THE ROTATIONAL LINES OF METHANOL IN COMETS AUSTIN 1990 V AND LEVY 1990 XX

We have observed rotational lines of methanol in comets Austin (1990 V ) and Levy (1990 XX) with the IRAM 30-m millimetre radio telescope. On comet Austin, several J (3-2) transitions around 145 GHz were detecte d and two J (2-1) lines near 97 GHz were marginally detected on May 25 , 1990. On comet Levy, observed in the August 26-31, 1990 period, 12 l ines were detected among 21 lines observed near 97, 145, 165 and 218 G Hz. As a first step, the observations are interpreted using the rotati onal diagram method commonly used for interstellar molecular line stud ies. The methanol rotational distribution is best described by tempera tures in the range 20 to 40 K. The corresponding column densities and methanol production rates are derived. In a second step, an out-of-equ ilibrium model is used where the methanol rotational distribution is g overned by collisions, spontaneous relaxation, and excitation of the f undamental vibrational bands by solar radiation. This model shows that the observed distribution is sensitive to the coma kinetic temperatur e, and suggests that the collisional region is underestimated when tak ing into account only collisions with water: collisions with electrons may play a major role in the excitation of methanol. The retrieved me thanol production rates are 2.0 and 2.2 10(27) s-1, corresponding to r elative abundances of 5 and 0.9% with respect to water, for comets Aus tin and Levy, respectively. These abundances are in agreement with ind ependent infrared observations of cometary methanol in the same comets . They show that methanol is indeed an important cometary volatile, an d that its abundance may vary from comet to comet. Since methanol is a lso found to be an abundant constituent of interstellar grains, its pr esence in comets is consistent with the scenario of formation of comet s from unaltered interstellar matter. The photolytic decay products of methanol, such as CH3O, should be important in cometary atmopheres, b ut they cannot account for the suspected distributed sources of CO or H2CO.