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 ${\mathrm{H}}_2\mathrm{O}$ and CO abundances, respectively. We derived OH production rates of $(1.32\pm 0.47)\times 10^{28}{\mathrm{s}}^{-1}$ at 2.27 au and $(1.89\pm 0.37)\times 10^{28}{\mathrm{s}}^{-1}$ at 1.96 au as well as an average CO production rate of $(5.75\pm 1.91)\times 10^{27}{\mathrm{s}}^{-1}$ between 2.33 and 1.75 au, inferring a CO/${\mathrm{H}}_2\mathrm{O}$ ratio of ($28\pm 11\%$). With the mean HCN production rate of $2.5\times 10^{25}{\mathrm{s}}^{-1}$ at 2.1 au reported by \citet{2025arXiv251120845R} and \citet{2025arXiv251002817C}, we infer a CO/HCN ratio of ($230\pm 76$). 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.

14 pages, 9 figures, accepted for publication in ApJL

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