A Catalog of Optically Selected Cores

We present a new catalogue of 406 dense cores optically selected by using theSTScI Digitized Sky Survey (DSS). In this catalogue 306 cores have neither anEmbedded YSO (EYSO) nor a Pre-Main-Sequence (PMS) star, 94 cores have EYSOs (1core has both an EYSO and a PMS star), and 6 cores have PMS star only. Oursample of dense cores in the catalogue is fairly complete within a category ofnorthern Lynds class 5, 6 clouds, and southern Hartley et al. (1986)'s class Aclouds, providing a database useful for the systematic study of dense cores.Most of the cores listed in the catalogue have diameters between $0.05 - 0.36$pc with a mean of $\sim 0.24$ pc. The sizes ($\sim 0.33$ pc in the mean) ofcores with EYSOs are found to be usually larger than the sizes ($\sim 0.22 $ pcin the mean) of starless cores. The typical mean gas density of the cores is$\sim7\times 10^3 cm^{-3}$. Most of the cores are more likely elongated thanspherical (mean aspect ratio: $\sim 2.4$). The ratio of the number of coreswith EYSOs to the number of starless cores for our sample is about 0.3,suggesting that the typical lifetime of starless cores is $0.3-1.6$ Myr, about3 times longer than the duration of the Class 0 and Class I phases. Thislifetime is shorter than expected from models of ambipolar diffusion, byfactors of 2-44.Comment: 22 pages, 8 figures, 3 tables, and to appear in ApJS. Harvard-Smithsonian Center for Astrophysic