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

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 [O I] $\lambda 6300$ emission shows a significant residual after subtracting the expected contributions from H${}_2$O, CO${}_2$, and CO, which may reflect systematic uncertainties in the photodissociation yields of those molecules or a contribution from additional oxygen-bearing parents.

20 pages, 4 figures, 6 tables, accepted for publication in ApJL