MOLECULAR ABSORPTION AS A PROBE OF THE STRUCTURE OF DENSE CLOUDS

Absorption studies of molecular clouds using external galaxies as back ground continuum sources are valuable tools for several reasons. They exploit the very small angular size of these distant sources, achievin g angular resolution comparable to and often better than that obtained with interferometers. The sensitivity can be very high, as the intens ity of many continuum sources is quite large, especially at low freque ncies. Absorption measurements can also yield different and complement ary information compared to that obtained from emission data. In this paper we discuss these issues with particular regard to absorption spe ctroscopy of molecular clouds using the Upgraded Arecibo telescope. We discuss plans for a survey of the Taurus molecular cloud region to st udy the molecular gas along the line of sight to extragalactic radio c ontinuum sources. Study with Arecibo should yield a set of high-sensit ivity, high-resolution measurements in the 1 to 10 GHz range of a numb er of molecular cloud cores, as well as less dense lines of sight in t his region. The observations of absorption by OH, H2CO, HC3N, HC5N and other molecular species should yield important information about the very small-scale structure of molecular clouds as well as the physical conditions within them.