Millimeter-wave absorption spectroscopy of molecular clouds

Abstract

We review recent observations of millimeter wave absorption line measurements in front of compact extragalactic radio sources. The availability of a new instrument with a sensivity gain of an order of magnitude would considerably imcrease the number of available sources, with the potentiality of improving our knowledge of the molecular phase of the interstellar medium, and to boost the study of chemistry in diffuse/translucent and possibly dense molecular clouds regions. Our survey of HCO⁺ absorption shows that the population of detected clouds has statistical properties resembling that of HI clouds. Isotopic abundances in the CO molecules show strong fractionation, giving and indication of rather low kinetic temperatures. A close relationship between OH and HCO⁺ abundances is found, which is not fully understood.