A catalog of small, optically-selected molecular clouds: Optical, infrared, and millimeter properties

Abstract

A catalog of 248 small molecular clouds selected from the northern hemisphere Palomar Observatory Sky Survey has been assembled. These clouds have angular sizes of less than 10 arcmin (the mean of the sample is 4 arcmin), are reasonably well-isolated, and centrally opaque. They most closely resemble small versions of the larger Bok globules and Barnard objects. Many (30%) of the smallest members of this catalog have not been previously cataloged or studied. Coordinates accurate to 0.5 arcmin, optical extents and orientation angles, and cross-references are given for all of the clouds. Radial velocities, obtained from CO observations, are listed for 158 of the clouds. Additionally, 339 associated or possibly associated IRAS point sources are listed for 147 of the clouds. As most of these clouds have few or no foreground stars and subtend small angular extents, they are likely the smallest (all < 1.5 pc) and simplest dark molecular clouds near the Sun. The mean ellipticity (b) of the clouds in the catalog is 2.0, indicating that spherical cloud models may not be appropriate for this sample. The mean orientations, with respect to the galactic plane, of the clouds showing ellipticities greater than unity has no angular dependence; that is, the local galactic plane does not seem to order the direction of the optically determined ellipticity for the clouds. The far-infrared properties of the clouds with associated IRAS sources are similar to those seen in the sample of dark cloud cores studied by Beichman et al. (1986) [Ap. J., 307, 337], but are somewhat cooler (19 K vs 28 K for the means) and less FIR bright. Millimeter 12CO (J = 2-1) observations of the centers of 93 of the cataloged clouds shows that most (70%) of the sample are cool (gas kinetic temperature ∼8 K) and quiescent (gas dynamical motions ≈ sonic). Some clouds exhibit warmer gas temperatures possibly driven by radiative heating from nearby stars. Another small group of clouds exhibit the cool temperatures and supersonic linewidths characteristic of active embedded protostellar collapse or outflow.