CH$_3$OH and HCN in Interstellar Comet 3I/ATLAS Mapped with the ALMA Atacama Compact Array: Distinct Outgassing Behaviors and a Remarkably High CH$_3$OH/HCN Production Rate Ratio

TL;DR

The paper presents ALMA ACA observations of CH$_3$OH and HCN in the interstellar comet 3I/ATLAS during pre-perihelion epochs, revealing distinct outgassing patterns and an exceptionally high CH$_3$OH/HCN production-rate ratio. Using non-LTE radiative-transfer modeling with Haser-like density distributions and two-component CH$_3$OH outgassing, the study separates nucleus-derived and coma-sourced production, finding HCN consistent with direct nucleus sublimation while CH$_3$OH includes a coma component with $L_p>258$ km. CH$_3$OH production rises sharply as $r_H$ decreases toward 2 au, and CH$_3$OH/HCN ratios of $124^{+30}_{-34}$ and $79^{+11}_{-14}$ place 3I among the methanol-rich comets known, suggesting chemical heterogeneity or grain-sourced methanol in the coma. The results underscore the value of high-resolution, multi-transition spectroscopy of interstellar visitors for constraining nucleus composition, coma chemistry, and the physical processes governing outgassing and grain processing. This work advances our understanding of exogenous material delivery and offers benchmarks for comparative studies with solar-system comets.

Abstract

We report the detection of methanol (CH$_3$OH) toward interstellar comet 3I/ATLAS using the Atacama Compact Array of the Atacama Large Millimeter/Submillimeter Array (ALMA) on UT 2025 August 28, September 18 and 22, and October 1, and of hydrogen cyanide (HCN) on September 12 and 15. These observations spanned pre-perihelion heliocentric distances ($r_H$) of 2.6 -- 1.7 au. The molecules showed outgassing patterns distinct from one another, with HCN production being depleted in the sunward hemisphere of the coma, whereas CH$_3$OH was enhanced in that direction. Statistical analysis of molecular scale lengths in 3I/ATLAS indicated that CH$_3$OH included production from coma sources at $L_p>258$ km at 99% confidence, although low signal-to-noise on long baselines prevented definitively ruling out CH$_3$OH as purely a parent species. In contrast, HCN production was indistinguishable from direct nucleus sublimation. The CH$_3$OH production rate increased sharply from August through October, including an uptick near the inner edge of the H$_2$O sublimation zone at $r_H$ = 2 au. Compared to comets measured to date at radio wavelengths, the derived CH$_3$OH/HCN ratios in 3I/ATLAS of $124^{+30}_{-34}$ and $79^{+11}_{-14}$ on September 12 and 15, respectively, are among the most enriched values measured in any comet, surpassed only by anomalous solar system comet C/2016 R2 (PanSTARRS).

Figures (16)

• Figure 1: (A)--(D). Spectrally integrated flux maps for HCN on September 12 and 15 and for CH$_3$OH on September 18 and October 1. 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. Contours are $5\sigma$ and $10\sigma$ for HCN and for CH$_3$OH on October 1, and $3\sigma$ and $5\sigma$ for CH$_3$OH on September 18. 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 ($\phi_{\mathrm{STO}} \sim$ 20$\degr$), as well as the direction of the Sun and dust trail, are indicated in the lower right. A spectrum of the HCN (J = $4-3$) transition from a $10\arcsec$ diameter aperture centered on the peak emission is shown in the upper right. The CH$_3$OH map on September 18 shows the same extract for the high resolution spectrum of the $J_K=1_1-0_0 A^+$ transition near 350 GHz, whereas the October 1 map shows this transition at low spectral resolution on October 1.
• Figure 2: (A). HCN (J = $4-3$) spectrum in 3I on Sept. 12, with each panel representing spectra extracted from differing baseline ranges (angular scales). The spectra are displayed with a frequency resolution of 122 kHz (velocity resolution 0.13 km s$^{-1}$). The best-fit 1D daughter and parent models are overplotted. A dashed vertical line at $v=0$km s$^{-1}$ emphasizes the redward offset of the line center. (B).$\Delta\chi^2$ analysis for the HCN $L_p$ on September 12 (Appendix \ref{['sec:fourier']}). (C--D). Plots for HCN (J = $4-3$) in 3I on September 15, with traces and labels as in panels (A) and (B).
• Figure 3: (A). CH$_3$OH ($J_K=1_1-0_0 A^+)$ spectrum in 3I on Sept. 18, with each panel representing spectra extracted from differing baseline ranges (angular scales). The spectra are displayed with a frequency resolution of 122 kHz (velocity resolution 0.21 km s$^{-1}$). The best-fit daughter and parent models are overplotted. A dashed vertical line at $v=0$km s$^{-1}$ emphasizes the blueward offset of the line center. (B).$\Delta\chi^2$ analysis for the CH$_3$OH $L_p$ on September 18 from the 1D and 3D models (Appendix \ref{['sec:fourier']}). (C). As in panel (A), but with a comparison of the best-fit 1D and 3D SUBLIME daughter models overplotted. Spectra are summed over a larger number of short baselines to emphasize the signal-to-noise ratio of the asymmetric line profile. (D). Integrated CH$_3$OH ($J_K=1_1-0_0A^+$) flux map on September 18, with contours for the red and blue components overlaid separately. Contours are in $1\sigma$ increments, with $2\sigma$ being the lowest contour level. The extracted spectral line profile shows the integration region for the blue and red maps across the line.
• Figure 4: (A). HCN (J = $4-3$) integrated flux map on September 12 with contours for the blue and red components of the line overlaid separately. The extracted spectral line profile shows the integration region for the blue and red maps across the line. Contours are in $3\sigma$ increments, with $3\sigma$ being the lowest contour level. (B). As in panel (A) for September 15. (C).$Q$(CH$_3$OH) in 3I/ATLAS as a function of rH measured with the ACA. Also shown in the best-fit power law curve. (D). CH$_3$OH/HCN ratio in solar system comets measured at radio wavelengths to date adapted from Biver2024. The values for 3I (CH$_3$OH/HCN=$124^{+30}_{-34}$ and $79^{+11}_{-14}$) are indicated with purple and yellow boxes for September 12 and 15, respectively, with the horizontal dashes indicating the $1\sigma$ uncertainty. The most enriched comet (far right) is C/2016 R2 (PanSTARRS), with CH$_3$OH/HCN=$280\pm72$Biver2018.
• Figure 5: (A)--(D). Spectrally integrated flux maps for CH$_3$OH on August 28, September 18 and 22, and October 1, with traces and labels as in Figure \ref{['fig:main-maps']}. Panel A shows a spectrum extracted in a $10\arcsec$ diameter aperture centered on the peak gas emission for the transition indicated. Panels B, C, and D show the integrated flux for all detected CH$_3$OH ($J_K=7_K-6_K$) transitions on a given date.