Physical Conditions in Quiescent Dark Cloud Cores Determined from Multitransition Observations of CCS
We have studied three transitions of the CCS molecule to determine physical conditions in L1498 and TMC-1D, two narrow-line dense cores in the Taurus region. We observed the NJ = 12 --> 01, 34 --> 23, and 78 --> 67 transitions at 22.3, 45.4, and 93.9 GHz, respectively, at 50" angular resolution. The intensities of the emission lines have been analyzed using statistical equilibrium calculations and collision rates calculated for the CCS-H2 system. These were obtained from the Molscat scattering code together with inclusion of spin dependence in Hund's case (b) model. We find that the kinetic temperature in both sources is extremely low, between 7 and 10 K. The L1498 emission appears to originate in a single velocity component with mean hydrogen density 3-14 × 104 cm-3. We analyzed three velocity components in TMC-1D separately, and find that the low-velocity component has a mean H2 density of 6 × 103 cm-3, while the two higher velocity components are denser by approximately a factor of 3. The L1498 core is close to virial equilibrium in that the magnitude of its gravitational energy is close to that of its kinetic energy. However, the cores corresponding to the three velocity components in TMC-1D are unbound by factors of 2-7.We dedicate this paper to the memory of Sheldon Green, who passed away in 1995 December. Sheldon was an outstanding chemist who made many significant contributions to molecular astrophysics.