Abstract
We have conducted a survey of the CCS JN = 32 - 21 line toward 7 dark clouds at 30" spatial resolution and 0.054 km/s spectral resolution. Previous studies (Suzuki et al. 1992; Wolkovitch et al. 1997) show that CCS is an abundant molecule in pre-protostellar cores with an appropriate critical density (104-105 cm-3) to determine the physical conditions in such cores. CCS is also an excellent probe of kinematic structure of dark clouds because of its lack of hyperfine splittings and its moderate optical depth, which is less than 1.5 in our samples. Our observations with higher spatial and spectral resolutions than previous work reveal detailed structure of sampled clouds and allow a more precise interpretation for these clouds. The surveyed clouds show strong emission (0.9-4.3 Jy/beam) of CCS and extremely narrow linewidthes (0.1-0.8 km/s). Virial masses of the cores in these clouds are 1-10 Msun. Assuming virial equilibrium, we estimate that the fractional abundance of CCS ranges from 10-11 to 9x 10-10, which brackets the value 4.9x 10-10 obtained from the LVG calculation for L1498 by Wolkovitch et al. The variation of the fractional abundance may indicate the evolutionary status of star formation in these cores. The most striking feature in our data is a ring structure in the position-velocity diagram of L1544 with an inner hole of 0.04 pcx0.13 km/s and an outer boundary of 0.16 pcx0.55 km/s. This position-velocity structure clearly indicates an edge-on disk or ring geometry, as opposed to the two-layer model adoped by Tafalla et al. 1998 for L1544. Our data further showseveral condensations in the disk or ring with mass 1Msunin each condensation. CCS not only is a good tracer of pre-protostellar cores, but also has a largeZeeman splitting factor comparable to that of HI and OH. Therefore, oursurvey provides necessary information for follow-up Zeeman study of surveyedclouds.