Low Abundance of Hydrogen Isocyanide in Comet 73P-B/Schwassmann-Wachmann 3

Details Low Abundance of Hydrogen Isocyanide in Comet 73P-B/Schwassmann-Wachmann 3 Low Abundance of Hydrogen Isocyanide in Comet 73P-B/Schwassmann-Wachmann 3

Sep 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006dps....38.1205l&link_type=abstract

American Astronomical Society, DPS meeting #38, #12.05; Bulletin of the American Astronomical Society, Vol. 38, p.503

Astronomy and Astrophysics

Astronomy

Scientific paper

We present a sensitive 3-σ upper limit of 5.1×1023 s-1 for the production rate of hydrogen isocyanide in comet 73P-B/Schwassmann-Wachmann 3, obtained on May 10-11, 2006 using Caltech Submillimeter Observatory. The production rate of hydrogen cyanide at the same time is 4.5×1025 s-1, implying a low HNC/HCN abundance ratio of <1.1% (3-σ upper limit). HNC has so far been detected in 11 long-period comets, not including the very active comet Hale-Bopp (Biver et al. 2005; Crovisier et al. 2005). The existing measurements of the HNC/HCN abundance ratio as a function of heliocentric distance are well described by a linear dependence, [HNC]/[HCN]=0.33-0.25*rh(AU), for rh between 0.5 and 1.2 AU. Our 3-σ upper limit of 1.1% at 1 AU in comet Schwassmann-Wachmann 3 is thus a factor of 7 times lower than the values measured previously in moderately active comets. Comet Schwassman-Wachmann 3 is the first Jupiter family comet, in which a sensitive upper limit for the HNC/HCN abundance ratio has been obtained. The low value implied by our observations may indicate that HNC is depleted in short-period comets. An alternative explanation is that the HNC emission originates from an extended source in the coma. The production rates given above are computed assuming a parent density distribution for both molecules (direct release from the nucleus). The HNC production rate would increase by a factor of 4 for a daughter density distribution with a parent scale-length of 3000 km, bringing the HNC/HCN abundance ratio within the range of previous measurements in moderately active long-period comets. The strong dependence of the HNC/HCN abundance ratio in comet Schwassmann-Wachmann 3 on the HNC parent scale-length is a result of the small geocentric distance of the comet at the time of our observations (the 25" CSO beam corresponds to 1300 km at a distance of 0.072 AU).

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