Pre-perihelion Volatile Evolution of Interstellar Comet 3I/ATLAS Indicating Significant Contribution from Extended Source in the Coma

Abstract

Interstellar comets provide rare opportunities for probing the diversity of refractory and volatile inventory around other stars. As the second ever interstellar comet, and the third interstellar object, 3I/ATLAS has been the focus of telescopic observations since its discovery in July 2025. Following the previous observations at multi-wavelengths, we present further radio observations of the 1665/1667 MHz ground-state OH lines and millimeter observations of the CO($J$=1-0) transition at 115.271 GHz that trace the coma $\rm H_2O$ and CO abundances, respectively. We derived OH production rates of $(1.32\pm0.47)\times10^{28}\ \rm s^{-1}$ at 2.27 au and $(1.89\pm0.37)\times10^{28}\ \rm s^{-1}$ at 1.96 au as well as an average CO production rate of $\rm (5.75\pm1.91) \times 10^{27}\, s^{-1}$ between 2.33 and 1.75 au, inferring a CO/$\rm H_2O$ ratio of ($28\pm11\%$). With the mean HCN production rate of $2.5\times 10^{25}\ \rm s^{-1}$ at 2.1 au reported by \citet{2025arXiv251120845R} and \citet{2025arXiv251002817C}, we infer a CO/HCN ratio of ($230\pm76$). By synthesizing water production rates measured with instruments of different apertures, we found that the sublimation from extended source in the coma contributes significantly to 3I's pre-perihelion water measurements, accounting for up to 80\% from 3 au to 2 au.

Figures (9)

• Figure 1: Averaged 18 cm OH lines (scaled to 1667 MHz) of 3I obtained in four observations with both a Gaussian and a trapezoid fit: Sep. 08 and 09 (top), Sep. 18 and 23 (bottom). Original observations are shown in Figure \ref{['fig:all2']} and \ref{['fig:all3']}.
• Figure 2: Averaged CO(1-0) line of 3I obtained from September observations. Original observations are shown in Figure \ref{['fig:allco']}.
• Figure 3: $\rm H_2O$ production rates of comet 3I as a function of heliocentric distance. The measurements from different sources are shown with different colors 2025AA...700L..10A2025arXiv250926053H2025ApJ...991L..43C2025ApJ...991L..50X2025arXiv251207318L2025CBET.5625....1C. Error bars represent 1$\sigma$ uncertainties. The symbol of circle, rectangle and triangle indicate the detection via OH, $\rm H_2O$ and upper limits, respectively. The red line shows the best-fit power-law curve with all data excluding those from VLT 2025arXiv250926053H and JWST 2025ApJ...991L..43C. The blue one shows the best-fit power-law curve with VLT measurements only.
• Figure 4: Contribution of extended source. (a) The power-law relationship between $\rm H_2O$ production rate and heliocentric distance. Black and blue solid lines are the fitted power-law relations with measurements done with large apertures ($\gtrsim20000$ km) and small apertures (3000--4000 km), respectively. Red dashed line is the curve of production rate directly from the nucleus inferred from the model fitting. (b) Aperture-dependent column density calculated based on the production rate shown in (a). (c) Best-fitted equivalent scale-length of $\rm H_2O$ extended source with an initial of $L_{\rm ext}=4000^{+1000}_{-1000}$ km at 2.9 au is derived from 2025ApJ...991L..50X. (d) Fraction of the extended source in the total water production within different apertures varying against heliocentric distances.
• Figure 5: Coma CO/$\rm H_2O$ mixing ratios as a function of heliocentric distance for Solar System comets and interstellar comets. The measurements of different dynamic group are shown with different colors. All data are summarized from the collections in previous works 2016Icar..278..301D2019AJ....158..128M2022PSJ.....3..247H and recent observations 2018AA...619A.127B2024AA...690A.271B2022AA...664A..95B2022PSJ.....3....6D2025arXiv251105662R2020NatAs...4..861C2025PSJ.....6..139W.A log-linear curve is fitted to the Solar System comets, with $1\sigma$ prediction band shown as dashed gray lines. The other two results of 3I/ATLAS are from 2025ApJ...991L..43C and 2025arXiv251207318L.