CIRCUM-PROTOSTELLAR ENVIRONMENTS .3. GAS DENSITIES AND KINETIC TEMPERATURES

We have surveyed a complete, flux-limited, IRAS-selected sample of protostars in Taurus whose infrared through millimeter-wave properties indicate them to be younger than T Tauri stars. We have observed CS J = 3-2, 5-4, and 7-6, and H2CO J(K-1K+1) = 3(03)-2(02) and 3(22)-2(21), toward the central positions of all 25 objects. CS traces the dense gas in the circumstellar envelope, while H2CO probes the kinetic temperature of the dense gas. Only three of the sources were detected in both transitions of H2CO, making it of limited use as a temperature probe of these objects. Combining the CS- and H2CO-derived properties with those previously derived from dust continuum emission, we have placed limits on the temperatures of the envelopes, typically 20 K less than or similar to T-K less than or similar to 50 K. Derived envelope gas densities and CS column densities were typically a few x 10(6) cm(-3) and a few x 10(12) cm(-2), respectively. Where CS 5-4 was detected (roughly half of the observed sources), the derived envelope masses were consistent with those derived from dust emission (assuming a CS/H-2 abundance of 10(-8)). Since most of the embedded (i.e., not optically visible) sources were detected in CS 5-4, and most of the visible sources were not, this may mean either that the CS-emitting envelope has dissipated in the more evolved objects (confirming Ohashi et al. 1991), or CS has become depleted. L1551NE may have an asymmetric, double-peaked line profile like that of B335, suggestive of a collapsing envelope. L1551NE may be in transition from the much younger ''class 0'' protostar stage to the somewhat more evolved ''class I'' protostar stage. Several of the sources have broad CS line wings probably originating from dense gas in a molecular outflow. In at least one case, the kinetic temperature of the outflowing gas may be greater than that in the envelope.