Methanol as a diagnostic tool of interstellar clouds - I. Model calculations and application to molecular clouds

We present a detailed analysis of the diagnostic properties of methanol, (CH3OH), in dense molecular clouds, made possible by the availability of new (CH3OH-He) collisional rate coefficients. Using a spherical Large Velocity Gradient (LVG) model, the dependence on kinetic temperature and spatial density of various millimeter and submillimeter line bands is investigated over a range of physical parameters typical of high- and low-mass star-forming regions. We find CH3OH to be a good tracer of high-density environments and sensitive to the kinetic temperature. Using our LVG model, we have also developed an innovative technique to handle the problem of deriving physical parameters from observed multi-line spectra of a molecule, based on the simultaneous fit of all the lines with a synthetic spectrum, finding the best physical parameters using numerical methods.