In a first step, the observations are interpreted using the rotational diagram method commonly used for interstellar molecular line studies. The methanol rotational distribution is best described by temperatures in the range 20 to 40 K. The corresponding column densities and methanol production rates are derived. In a second step, an out-of-equilibrium model is used where the methanol rotational distribution is governed by collisions, spontaneous relaxation, and excitation of the fundamental vibrational bands by solar radiation. This model shows that the observed distribution is sensitive to the coma kinetic temperature, and suggests that the collisional region is underestimated when taking into account only collisions with water: collisions with electrons may play a major role in the excitation of methanol.
The retrieved methanol production rates are 2.0 and 2.2 10 s, corresponding to relative abundances of 5 and with respect to water, for comets Austin and Levy, respectively. These abundances are in agreement with independent infrared observations of cometary methanol in the same comets. They show that methanol is indeed an important cometary volatile, and that its abundance may vary from comet to comet. Since methanol is also found to be an abundant constituent of interstellar grains, its presence in comets is consistent with the scenario of formation of comets from unaltered interstellar matter.
The photolytic decay products of methanol, such as CHO, should be
important in cometary atmospheres, but they cannot account for the
suspected distributed sources of CO or HCO.
(accepted for publication in Astronomy and Astrophysics, Main Journal)