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Study of chemical compositions of Solar System comets via spectral observations of various cometary molecules may provide us essential information on proto-Solar Nebula and even on the origin of life. On 2007 October 24, Comet 17P/Holmes, a short-period Jupiter-family comet, underwent an apparent gas outburst followed by a sudden, dramatic increase of its brightness nearly one million-fold, and became a naked-eye object in two days. In this study, we therefore used the Submillimeter Telescope (SMT) and the Kitt Peak 12m millimeter telescope (KP12M) of the Arizona Radio Observatory (ARO) to observe the rotational transitions of cometary molecules HCN, HDO, H2CO, HDCO, c-C3H2, and CH3OH toward Comet 17P/Holmes post-outburst in November 2007.

We detected successfully HCN J = 1–0 and 3–2 emission lines, HDO 21,1–21,2 line, and the CH3OH lines of J = 3–2 and 5–4 bands. It is noteworthy that HDO has never been observed in any Jupiter-family comet; our detection of HDO is hence the first ever in Jupiter-family comets. The deuterium-fractionation ratio, [D/H], thus derived is of 7.34×10-4, a deuterium-enrichment factor of 4.7 higher than that of Earth’s ocean and a factor of 2 enriched than those of other comets. Compare with other known interstellar measurements, our fractionation ratio appears to be closer to the interstellar [D/H] value of primordial interstellar ice, with HDO sublimated from cometary nuclear surface directly.

In addition, the c-C3H2 doublet line of transitions 70,7–61,6 and 71,7–60,6 was detected marginally; on the other hand, barely noticeable HDCO 21,1–21,0 line was “visible” at 2-σ level. A search for the H2CO 21,1–21,0 line was also conducted but yielded no positive result. Furthermore, rotational excitation temperatures of ~21 K and ~15 K were derived, respectively, for multiple HCN and CH3OH transitions. Column densities of ~1 and ~10×1012 cm-2 of HCN and CH3OH, respectively, were also obtained accordingly. Corresponding gas production rates of ~5.7×1025 sec-1 and 1.4 – 2.1×1026 sec-1 at a distance of 2.5 AU from the Sun are inferred likewise for HCN and CH3OH, respectively. Compare with other average comets, 17P/Holmes, a Jupiter-family comet, appears to be hydrogen cyanide and methanol deficient.

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