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 from December 2025 to January 2026 (heliocentric distances 1.8 to 3.3 au). From these spectra, we derive post-perihelion production rates and/or mixing ratios for daughter molecules (CN, C$_3$, C$_2$, and CH) and gaseous metals (Fe I and Ni I). We also estimate a lower limit on the CO abundance based on the [O I] $\lambda6300$ line. The resulting outgassing profiles reveal a pronounced perihelion asymmetry, with volatile production declining more gradually outbound than during the inbound phase. In addition, the coma becomes less depleted in C$_2$ following perihelion, and shows a substantial increase, relative to H$_2$O, in the production rates of metals and potentially also CO. These trends may indicate the activation of subsurface material or compositional heterogeneity revealed by seasonal effects. The inferred high CO abundance further suggests possible decoupling of CO from H$_2$O and CO$_2$, similar to the behavior observed in solar system comets. The potential post-perihelion enhancement of both CO and metal production, if confirmed, would also be consistent with metal carbonyls contributing to the release of gaseous metals in cometary comae.

Figures (3)

• Figure 1: Eimission spectra of 3I/ATLAS taken with BFOSC and YFOSC. For clarity, the full spectral range is divided into three panels with different vertical offsets to emphasize specific features. The left panel highlights emission lines of Ni i (blue dashed lines) and Fe i (red dashed lines), and the CN violet band (blue shaded). The middle panel shows molecular bands of C$_3$ (green shaded), CH (orange shaded), and C$_2$ (red shaded), and lists the heliocentric distance for each spectrum. The right panel displays the [O i] $\lambda\lambda6300,6364$ doublet (green dashed lines).
• Figure 2: Production rates of CN, Ni i, and Fe i as a function of heliocentric distance for 3I/ATLAS. Post-perihelion measurements from this work are shown as filled circles, while pre-perihelion measurements from Hutsemekers:2026 are shown as open circles. The species are color-coded as CN---grey, Ni i---blue, and Fe i---red. Dashed lines indicate the corresponding power-law dependencies ($r_{\rm h}^{-n}$) derived from weighted fits.
• Figure 3: Correlations. (a): $\log(Q_{\rm Ni\,I}/Q_{\rm Fe\,I})$ versus $\log(Q_{\rm C_2}/Q_{\rm CN})$; (b): $\log(Q_{\rm Ni\,I+Fe\,I})$ versus $\log(Q_{\text{H}_2\text{O}})$; (c): $\log(Q_{\rm Ni\,I+Fe\,I})$ versus $\log(Q_{\text{CO}})$. The red arrows in panel (a) indicate the temporal evolution of 3I. The black lines in panel (b) and (c) denote the best-fit correlations with $Q_{\text{H}_2\text{O}}$ (including all solar system comets except C/2016 R2) and with $Q_{\text{CO}}$ (including all comets). Post-perihelion measurements of 3I (red circles) are from this work, except for $Q_{\rm H_2O}$, which is adopted from Tan:2026. Pre-perihelion measurements of 3I (open squares) are taken from Hutsemekers:2026 and Cordiner:2025. Comparison data are from Hutsemekers:2021, Manfroid:2021, and Opitom:2021, with colors and marker styles following Manfroid:2021 for consistency.