Formaldehyde in comets. II - Excitation of the rotational lines

Astronomy and Astrophysics – Astrophysics

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Cometary Atmospheres, Comets, Formaldehyde, Molecular Excitation, Molecular Rotation, Abundance, Austin Comet, Brorsen-Metcalf Comet, Infrared Radiation, Okazaki-Levy-Rudenko Comet, Solar Radiation, Spectral Emission

Scientific paper

In order to prepare and interpret microwave observations of formaldehyde in cometary atmospheres, we investigate the excitation and emission of H2CO rotational lines. The model includes collisional excitation and radiative excitation of the vibrational bands by solar infrared radiation. It computes the evolution of the rotational population distribution, as the molecules expand throughout the coma. Centimeter line emissions are predicted to be very weak and presently unreachable even with large telescopes: this is in contradiction with the claimed detection of the 6 cm line in Comet P/Halley with the VLA. Several H2CO millimeter and submillimeter lines should be easily detected in bright comets if H2CO is present in cometary atmospheres at a 1 percent abundance level. On the basis of these predictions, the observations of the 226-GHz 3(12)-2(11) line undertaken in Comets P/Brorsen-Metcalf (1989 X), Austin (1990 V) and Levy (1990 XX) with the IRAM 30-m telescope were successful and led to the first detection of cometary formaldehyde at millimeter wavelengths.

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