| Description |
Molecular clouds do not exist in a vacuum, but are embedded in a warm, diffuse interstellar medium containing hydrogen largely in atomic form, and ionized carbon. Based on theoretical modeling, dense, cold clouds are surrounded by an intermediate temperature envelope in which the hydrogen is molecular (due to efficient self-shielding) and in which carbon changes from ionized to atomic, to molecular (primarily CO) as one moves to regions of greater extinction. This cloud envelope is expected to have a major impact on the structure of dense cloud in which star formationtakes place, as it can add to the pressure support confining them, and can serve as a conduit for energy flowing into the molecular cloud that can be critical for sustaining observed turbulence. This boundary layer is not readily observable in CO since the abundance of this species has dropped dramatically, and it is also difficult to study in molecular hydrogen emission, as the temperature is too low to significantly populate even the lowest excited rotational states. The boundary, sometimes called dark gas, possibly contains a significant fraction of the total mass of the dense ISM. Based on detection of weak H2 emission from the boundary of the Taurus molecular cloud by Goldsmith et al. (2010), we here propose to use the unique capabilities of Herschel to make a well-calibrated cut through the linear edge boundary region in Taurus in the 158 micron fine structure line of CII, and both the 492 GHz and 810 GHz fine structure lines of CI. Accurate calibration is essential and cannot be achieved using ground-based facilities. We propose to use the HIFI instrument to resolve the line widths and probe the kinematics in the boundary layer. The ratio of the CI lines yields the density, and these lines, together with the distribution of intensity of CII and H2 will allow us to develop a well-defined model for the boundary layer. This will address important questions about molecular cloud structure, total mass, and evolution. |
