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

Abstract

3I/ATLAS is only the third known interstellar object to traverse the Solar System. We obtained high-resolution ($R\simeq 140000$) 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 $B^2\Sigma^+-X^2\Sigma^+$ 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 $Q_{CN}=a(r_{h}/1\text{au})^b$ to the production rate versus heliocentric distance relation. The pre-perihelion CN power-law index is $b_{pre}=-4.62^{+1.25}_{-1.22}$, 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 $r_{h}\simeq 2.4$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.