DENSE GAS IN HIGH-LATITUDE MOLECULAR CLOUDS

The nearby molecular clouds MBM 7, 12, 30, 32, 40, 41, and 55 were sur veyed for tracers of dense gas, including the (1-0), (2-1), and (3-2) rotational lines of CS and the (1-0) lines of HCO+ and HCN. MBM 7 and MBM 12 contain dense cores, while the other clouds contain little or n o traces of dense gas. Comparison of the emission from dense gas trace rs to that of CO-13 reveals that the former are more compact in angula r size as well as line width. An extensive CS(2-1) survey of part of M BM 12 reveals that the emission is characterized by clumps on approxim ately 3' scales as well as extended emission. Observations of the CS(1 -0) and (3-2) lines using telescopes with matched beam sizes reveal th at the volume density must be at least approximately 10(4.5) cm-3 with in the (3-2) emitting regions, which are approximately 0.03 pc in radi us. Electron excitation of the CS rotational levels is ruled out (in t he cores) by comparing the (3-2)/(1-0) line ratios with models includi ng H2 and electron collisions. The volume density in the cores is subs tantially larger than in the portions of the cloud traced by CO emissi on. The density increases into the cores as r-2, suggesting dynamical collapse. The masses of the cores are close to the virial mass, sugges ting they are dynamically bound. The cores in MBM 7 and MBM 12 are thu s likely to form stars; they are the nearest sites of star formation.