Coma Volatile Composition and Thermal Physics in Comet C/2022 E3 (ZTF) Measured Near Closest Approach to Earth with NASA-IRTF

TL;DR

This work presents high-resolution near-infrared spectroscopy of comet C/2022 E3 (ZTF) near its closest approach to Earth, mapping the abundances and spatial distributions of coma volatiles with iSHELL at the NASA-IRTF. By observing multiple ro-vibrational transitions and employing dual slit orientations, the authors derive production rates $Q$ and nucleus-centered rates $Q_{NC}$, along with rotational temperatures $T_{rot}$ for a suite of molecules (H$_2$O, CO, OCS, CH$_3$OH, CH$_4$, C$_2$H$_6$, C$_2$H$_2$, HCN) and OH$^*$ as a proxy. They find overall depletion of most volatiles relative to typical Oort cloud comets, except OCS, and uncover distinct outgassing sources for certain species via spatial profiles; notably, H$_2$O $T_{rot}$ peaks off-nucleus and shows slower cooling in the anti-sunward hemisphere, consistent with icy-grain sublimation in the coma. A radiative-sublimation analysis suggests E3 could be hyperactive if its nucleus is sufficiently small ($R_N \lesssim 1.35$ km). Collectively, the results illustrate the NASA-IRTF’s capability to dissect coma chemistry and thermal physics at sub-1000 km scales, complementing JWST and ALMA findings and advancing our understanding of OCCs near perihelion.

Abstract

The 2023 perihelion passage of comet C/2022 E3 (ZTF) afforded an opportunity to measure the abundances and spatial distributions of coma volatiles in an Oort cloud comet at high spatial resolution near its close approach to Earth ($Δ_\mathrm{min}\sim 0.28$ au on UT February 1). We conducted near-infrared spectroscopic observations of C/2022 E3 (ZTF) using iSHELL at the NASA Infrared Telescope Facility on UT 2023 February 9. Our measurements securely detected fluorescent emission from H$_2$O, CO, OCS, CH$_3$OH, CH$_4$, C$_2$H$_6$, C$_2$H$_2$, and HCN. For each instrumental setting we took exposures with the slit oriented parallel and also perpendicular to the projected Sun-comet vector, thereby enabling a test of the spatial distributions of these molecules. We report rotational temperatures, production rates, and abundance ratios (i.e., mixing ratios) for all sampled species. Our measurements found that molecular abundances in C/2022 E3 were depleted compared to their average values in Oort cloud comets with the exception of OCS, which was consistent. The H$_2$O production rate varied significantly and was likely tied to nucleus rotation effects. Measurements at the two slit orientations showed distinct column density and rotational temperature profiles for H$_2$O. Peak temperatures occurred off-nucleus and slower cooling was present in the anti-sunward hemisphere, consistent with the presence of icy grain sublimation in the coma.

Figures (9)

• Figure 1: Upper. Detections of H$_2$O, HCN, C$_2$H$_2$, and OH$^*$ in E3 covering iSHELL echelle order 170--172. The uppermost trace is the observed cometary spectrum with the telluric transmittance model superimposed. Below are individual molecular fluorescence models color-coded by species. The bottom trace shows the residual (cometary spectrum minus all models) with the $1\sigma$ uncertainty envelope overlaid and shaded. Lower. Zoomed plots covering the gray shaded regions indicated in the upper panel. In this case, the uppermost trace is the telluric-subtracted comet spectrum, with individual fluorescence models plotted below. We searched for NH$_2$, a potentially confounding species in this spectral region, but did not detect it. Our $3\sigma$ upper limit is NH$_2$/H$_2$O $<$ 0.03%.
• Figure 2: Upper, Lower. Detections of H$_2$O and CO in E3 covering iSHELL echelle orders 109--112, with traces and labels as in Figure \ref{['fig:h2o']}.
• Figure 3: Upper, Lower. Detections of H$_2$O (OH$^*$), C$_2$H$_6$, CH$_3$OH, and CH$_4$ in E3 covering iSHELL echelle orders 154--158, with traces and labels as in Figure \ref{['fig:h2o']}.
• Figure 4: Upper, Lower. Detections of H$_2$O, OCS, and CN in E3 covering iSHELL echelle order 106, with traces and labels as in Figure \ref{['fig:h2o']}.
• Figure 5: Upper. Spatial profiles of emissions for co-measured H$_2$O (blue), HCN (orange), and continuum (dashed) in E3. Color-coded 1$\sigma$ uncertainties are shown for the profile of each species. The slit was oriented along the projected Sun-comet line (position angle 88$\degr$), with the Sun-facing direction to the left as indicated. Also shown is the Sun-comet-Earth angle (phase angle, $\beta$) of 48$\degr$. "Smooth" indicates the width (pixels) of a boxcar smoothing kernel applied to HCN. Lower. Spatial profiles of co-measured H$_2$O, HCN, and continuum in E3 with the slit oriented perpendicular to the projected Sun-comet line (position angle 178$\degr$).