A CS survey of small molecular cores
We are addressing the sites of isolated low mass star formation in the solar neighbourhood, i.e. small cloud cores within one kiloparsec. We aim at determining the physical parameters of the cores, i.e., temperature, volume density, column density and (radial) velocity fields, and the status of star formation, i.e., whether embedded objects are present within the cores. Surveying small dark clouds in both celestial hemispheres we study the physical conditions of low-mass star formation for detectable core masses M>0.01Msun. The target list is drawn from catalogues of optically selected dark cloud cores, where the visual extinction exceeds 5 magnitudes. The selected probe is the CS molecule that needs high densities for excitation of its rotational levels. To gauge the state of excitation, the cores were observed in two transitions. In a limited number of cases, optical depths were derived from complementing lines of the rarer isotopologue C34S for the (2-1) and (3-2) transitions. Making small (3arcmin by 3arcmin) maps, the 471 optically selected cores were searched for CS(2-1) and 315 (67%) were detected (T_A*>3sigma). In general, the position of peak CS emission does not coincide with the optically determined centre of the cores. The cores appear cold (T<10K) and, in the majority of cases, the CS emission is optically thin (tau<1). On the arcminute scales of the observations, the median column density of carbon monosulfide is N(CS)=7.E12/cm2. For an average abundance of N(CS)/N(H2)=1.E-8, the median mass of the detected cores is 1.0Msun. The line shapes are most often Gaussian with widths exceeding that due to thermal broadening of <0.1km/s. The observed median FWHM=0.7km/s, i.e. non-thermal turbulence contributes dominantly to the line widths