Title: Peculiar Near-Nucleus Outgassing of Comet 17P/Holmes During Its 2007 Outburst

Abstract:
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", or 
~3000 km at the comet distance.  The observations resulted in detections of the rotational lines 
CO 3-2, HCN 4-3, H13CN 4--3, CS 7--6, H2CO 3(1,2)-2(1,1), H2S 2(2,0)-2(1,1), and multiple 
CH3OH lines, along with the associated dust continuum at 221 and 349 GHz.  The continuum 
has a spectral index of 2.7+/-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 (~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 to 30 degrees 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+/-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.

http://arxiv.org/abs/1411.4632