The distribution, excitation, and formation of cometary molecules: Methanol, methyl cyanide, and ethylene glycol

We present an interferometric and single-dish study of small organic species toward Comets C/1995 01 (Hale-Bopp) and C/2002 T7 (LINEAR) using the BIMA interferometer at 3 mm and the ARO 12 m telescope at 2 mm. For Comet Hale-Bopp, both the single-dish and interferometer observations of CH3OH indicate an excitation temperature of 105 ± 5K and an average production rate ratio Q2(CH₃OH)/Q(H₂O) ̃ 1.3% at ̃1 AU. In addition, the aperture synthesis observations of CH₃OH suggest a distribution well described by a spherical outflow and no evidence of significant extended emission. Single-dish observations of CH₃CN in Comet Hale-Bopp indicate an excitation temperature of 200 ± 10 K and a production rate ratio of Q(CH₃CN)/Q(H₂O) ̃ 0.017% at ̃1 AU. The non detection of a previously claimed transition of cometary (CH₂OH)₂ toward Comet Hale-Bopp with the 12 m telescope indicates a compact distribution of emission, D < 9" (<8500 km). For the single-dish observations of Comet T7 LINEAR, we find an excitation temperature of CH₃OH of 35 ± 5 K and a CH₃OH production rate ratio of Q(CH₃OH)/Q(H₂O) ̃ 1.5% at ̃0.3 AU. Our data support current chemical models that CH₃OH, CH₃CN, and (CH₂OH)₂ are parent nuclear species distributed into the coma via direct sublimation off cometary ices from the nucleus with no evidence of significant production in the outer coma.