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
