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

We have surveyed a complete, flux-limited, IRAS-selected sample of pro tostars in Taurus whose infrared through millimeter-wave properties in dicate 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), t oward the central positions of all 25 objects. CS traces the dense gas in the circumstellar envelope, while H2CO probes the kinetic temperat ure 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 o f these objects. Combining the CS- and H2CO-derived properties with th ose previously derived from dust continuum emission, we have placed li mits 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 den sities 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 (rough ly half of the observed sources), the derived envelope masses were con sistent with those derived from dust emission (assuming a CS/H-2 abund ance of 10(-8)). Since most of the embedded (i.e., not optically visib le) sources were detected in CS 5-4, and most of the visible sources w ere not, this may mean either that the CS-emitting envelope has dissip ated in the more evolved objects (confirming Ohashi et al. 1991), or C S has become depleted. L1551NE may have an asymmetric, double-peaked l ine profile like that of B335, suggestive of a collapsing envelope. L1 551NE may be in transition from the much younger ''class 0'' protostar stage to the somewhat more evolved ''class I'' protostar stage. Sever al of the sources have broad CS line wings probably originating from d ense gas in a molecular outflow. In at least one case, the kinetic tem perature of the outflowing gas may be greater than that in the envelop e.