A NEW METHOD FOR DETERMINING THE CO TO H-2 CONVERSION FACTOR FOR TRANSLUCENT CLOUDS

In this paper we describe a new technique for obtaining the conversion factor between the molecular hydrogen column density and the CO(J = 1 -0) integrated antenna temperature. This factor, typically known as X( CO), is often taken to be of order a few times 10(20) cm(-2) [K km s(- 1)](-1) for the molecular clouds in the Galaxy and is one of the prima ry means of determining the molecular cloud mass from CO observations. However, for the low-extinction interstellar clouds known as the tran slucent molecular clouds, estimates of X(CO) vary by up to a factor of 60 depending on the object and techniques employed to calibrate X(CO) . Since the cloud mass is directly proportional to X(CO), uncertaintie s in mass estimates of translucent clouds can be more than an order of magnitude. We calibrate the H-2 content in translucent clouds by usin g the linear relationship between the CH and H-2 column densities. The CH column density is readily determined from observations of the CH g round-state hyperfine main-line transition at 3335 MHz. Using CH as a surrogate tracer for H-2 and CO(J = 1-0) observations of a sample of t ranslucent and dark molecular clouds, we find a wide variation in valu es for X(CO). For translucent clouds, X(CO) ranges from 0.3 to 6.8 x 1 0(20) and for dark clouds the values range from 0.8 to 8.6. Although t he average values for both types of cloud are similar to the canonical value determined for the Galactic molecular cloud ensemble (2-4 x 10( 20)), the scatter in individual X(CO) values may indicate that X(CO) f or a given translucent cloud cannot be determined a priori and must be obtained for each cloud so that a reliable mass determination may be made.