Preperihelion CN Production Rate of Interstellar Comet 3I/ATLAS

Abstract We present spectroscopic observations of the third interstellar comet 3I/ATLAS obtained with the DOLORES instrument on the 3.58 m Telescopio Nazionale Galileo on 2025 August 16 ( r h = 2.98 au). The spectrum revealed a red slope of 12.68% ± 0.05%/1000 Å across the 5000–8000 Å range, consistent with previous findings. Observations show the CN outgassing violet band, which yields a production rate of (4.28 ± 0.83) · 10 23 mol s −1 . We derived upper limits for C 2 and C 3 production rates since no clear detection was possible and used those to compute C 2 /CN and C 3 /CN ratios. The resulting, notably low, values of −0.86 and −1.47, respectively, for C 2 /CN and C 3 /CN ratios classify 3I as a strongly carbon-chain depleted comet in the preperihelion orbit. Notably, recent postperihelion observations found a typical solar system cometary composition, probably suggesting new innermost material being exposed to solar radiation.