We present high angular resolution Submillimeter Array observations of the outbursting Jupiter family comet 17P/Holmes on 2007 October 26–29 , achieving a spatial resolution of 2.5 ^ { \prime \prime } , or \sim 3000 km at the comet distance .
The observations resulted in detections of the rotational lines CO 3–2 , HCN 4–3 , H ^ { 13 } CN 4–3 , CS 7–6 , H _ { 2 } CO 3 _ { 1 , 2 } -2 _ { 1 , 1 } , H _ { 2 } S 2 _ { 2 , 0 } –2 _ { 1 , 1 } , and multiple CH _ { 3 } OH lines , along with the associated dust continuum at 221 and 349 GHz .
The continuum has a spectral index of 2.7 \pm 0.3 , slightly steeper than blackbody emission from large dust particles .
From the imaging data , we identify two components in the molecular emission .
One component is characterized by a relatively broad line width ( \sim 1 km s ^ { -1 } FWHM ) exhibiting a symmetric outgassing pattern with respect to the nucleus position .
The second component has a narrower line width ( < 0.5 km s ^ { -1 } FWHM ) with the line center red-shifted by 0.1–0.2 km s ^ { -1 } ( cometocentric frame ) , and shows a velocity shift across the nucleus position with the position angle gradually changing from 66 \arcdeg to 30 \arcdeg within the four days of observations .
We determine distinctly different CO/HCN ratios for each of the components .
For the broad-line component we find CO/HCN < 7 , while in the narrow-line component , CO/HCN = 40 \pm 5 .
We hypothesize that the narrow-line component originates from the ice grain halo found in near-nucleus photometry , believed to be created by sublimating recently released ice grains around the nucleus during the outburst .
In this interpretation , the high CO/HCN ratio of this component reflects the more pristine volatile composition of nucleus material released in the outburst .