Abstract. We present a detailed analysis of the diagnostic properties of methanol, (CH 3 OH), in dense molecular clouds, made possible by the availability of new (CH 3 OH-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 CH 3 OH 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.

In this paper, we present the results of calculations of cross‐sections and rate coefficients for rotationally inelastic transitions in methanol (CH3OH), in its ground torsional state, induced by collisions with para‐H2 in its ground rotational state. The interaction potential was calculated by means of many‐body perturbation theory, and the corresponding cross‐sections computed using the coupled states approximation to the Schrödinger equation for the CH3OH–H2 system. The rate coefficients have been calculated for kinetic temperatures in the range 5 ≤T≤ 200 K. We found that, although the rate coefficients for para‐H2 are qualitatively similar to those computed for He, the collisional propensity rules are less pronounced when the perturber is para‐H2 than when it is He. The rate coefficients for CH3OH–H2 tend to be larger than for CH3OH–He, a tendency which is anticipated from a comparison of the corresponding interaction potentials. Although our discussion is in terms of E‐type methanol, the analogous results for A‐type methanol and para‐H2 are also available. Our calculations are the first to consider H2 as a collision partner for CH3OH and are directly relevant to the interpretation of observations of methanol in the interstellar medium.

We have improved and extended our previous calculations of cross sections for the rotational excitation of methanol by helium. In the case of methanol in its torsional ground state, we extended our coupled states computations of the cross sections to higher collision energies and larger rotational basis sets. The accuracy of the rotational energy levels and eigenfunctions has been reviewed and improved for A-type methanol. The calculations have been extended to A- and E-type methanol in their first excited torsional states by averaging the CH3OH-He interaction potential over the excited state torsional eigenfunctions. Thermal rate coefficients have been calculated at low temperatures. Predictions are made of line intensity ratios which are sensitive to the density of the He perturber and which lend themselves to the determination of the perturber densities in the dark molecular clouds of the interstellar medium.

We report the results of coupled channels calculations of cross-sections for torsionally elastic and inelastic transitions in E-type methanol (CH 3 OH), with helium as the colliding partner. The dependence of the CH 3 OH-He interaction potential on the internal rotation (torsional) angle was determined using secondorder many-body perturbation theory. The methanol basis comprised levels belonging to the ground torsional state (ν = 0) and the first excited torsional state (ν = 1). The collisional 'propensity rules' observed in the case of torsionally elastic collisions were found not to apply to torsionally inelastic transitions between states of ν = 0 and 1. We assessed the effect of the torsional coupling on the torsionally elastic cross-sections and found changes of no more than about 30% at the highest collision energy considered (500 cm −1 ). The cross-sections for torsionally inelastic transitions were found to be typically two orders of magnitude smaller than for torsionally elastic transitions.

We report new calculations of the CH3OH-He interaction potential. The results of these computations have been fitted by an expansion in terms of the coordinates of the atom with respect to a coordinate system fixed in the molecule, and the internal rotation angle of the methyl radical. The potential was then used to determine rotational excitation cross sections by means of the quantum mechanical, coupled states method. It was assumed (i) that the methyl radical was fixed at its minimum energy conformation, or (ii) that the interaction potential was an average with respect to the ground state torsional eigenfunctions of A- and E-type methanol. From the cross sections, thermally averaged rate coefficients were calculated at kinetic temperatures T = 10 and 20 K. The `propensity rules' governing the collisional transitions were examined and compared with the results of microwave double-resonance experiments. Finally, the rate coefficients have been used to compute the excitation temperature of the 12.18 GHz transition of interstellar methanol, which has been observed in absorption against the 2.7 K cosmic microwave background radiation.