The Evolution in Coma Molecular Composition of Comet C/2017 K2 (PanSTARRS) Across the H$_2$O Sublimation Zone: ALMA Imaging of an H$_2$O-Dominated Coma

TL;DR

The paper presents ALMA Band 7 observations of C/2017 K2 as it traverses the $H_2O$ sublimation zone, revealing a CO- and CH$_3$OH-rich, $H_2O$-dominated coma with mixed production pathways. Using Fourier-domain radiative transfer with the SUBLIME code, the authors derive a two-region outgassing geometry, a three-segment radial temperature profile for CH$_3$OH, and distinct source mechanisms for CO, CH$_3$OH, HCN, CS, and H$_2$CO, including evidence for icy-grain sublimation and extended coma sources. They measure an ortho-to-para ratio for H$_2$CO of $OPR=2.9\pm0.4$ and constrain the nucleus size to an upper limit of $d<6.6-7.4$ km along with a coma dust mass of $(1.2-2.4)\times10^{11}$ kg. The results, when compared to other comets, show K2’s enrichment in CO and CH$_3$OH and highlight the role of icy grains in shaping coma composition and thermal structure. These findings establish a baseline for multi-epoch ALMA studies tracking molecular evolution as comets cross sublimation zones, with implications for understanding volatile inheritance and grain chemistry in the early solar system.

Abstract

We report a survey of molecular emission from cometary volatiles using the Atacama Large Millimeter/Submillimeter Array (ALMA) toward comet C/2017 K2 (PanSTARRS) carried out on UT 2022 September 21, 22, and 23 at a heliocentric distance (\rh{}) of 2.1 au. These measurements of HCN, CS, CO, CH$_3$OH, and H$_2$CO (along with continuum emission from dust) sampled molecular chemistry in C/2017 K2 at the inner edge of the H$_2$O sublimation zone, discerning parent from daughter or extended source species. This work presents spectrally integrated flux maps, production rates, and parent scale lengths for each molecule. CH$_3$OH, CO, and HCN were produced within $\sim$250 km of the nucleus, potentially including contributions from sublimation of icy grains. CS was consistent with production from CS$_2$ photolysis, and H$_2$CO required production from extended sources in the coma. An ortho-to-para ratio OPR=$2.9\pm0.4$ for H$_2$CO was derived from simultaneously measured transitions of each spin species. The continuum was extended and spatially resolved, consistent with thermal emission from dust in the coma. Analysis of the continuum visibilities provided an upper limit on the nucleus diameter $d<6.6$ km and coma dust masses of $1.2-2.4\times10^{11}$ kg.

Figures (12)

• Figure 1: (A)--(D). Spectrally integrated flux maps for HCN, CO, CH$_3$OH, and CS in K2. Contour intervals in each map are given in multiples of the rms noise. The rms noise ($\sigma$, mJy beam$^{-1}$ km s$^{-1}$) is indicated in the upper left corner of each panel. Sizes and orientations of the synthesized beam (Table \ref{['tab:obslog']}) are indicated in the lower left corner of each panel. The comet's observer-centered illumination ($\phi \sim$ 25$\degr$), as well as the direction of the Sun and dust trail, are indicated in the lower right. The black cross indicates the position of the peak continuum flux. A spectrum extracted from a $10\arcsec$ diameter aperture centered on the nucleus is shown in the upper right. Contours are in 10$\sigma$ increments for HCN and 5$\sigma$ increments for CO, CH$_3$OH, and CS.
• Figure 2: (A)--(D). Spectrally integrated flux map for CH$_3$OH and H$_2$CO in K2, with traces and labels as in Figure \ref{['fig:hcn-maps']}. Contours are in 10$\sigma$ increments for CH$_3$OH and 5$\sigma$ increments for H$_2$CO ($J_{Ka,Kc}=5_{1,5}-4_{1,4}$). Contours are 3$\sigma$ and $5\sigma$ for H$_2$CO ($J_{Ka,Kc}=5_{0,5}-4_{0,4}$). In contrast to the other species, H$_2$CO showed asymmetric, clumpy, and spatially extended emission.
• Figure 3: (A)--(D). Continuum flux maps in K2, with traces and labels as in Figure \ref{['fig:hcn-maps']}. Contours are in 5$\sigma$ increments.
• Figure 4: 345 GHz continuum visibilities in C/2017 K2, with best-fit coma model overlaid. The $uv$ distances have been plotted in units of k$\lambda$.
• Figure 5: Left. Sunward and anti-sunward best-fit radial kinetic temperature profiles for CH$_3$OH in C/2017 K2. The shaded regions show the $1\sigma$ uncertainties. Right. Sunward and anti-sunward radial rotational temperature profiles calculated from the best-fit SUBLIME model rotational populations.