The rotational excitation of methanol by molecular hydrogen

Abstract

We have computed cross‐sections and rate coefficients for the rotational excitation of A‐ and E‐type methanol by molecular hydrogen. Calculations were performed for rotational transitions within the torsional ground state, ν= 0, and within the first and second excited torsional states, ν= 1 and ν= 2. For collisions of methanol with para‐H2 in its rotational ground state, j2= 0, the methanol basis included rotational states j1≤ 15, thereby extending previous calculations, which included states j1≤ 9 only. For the first time, calculations have also been performed for ortho‐H2 in its rotational ground state, j2= 1, although it was necessary to revert to the smaller methanol basis, j1≤ 9, owing to the coupling to states of molecular hydrogen with j2 > 0. The coupled states approximation was used in the production calculations, to generate the thermal rate coefficients at temperatures 10 ≤T≤ 200 K, but some limited comparisons, at a few collision energies, of cross‐sections obtained using the full coupled channels (CC) method have been made. The propensities of the collisions, with respect to changes in the rotational quantum number, j1, and its projection, K, on the symmetry axis of the methanol molecule, were investigated. There are qualitative differences between the K propensities for collisions with ortho‐ and para‐H2, which relate to the fact that the inelastic cross‐sections tend to be significantly larger when ortho‐H2 is the perturber.

Observational tests of interstellar methanol formation

“…This model will allow us to include an additional radiation field as experienced by the molecules, corresponding to the dust continuum radiation from the embedded YSO, as well as a possibility to control the overall E/A ratio of the model via assumptions on formation and destruction. It will also incorporate new collision rates published by Rabli & Flower (2010). This work will be presented in a forthcoming paper.…”

Deuterated methanol in the pre-stellar core L1544

“…The 2 0,2 and 2 1,2 upper levels are connected to their corresponding lower state within the K a = 0 or K a = 1 manifolds by "strong" collisional transitions (i.e., upward rate >10 −11 cm 3 s −1 at 10 K), and other weaker E 1 − E 2 connections also exist. On the other hand, the collisional transitions connecting 0 0,0 to the lower 1 1,1 and 2 1,2 states have rate coefficients that are set to zero in the present E-CH 3 OH/p-H 2 data set (Rabli & Flower 2010a). As discussed in Rabli & Flower (2010b), this is an artefact of the coupled state (CS) approximation used in the production of the collision cross sections, and it is very likely that the collisional 0 0,0 −1 1,1 and 0 0,0 −2 1,2 transitions actually have small, but non-zero, rate coefficients.…”

“…Even if the L1544 emission maps show that the cloud is not spherical but instead has an elongated shape, the adopted model is simple, physically motivated and certainly adequate to model observation data averaged over a single dish beam profile. For the statistical equilibrium calculation, we used de-excitation rates for p-H 2 /A-CH 3 OH and p-H 2 /E-CH 3 OH collisional systems (Rabli & Flower 2010a) available at the LAMDA database (Schöier et al 2005). Collisional data for o-H 2 are lacking, but this does not represent a problem for our modelling because the H 2 ortho-topara ratio (OPR) is expected to be very low in pre-stellar cores (see, e.g., Walmsley et al 2004;Sipilä et al 2013).…”

The HIFI spectral survey of AFGL 2591 (CHESS)

“…This estimation is based on only five detected HCl lines, three hyper-fine components from the energy level of E up = 30 K and two from the higher state E up = 90.1 K, thus the uncertainty of this estimation is higher, within a factor of 8. The collisional rates for CH 3 OH with H 2 were taken from Rabli & Flower (2010). Data are available for both A-and E-type CH 3 OH, but given the many CH 3 OH lines in our spectra, we used only transitions of A-type CH 3 OH, assuming a ratio of A/E = 1:1, which is appropriate at the high (>30 K) temperature in the AFGL 2591 envelope.…”