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 is presented. These clouds have angular sizes of less than 10' (the mean of the sample is 4'), are reasonably well isolated, and are 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, optical extents and orientation angles, and cross references are given for all of the clouds. Radial velocities, obtained from CO observations, are Usted for 242 of the clouds. Additionally, 346 associated or possibly associated IRAS point sources are Usted for 149 of the clouds. Since 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 in the Galaxy. The mean elhpticity (a/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 elhpticities greater than unity has no angular dependence; that is, the local Galactic plane does not seem to order the direction of the optically determined elUpticity 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 and coworkers, but are somewhat cooler (19 K versus 28 K for the mean dust temperatures derived from 60-100 ¡im flux ratios) and less FIR-bright. Millimeter 12 CO .7=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 (gasdynamical motions approximately sonic). Some clouds exhibit warmer gas temperatures possibly driven by radiative heating from nearby stars. Another small group of clouds exhibits the cool temperatures and supersonic line widths characteristic of possible embedded protostellar activity.