COMET Kiwi: Oort Cloud Comet Activity

• COMET Kiwi is a dynamically new Oort Cloud comet noted for early hypervolatile activity and the development of an extensive coma at large heliocentric distances.
• High-resolution spectroscopy using instruments like McDonald Observatory’s Tull coudé and Keck I’s HIRESb enabled the precise detection of sequential radical emissions as the comet approaches the Sun.
• The study reveals a transition from hypervolatile-driven to water-driven outgassing, offering key insights into volatile layering and the compositional evolution of pristine cometary material.

Comet C/2017 K2 (PanSTARRS), referred to herein as "COMET Kiwi" (Editor's term), is a dynamically new long-period comet originating from the Oort Cloud, first observed at an unusually large heliocentric distance (16 au). Notable for its extensive coma at discovery and for exhibiting early dust and gas activity before perihelion, COMET Kiwi has become an important case study in the evolution of hypervolatiles and the onset of cometary activity with increasing solar proximity. Recent high-resolution spectroscopic studies have allowed detailed investigation of both dust and gas components, including the sequence in which key molecular species become detectable as the comet approaches the Sun, and the physical implications regarding compositional layering and drivers of cometary activity (Cochran et al., 31 Oct 2025).

1. Observational Methodology and Instrumentation

COMET Kiwi was observed using the McDonald Observatory 2.7 m telescope (Tull coudé spectrograph, $R = \lambda/\Delta\lambda \approx 60,\!000$) and the W. M. Keck I 10 m telescope (HIRESb spectrograph, $R \approx 48,\!000$). The Tull coudé provided 63 echelle orders covering $3400$–$10,\!000$ Å, with a $1.2''\times 8.2''$ slit, while HIRESb used three detectors over 53 orders, with a $0.86''\times 7''$ slit.

Ten observational epochs spanned from 18 Oct 2021 ($R_\mathrm{h}=5.06$ au) to 16 Nov 2023 ($R_\mathrm{h}=4.20$ au). Calibration included master bias and flat-field corrections, ThAr lamp wavelength calibration (precision $\lesssim 0.002$ Å), and order extraction. Observed spectra were flux-calibrated relative to standard stars or lamps, and the solar continuum was removed using high-S/N solar analog spectra (day-sky for Tull, HD 28099 for HIRESb). Doppler-shift corrections aligned emission features to the cometary rest frame. This workflow ensured detection of weak molecular emission lines over the reflected-solar continuum [(Cochran et al., 31 Oct 2025), Fig. 2].

2. Spectroscopic Discrimination: Dust and Gas Regimes

At large heliocentric distance ($R_\mathrm{h}\approx16$ au) and at $R_\mathrm{h}=5.06$ au, only a reflected-solar continuum with Fraunhofer absorption lines was observed; no gas emissions rose above the continuum after optimal subtraction of the solar analog. Gas emission identification relied on detecting narrow, positive residuals at wavelengths corresponding to known cometary radicals (CN, C$_2$, C$_3$, NH$_2$), following solar-spectrum subtraction. High spectral resolving power ($R\gtrsim48,000$) enabled weak gas features to be distinguished from noise, a feat unachievable at lower resolutions due to line broadening.

3. Sequence of Gas Turn-On: Radical Detection with Heliocentric Distance

The transition from dust-only to dust-and-gas activity was marked by the sequential detection of molecular emissions as the comet approached the Sun. The table below summarizes the heliocentric distances at which key radicals were first identified inbound:

Species	Transition (λ)	First Detection $R_\mathrm{h}$ (au)
CN	B–X (3883 Å)	3.39
C$_3$	A–X Q-branch (4050 Å)	3.24
C$_2$	Δv=0 (0–0) (5165 Å)	3.24
CH	A–X (4313 Å)	2.95
NH$_2$	(approx. 5731 Å)	2.82

CN was first detected on 22 Apr 2022 at $R_\mathrm{h}=3.39$ au, with subsequent detections of C$_3$ and C$_2$ as the comet approached $R_\mathrm{h}=3.24$ au. CH and NH$_2$ emissions followed at $R_\mathrm{h}<3.0$ au. This stratification indicates a hierarchy in volatile activation and outgassing, reflecting the parent molecule sublimation temperatures and column densities required for detection [(Cochran et al., 31 Oct 2025), Table A, Figs. 3–7].

