Previous
abstract 
Next abstract
To date, no star formation has been detected in the nearest molecular clouds. We are attempting to determine whether the clouds contain suitable sites for future star formation by searching them for concentrations of dense ($n({\rm H}_2) > 10^4 {\rm cm}^{-3}$) gas. Molecules with large dipole moments (e.g. CS, HCN, and HCO$^+$) are most likely to trace dense gas because of the large volume density required for thermal collisional excitation. However, detection of rotational transitions of these molecules does not guarantee the presence of dense gas, because their large dipole moments make them also susceptible to electron collisional excitation. Electron abundances are expected to be relatively high in these clouds, because the interstellar radiation field can ionize free C except deep in the cores. We have surveyed several high-latitude molecular clouds (MBM 12, 7, 55, 40) in the 2--1 and 3--2 rotational transitions of CS (and 1--0 observations are scheduled for this May). An extensive CS(2--1) line map of the nearby ($d\simeq 65$ pc) cloud MBM 12 reveals that much of the CS(2--1) emission originates from a relatively smooth, continuous `horseshoe' structure (about 0.1 pc wide and 1 pc long). We mapped the CS(3--2) emission around several prominent CS(2--1) peaks in the clouds, and also made a set of deep spectra in a slice perpendicular to the `horseshoe' of MBM 12. The 3--2 lines are much brighter than expected from electron excitation. Electron collisions cannot produce the observed line strengths unless the electron abundance is larger than $5\times 10^{-4}$---the entire cosmic abundance of C. We are thus led to the conclusion that the CS rotational transitions are probably tracing dense gas. The presence of dense gas is a necessary but not sufficient condition for star formation, and it remains to be explained which form of internal pressure (turbulent, magnetic, etc.) keeps the cloud cores from collapsing.
