses of ∼100 solar masses, and kinetic temperatures of ∼20 K. The observed data are not inconsistent with the molecular clouds being in a state of hydrostatic equilibrium.