EN ROUTE TO DESTRUCTION: THE EVOLUTION IN COMPOSITION OF ICES IN COMET D/2012 S1 (ISON) BETWEEN 1.2 AND 0.34 AU FROM THE SUN AS REVEALED AT INFRARED WAVELENGTHS*

Abstract

ABSTRACT We report production rates for H2O and eight trace molecules (CO, C2H6, CH4, CH3OH, NH3, H2CO, HCN, C2H2) in the dynamically new, Sun-grazing Comet C/2012 S1 (ISON), using high-resolution spectroscopy at Keck II and the NASA IRTF on 10 pre-perihelion dates encompassing heliocentric distances R h = 1.21–0.34 AU. Measured water production rates spanned two orders of magnitude, consistent with a long-term heliocentric power law Q(H2O) ∝ R h ( − 3.1 ± 0.1 ) ?> . Abundance ratios for CO, C2H6, and CH4 with respect to H2O remained constant with R h and below their corresponding mean values measured among a dominant sample of Oort Cloud comets. CH3OH was also depleted for R h > 0.5 AU, but was closer to its mean value for R h ≤ 0.5 AU. The remaining four molecules exhibited higher abundance ratios within 0.5 AU: for R h > 0.8 AU, NH3 and C2H2 were consistent with their mean values while H2CO and HCN were depleted. For R h < 0.5 AU, all four were enriched, with NH3, H2CO, and HCN increasing most. Spatial profiles of gas emission in ISON consistently peaked sunward of the dust continuum, which was asymmetric antisunward and remained singly peaked for all observations. NH3 within 0.5 AU showed a broad spatial distribution, possibly indicating its release in the coma provided that optical depth effects were unimportant. The column abundance ratio NH2/H2O at 0.83 AU was close to the “typical” NH/OH from optical wavelengths, but was higher within 0.5 AU. Establishing its production rate and testing its parentage (e.g., NH3) require modeling of coma outflow.