Methanol: A Diagnostic Tool for High-Mass Star-Forming Regions

Kinetic temperature and density are fundamental parameters for our understanding of the interstellar medium (ISM). Usually, symmetric rotors such as NH3 are used to probe a cloud’s kinetic temperature, while linear molecules, e.g. CS, probe its density. However, different spatial distributions of the tracers (“chemistry”) often complicate the picture (see, e.g., [8]) as they often trace physically different and spatially non-coexisting gas components. It is thus desirable to trace all relevant physical parameters with a single molecule. Promising candidates exist among slightly asymmetric rotors, which have properties qualifying them as tracers for physical conditions. Methanol, CH3OH, is a slightly asymmetric rotor. It is ubiquitous and associated with different regimes of star formation, from quiescent, cold (T ∼ 10 K), dark clouds, to “hot core” sources near high-mass (proto)stellar objects, where [CH3OH/H2] values ∼ 10-7 -10-6 are observed [3]. Up to now an extremely poor knowledge of the CH3OH collisional rates and of their propensity rules has prevented realistic systematic studies exploiting methanol’s full potential as an interstellar tracer. Recently, t his situation has changed with the calculation of collisional rate coefficients by [5, 6], for collisions with helium, for both CH3OH-A and CH3OH-E, for levels up to (J,K) = 9.