Post-perihelion Coma Composition of the Interstellar Comet 3I/ATLAS from Optical Spectroscopy

TL;DR

This study presents optical spectroscopy data of interstellar comet 3I/ATLAS, revealing compositional heterogeneity, activity asymmetry around perihelion, and insights into volatile and metal release mechanisms.

Contribution

It provides the first detailed post-perihelion compositional analysis of 3I/ATLAS, highlighting heterogeneity and linking metal release to CO-bearing volatiles.

Findings

C$_2$ depletion decreases after perihelion, indicating subsurface activity.

Outgassing rates decline more gradually outbound, showing perihelion asymmetry.

Metal release correlates with CO, not just H$_2$O, suggesting specific volatile reservoirs.

Abstract

We present multi-epoch optical spectroscopy of the interstellar comet 3I/ATLAS obtained between December 2025 and January 2026 (heliocentric distances 1.8-3.3 au), yielding post-perihelion production rates and mixing ratios for CN, C$_3$, C$_2$, CH, and gaseous metals (Fe I and Ni I). Our results show that the coma is less depleted in C$_2$ after perihelion than before, indicative of subsurface activation or compositional heterogeneity. The outgassing profiles reveal a pronounced perihelion asymmetry: CN and metal production rates decline more gradually outbound than inbound, consistent with the reported behavior of H$_2$O and implying a change in the comet's activity pattern across perihelion. Despite being metal-rich relative to its H$_2$O content, 3I follows the metal-CO correlation observed in comets of diverse origins, suggesting that gaseous metal release is more closely linked to a CO-bearing volatile reservoir than to H$_2$O, potentially in the form of metal carbonyls. In addition, the significant residual in the [O I] $\lambda6300$ emission cannot be explained by H$_2$O, CO$_2$, and CO alone, indicating a contribution from additional oxygen-bearing parents.