Post-Perihelion Integral Field Spectroscopy of the Interstellar Comet 3I/ATLAS

TL;DR

This study presents the first post-perihelion blue-sensitive integral-field spectroscopy of the interstellar object 3I/ATLAS using KCWI, confirming ongoing CN, Ni, and Fe outgassing with new detections of C2 and C3 and CH emission. By applying a Haser-model framework to the spatially resolved data, the authors derive production rates for CN, C2, C3, Ni, and Fe, and quantify their radial distributions, finding notably larger e-folding radii post-perihelion than before. The Ni/Fe abundance ratio evolves with heliocentric distance, with log(Q_Ni/Q_Fe) ≈ -0.16 post-perihelion, approaching Solar System values and extending the pre-perihelion trend of Ni enrichment decline; this evolution provides a unique probe of exoplanetary material. Together, these results demonstrate that 3I/ATLAS continues to behave like a Solar-System comet in its post-perihelion phase and highlight Ni/Fe as a potential diagnostic of primordial metallicity in other planetary systems, while underscoring the value of spatially resolved, multi-epoch observations for interstellar small bodies and the outlook for future discoveries by LSST and similar surveys.

Abstract

The environs of other stellar systems may be directly probed by analyzing the cometary activity of interstellar objects. The recently discovered interstellar object 3I/ATLAS was the subject of an intensive worldwide follow-up campaign in its pre-perihelion approach. Now, 3I/ATLAS has begun its post-perihelion departure from the Solar System. In this letter, we report the first post-perihelion blue-sensitive integral-field unit spectroscopy of 3I/ATLAS using the Keck Cosmic Web Imager on November 16, 2025. We confirm previously reported CN, Fe, and Ni outgassing along with detections of carbon chain molecules $\mathrm{C}_2$ and $\mathrm{C}_3$. We calculate production rates for each species. We find Fe and Ni production rates of $\mathrm{Q_{Fe}} = (9.55\pm3.96)\times10^{25}$ atoms s$^{-1}$, and $\mathrm{Q_{Ni}} = (6.61\pm2.74)\times10^{25}$ atoms s$^{-1}$, resulting in a ratio of $\log(\mathrm{Q_{Ni}} / \mathrm{Q_{Fe}}) = -0.16\pm0.03$, which matches Solar System comets well and continues the pre-perihelion trend of declining $\log(\mathrm{Q_{Ni}} / \mathrm{Q_{Fe}})$ with $r_h$. We investigate the radial distributions of these elemental species and find characteristic $e$-folding radii of 3880$\pm$39 km for Ni, 6053$\pm$68 km for CN, 4194$\pm$45 km for $\mathrm{C}_2$, and 3833$\pm$45 km for $\mathrm{C}_3$. Compared to pre-perihelion measurements, these radii have increased by a factor of $\sim$6.5--7. Our post-perihelion observations reveal that 3I/ATLAS continues to exhibit cometary behavior broadly consistent with Solar System comets.

Figures (8)

• Figure 1: The continuum-subtracted KCWI spectrum of 3I/ATLAS between 3325 Å and 5225 Å, extracted from a 2 aperture centered on the comet. Cometary emission species are denoted as follows: Ni as red dotted lines, Fe as green dashed lines, CN as solid blue lines, $\mathrm{C}_2$ as dotted light purple lines, and $\mathrm{C}_3$ as dashed dark purple lines. The $\mathrm{C}_2$ and $\mathrm{C}_3$ lines shown may not be individually resolved.
• Figure 2: The same as Figure \ref{['fig:spec1d']}, but with individual panels for each emission feature.
• Figure 3: The continuum-subtracted KCWI spectrum of 3I/ATLAS between 7500 Å and 8600 Å, extracted from a 3 aperture centered on the comet. The red CN system is shown in blue.
• Figure 4: Post-perihelion production rate evolution as a function of heliocentric distance using data from this work at $r_h=1.509$ au and rates reported from TRAPPIST measurements Jehin25_ATelJehin25_ATel_2. CN and $\mathrm{C_2}$ have similar exponential decays, whereas $\mathrm{C_3}$ is nearly constant. Best-fit power laws are shown as the solid (CN), dotted ($\mathrm{C_2}$), and dashed ($\mathrm{C_3}$) lines.
• Figure 5: Comparison of narrow-band images from the KCWI data cube in the spectral regions corresponding to Ni, Fe, CN, $\mathrm{C_3}$, and $\mathrm{C_2}$. The maximum value of each panel is set individually.