Perihelion Asymmetry in the Water Production Rate of the Interstellar Object 3I/ATLAS

Abstract

3I/ATLAS is an interstellar object whose activity provides critical insights into its composition and origin. However, due to its orbital geometry, the object is too close to the Sun near perihelion to be observed from the ground, and space-based measurements are therefore required. Here we characterize the water production rate of 3I/ATLAS using SOHO/SWAN Lyman-$\alpha$ observations from 2025 November to December (heliocentric distances 1.4 to 2.2 au) with 3D Monte Carlo modeling. We report a peak post-perihelion water production rate of $Q_{{\mathrm{H}}_2\mathrm{O}}\approx 4\times 10^{28}$ molecules~s${}^{-1}$, corresponding to a minimum active fraction of $\sim$30\% (assuming a maximum nucleus radius of 2.8 km). Comparison of our post-perihelion measurements with published pre-perihelion results reveals a heliocentric asymmetry, with an $r^{-5.9\pm 0.8}$ scaling for the inbound rise, followed by a shallower $r^{-3.3\pm 0.3}$ scaling during the outbound decline, where $r$ is heliocentric distance. The post-perihelion behavior indicates that the water production of 3I/ATLAS was driven primarily by the varying solar insolation acting on a stable active area. Combined with other evidence, including comparison with the hyperactive comet 103P/Hartley 2, our findings suggest that its water production is likely dominated by a distributed source of icy grains. Furthermore, it displayed remarkable stability in the activity with no signs of outbursts or rapid depletion of water production.

arXiv comment13 pages, 7 figures. Accepted for publication in The Astrophysical Journal Letters (ApJL)

References in corpus (23)

• SciPy 1.0--Fundamental Algorithms for Scientific Computing in Python
• Astropy: A Community Python Package for Astronomy
• The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package
• Binary companions of evolved stars in APOGEE DR14: Search method and catalog of ~5,000 companions
• The carbon monoxide-rich interstellar comet 2I/Borisov
• Terrestrial deuterium-to-hydrogen ratio in water in hyperactive comets
• A Survey of Water Production in 61 Comets from SOHO/SWAN Observations of Hydrogen Lyman-alpha: Twenty-One Years 1996-2016
• The Evolving Activity of the Dynamically Young Comet C/2009 P1 (Garradd)
• On the Origin and Thermal Stability of Arrokoths and Plutos Ices
• Water production rates and activity of interstellar comet 2I/Borisov
• Uncorrelated Volatile Behavior During the 2011 Apparition of Comet C/2009 P1 Garradd
• Water Production by Comet 103P/Hartley 2 Observed with the SWAN Instrument on the SOHO Spacecraft
• A rapid decrease in the rotation rate of comet 41P/Tuttle-Giacobini-Kresák
• Outburst and Splitting of Interstellar Comet 2I/Borisov
• Monitoring of the activity and composition of comets 41P/Tuttle-Giacobini-Kresak and 45P/Honda-Mrkos-Pajdusakova
• Compact pebbles and the evolution of volatiles in the interstellar comet 2I/Borisov
• Pre-perihelion evolution of the NiI/FeI abundance ratio in the coma of the interstellar comet 3I/ATLAS. From extreme to normal
• Activity and composition of the hyperactive comet 46P/Wirtanen during its close approach in 2018
• New Insights into Interstellar Object 1I/2017 U1 (`Oumuamua) from SOHO/STEREO Nondetections
• Water Production Rate of C/2020 F3 (NEOWISE) from SOHO/SWAN over Its Active Apparition
• Low NH${}_3$/H${}_2$O ratio in comet C/2020 F3 (NEOWISE) at 0.7 au from the Sun
• Rapid Brightening of 3I/ATLAS Ahead of Perihelion
• Ionic emissions and activity evolution in comet C/2020 F3 (NEOWISE): Insights from long-slit spectroscopy and photometry

Cited by in corpus (4)

• Perihelion observations of interstellar comet 3I/ATLAS with the IRAM 30-m telescope
• Interstellar Object 3I/ATLAS Observed from Mars by China's Tianwen-1 Spacecraft
• Pre-perihelion Volatile Evolution of Interstellar Comet 3I/ATLAS Indicating Significant Contribution from Extended Source in the Coma
• Dust Activity Evolution and Constraints on Icy Grains in the Interstellar Comet 3I/ATLAS