PHYSICAL CONDITIONS IN QUIESCENT DARK CLOUD CORES DETERMINED FROM MULTITRANSITION OBSERVATIONS OF CCS

We have studied three transitions of the CCS molecule to determine phy sical conditions in L1498 and TMC-1D, two narrow-line dense cores in t he Taurus region. We observed the N-J = 1(2) --> 0(1), 3(4) --> 2(3), and 7(8) --> 6(7) transitions at 22.3, 45.4, and 93.9 GHz, respectivel y, at 50 '' angular resolution. The intensities of the emission lines have been analyzed using statistical equilibrium calculations and coll ision rates calculated for the CCS-H-2 system. These were obtained fro m the Molscat scattering code together with inclusion of spin dependen ce in Hund's case (b) model. We find that the kinetic temperature in b oth sources is extremely low, between 7 and 10 K. The L1498 emission a ppears to originate in a single velocity component with mean hydrogen density 3-14 x 10(4) cm(-3). We analyzed three velocity components in TMC-1D separately, and find that the low-velocity component has a mean H-2 density of 6 x 10(3) cm(-3), while the two higher velocity compon ents are denser by approximately a factor of 3. The L1498 core is clos e to virial equilibrium in that the magnitude of its gravitational ene rgy is close to that of its kinetic energy. However, the cores corresp onding to the three velocity components in TMC-1D are unbound by facto rs 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 m ade many significant contributions to molecular astrophysics.