Very Large Telescope observations of interstellar comet 3I/ATLAS III: High-resolution monitoring of CN and forbidden oxygen emission across the perihelion passage with ESPRESSO

Baltasar Luco; Thomas H. Puzia; Rohan Rahatgaonkar; Juan Pablo Carvajal; Prasanta K. Nayak

paper

Very Large Telescope observations of interstellar comet 3I/ATLAS III: High-resolution monitoring of CN and forbidden oxygen emission across the perihelion passage with ESPRESSO

arXiv:2607.15355

Abstract

3I/ATLAS is only the third known interstellar object to traverse the Solar System. We obtained high-resolution ( $R ≃ 140000$ ) spectroscopy of 3I/ATLAS on 23 nights with VLT/ESPRESSO ( $r_{h} = 1.67 - 2.45$ au). CN production rates were derived by fitting the $B^{2} Σ^{+} - X^{2} Σ^{+}$ band, using CometSpec, a publicly released Python package for flexible fluorescence modeling. The three forbidden [OI] lines were modeled to derive the green-to-red (G/R) flux ratio and a CO $_{2}$ /H $_{2}$ O proxy. We fit $Q_{C N} = a (r_{h} /1 au)^{b}$ to the production rate versus heliocentric distance relation. The pre-perihelion CN power-law index is $b_{p r e} = - 4.6 2_{- 1.22}^{+ 1.25}$ , within the range observed for solar-system comets. The CN residuals about the smooth model show substantial scatter that we attribute to a combination of systematics and possible physical drivers that the present cadence cannot disentangle. The G/R ratio decreases from $\sim 0.42 \pm 0.14$ at $r_{h} ≃ 2.4$ au to $\sim 0.114 \pm 0.004$ near perihelion. The asymmetric piecewise oxygen fit is strongly preferred over the symmetric one, consistent with thermal-inertia models, although denser post-perihelion sampling would be needed to confirm this. The CO $_{2}$ /H $_{2}$ O proxy spans $\sim 0.06 - 1.06$ and is comparable to the values reported by Subaru/HDS and SPHEREx, and to those measured for 2I/Borisov. We additionally identify NiI and FeI, and report post-perihelion production rates for C $_{2}$ and CH. Our high-resolution monitoring of 3I/ATLAS reveals a CO $_{2}$ -rich coma that becomes progressively H $_{2}$ O-dominated near perihelion, with a G/R asymmetry consistent with delayed H $_{2}$ O sublimation predicted by thermal-inertia models. Together with the public release of CometSpec and the reduced data, these results provide both an empirical reference and a methodological framework for the next generation of interstellar-object monitoring campaigns.

19 pages, 11 figures, accepted for publication in A&A