Ion abundances in the plasma tail of 3I/ATLAS show that it is N2-rich

Abstract

Interstellar objects allow us to compare the early planetary formation process in the Solar System and around other stars. The second and third interstellar objects, 2I/Borisov and 3I/ATLAS, showed visible signs of activity, making it possible to assess their composition via spectroscopic analysis of their gas coma. While neutral volatiles surrounding comets are easily probed by common spectrographs, ion emissions emanate mostly from the tail, making them easier to detect with integral-field units. We present WHT/WEAVE-LIFU observations of 3I/ATLAS, performed post-perihelion on 2025 November 30 and 2025 December 02. We simultaneously detect the ions N2+, CO+, CO2+, H2O+ and CH+ located in an anti-solar tail. Using common fluorescence factors, we determine abundance ratios of these species in the tail. In particular, the N2+/CO+ ratio allows us to constrain the lower-limit N2/CO>0.023+/-0.001, indicating that 3I/ATLAS is N2-rich compared to Solar System comets, a property which can be linked to cold formation conditions. Abundance ratios between other ions cannot directly be used to constrain abundances of neutrals but seem consistent with the fact that 3I/ATLAS is enriched in hypervolatiles. We also investigate whether these ratios vary along the tail, and only detect a marginal decrease of CH+ with cometocentric distance.

7 pages, 4 figures

References in corpus (11)

• The wide-field, multiplexed, spectroscopic facility WEAVE: Survey design, overview, and simulated implementation
• The carbon monoxide-rich interstellar comet 2I/Borisov
• Unusually High CO Abundance of the First Active Interstellar Comet
• High resolution optical spectroscopy of the ${\mathrm{N}}_2$-rich comet C/2016 R2 (PanSTARRS)
• The similarity of the interstellar comet 2I/Borisov to solar system comets from high resolution optical spectroscopy
• Activity of the first interstellar comet 2I/Borisov around perihelion: Results from Indian observatories
• Pre-perihelion evolution of the NiI/FeI abundance ratio in the coma of the interstellar comet 3I/ATLAS. From extreme to normal
• Very Large Telescope Observations of Interstellar Comet 3I/ATLAS. II. From Quiescence to Glow: Dramatic Rise of Ni i Emission and Incipient CN Outgassing at Large Heliocentric Distances*
• Ionic emissions and activity evolution in comet C/2020 F3 (NEOWISE): Insights from long-slit spectroscopy and photometry
• Post-perihelion Coma Composition of the Interstellar Comet 3I/ATLAS from Optical Spectroscopy
• Near-perihelion activity and composition of 3I/ATLAS from JUICE/MAJIS observations