4. Production Rate Formalism

While absolute production rates were not quantified for COMET Kiwi, the standard fluorescence efficiency (g-factor) formalism applies for all detected bands. The molecular production rate, $Q$, is given by:

$Q = \frac{4\pi\,\Delta^{2}\,F}{g(r_\mathrm{h})}$

where $\Delta$ is the geocentric distance (cm), $F$ is the integrated, continuum-subtracted band flux (erg$\,$cm$^{-2}\,$s$^{-1}$), and $g(r_\mathrm{h})$ is the fluorescence efficiency, scaling $\propto r_\mathrm{h}^{-2}$. Typical $g$-factors at $1$ au include:

• $$g_\mathrm{CN}(1\,\mathrm{au}) \approx 2.7\times10^{-13}$$ erg s$^{-1}$ molecule$^{-1}$
• $$g_\mathrm{C_2}(1\,\mathrm{au}) \approx 3.3\times10^{-13}$$ erg s$^{-1}$ molecule$^{-1}$
• $$g_\mathrm{C_3}(1\,\mathrm{au}) \approx 0.5\times10^{-13}$$ erg s$^{-1}$ molecule$^{-1}$
• $$g_\mathrm{CH}(1\,\mathrm{au}) \approx 0.6\times10^{-13}$$ erg s$^{-1}$ molecule$^{-1}$
• $$g_\mathrm{NH_2}(1\,\mathrm{au}) \approx 1.0\times10^{-13}$$ erg s$^{-1}$ molecule$^{-1}$

A detailed analysis would require measurement of integrated fluxes, propagation of photon and calibration uncertainties ($\sim10$\% for calibration, $\lesssim20$\% for $g$-factors), and application of the above formula for each epoch and species (Cochran et al., 31 Oct 2025).

5. Physical Drivers of Cometary Activity: Sublimation and Outgassing

COMET Kiwi’s activity profile reveals a transition from hypervolatile-driven to water-driven outgassing. H$_2$O ice sublimates efficiently inside $R_\mathrm{h}\lesssim3$ au ($T_\mathrm{sub}\sim150$ K), whereas the parents of CN (likely HCN or related nitriles) have lower sublimation temperatures or are released from CO/CO$_2$-rich dust. This accounts for CN appearance at $3.39$ au, prior to significant water activation. C$_2$ and C$_3$ signals require denser gas columns and arise slightly closer to the Sun ($\sim3.24$ au). CH and NH$_2$, secondary products of CH$_4$/CH$_3$OH and H$_2$O photodissociation, activate inside $3$ au.

A hierarchical picture emerges:

• CO ($T_\mathrm{sub} \approx 25$ K) and CO$_2$ ($T_\mathrm{sub}\approx80$ K) support activity beyond $6$ au [NEOWISE, Yang et al. 2021].
• H$_2$O becomes dominant inside $3$ au, consistent with atomic oxygen line diagnostics and corroborated by JWST findings of H$_2$O$\gg$(CO+CO$_2$) [Woodward et al. 2025].

Oxygen forbidden line ratios, $R_\mathrm{OI} = F_{5577}/(F_{6300} + F_{6363})$, in the range $0.11$–$0.25$ confirm H$_2$O photodissociation as the primary atomic O source near perihelion. Gaussian deblending separates cometary from telluric [O I] emission, providing a robust physical proxy for in situ water abundance [(Cochran et al., 31 Oct 2025), Table 5, Fig. 9].

6. Compositional Evolution and Outgassing Structure

COMET Kiwi retains a "pristine" layer of hypervolatiles, as expected from an Oort Cloud first-entry object. Early hypervolatile (CO, CO$_2$, HCN) outgassing produces detectable CN well outside the water-ice sublimation zone. Inside $\sim3$ au, the coma chemistry shifts to H$_2$O-typical composition, evidenced both by molecular detection sequence and atomic oxygen diagnostics. Heterogeneous, clumpy outgassing is implied by slightly staggered "turn-on" distances among different species and by reappearance of CN emission at $R_\mathrm{h}=4.11$ au on the outbound leg, suggesting persistent or episodic localized activity. These findings offer insight into both the layering of volatile reservoirs and the mechanisms of dust and gas emission in dynamically new comets (Cochran et al., 31 Oct 2025).

7. Implications for Cometary Science and Future Investigations

The high spectral resolving power achieved in these observations has resulted in a detailed chronology and chemical stratigraphy of activity onset in COMET Kiwi. The data reinforce the conceptual model where early activity is maintained by hypervolatile species until water sublimation becomes thermodynamically viable closer to the Sun. These results serve as benchmarks for models of cometary coma evolution, compositional layering, and outgassing physics in Oort Cloud comets entering the inner Solar System for the first time. A plausible implication is that detailed multi-species, multi-epoch spectroscopy, as exemplified here, is indispensable for deconvolving the volatile composition and physical evolution of pristine cometary material (Cochran et al., 31 Oct 2025).