Methanol as a diagnostic tool of interstellar clouds

I. Model calculations and application to molecular clouds

¹ Max-Planck-Institut für Radioastronomie, Auf Dem Hügel 69, 53121, Bonn, Germany e-mail: [sleurini;schilke;kmenten]@mpifr-bonn.mpg.de
² Physics Department, University of Durham, DH1 3LE, UK e-mail: [david.flower;j.t.pottage]@durham.ac.uk
³ Physical Sciences Department, University of New Brunswick, Saint John, NB, E2L 4L5, Canada e-mail: xuli@unb.ca

Received: 9 January 2004
Accepted: 26 March 2004

Abstract

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.