2026/07/16 by Baltasar Luco, Thomas H. Puzia, Rohan Rahatgaonkar +2
#astro-ph.EP #astro-ph.GA
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 (rh=1.67-2.45au). CN production rates were derived by fitting the B2Σ+-X2Σ+ 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 CO2/H2O proxy. We fit QCN=a(rh/1au)b to the production rate versus heliocentric distance relation. The pre-perihelion CN power-law index is bpre=-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 ∼ 0.42± 0.14 at rh≃ 2.4au to ∼ 0.114 ± 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 CO2/H2O proxy spans ∼ 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 C2 and CH. Our high-resolution monitoring of 3I/ATLAS reveals a CO2-rich coma that becomes progressively H2O-dominated near perihelion, with a G/R asymmetry consistent with delayed H2O 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.