Far-ultraviolet Observations of Comet C/2012 S1 (ISON) from FORTIS

TL;DR

This study used far-UV imaging from a sounding rocket to observe comet C/2012 S1 (ISON), measuring water and carbon production rates, and analyzing the composition and dissociation processes in the comet's coma.

Contribution

First far-UV imaging observations of comet ISON from a sounding rocket, providing new measurements of its gas production rates and insights into its molecular dissociation processes.

Findings

Water production rate ~8e29 s^-1

Carbon production rate ~4e28 s^-1

Upper limit on CO/H2O ratio < 6%

Abstract

We have used the unique far-UV imaging capability offered by a sounding rocket borne instrument to acquire observations of C/2012 S1 (ISON) when its angular separation with respect to the sun was 26.3deg, on 2013 November 20.49. At the time of observation the comet's heliocentric distance and velocity relative to the sun were rh = 0.43 AU and rh_dot = -62.7 km s^-1. Images dominated by C I 1657 A and H I 1216 A were acquired over a 1e6 x 1e6 km^2 region. The water production rate implied by the Lyman alpha observations is constrained to be Q_H2O approximately 8e29 s^-1 while the neutral carbon production rate was Q_C approximately 4e28 s^-1. The radial profile of C I was consistent with it being a dissociation product of a parent molecule with a lifetime approximately 5e4 seconds, favoring a parent other than CO. We constrain the Q_CO production rate to 5(+1.5, -7.5)e28 s^-1 with 1sigma errors derived from photon statistics. The upper limit on the Q_CO/Q_H2O < 6%.