Onset of CN Emission in 3I/ATLAS: Evidence for Strong Carbon-chain Depletion

Interstellar objects provide a direct window into the environmental conditions around stars other than the Sun. The recent discovery of 3I/ATLAS, a new interstellar comet, offers a unique opportunity to investigate the physical and chemical properties of interstellar objects and to compare them with those of comets in our own solar system. In this Letter we present the results of a 10 night spectroscopic and photometric monitoring campaign with the 2.4 m Hiltner and 1.3 m McGraw–Hill telescopes at the MDM Observatory. The campaign was conducted between August 8 and 17 while 3I/ATLAS was inbound at heliocentric distances of 3.2–2.9 au. Our observations captured the onset of optical gas activity. Nightly spectra reveal a weak CN emission feature in the coma of 3I/ATLAS, absent during the first nights but steadily strengthening thereafter. We measure a CN production rate of Q (CN) ∼ 6 × 10 ^24 s ^−1 , toward the lower end of activity observed in solar system comets. Simultaneous photometry also indicates a small but measurable increase in the coma’s radial profile and increasing r -band Afρ with values in the order of ∼300 cm. We derived a gas-to-dust production ratio of ${\mathrm{log}}\,Q({\rm{CN}})/Af\rho \sim 22.4$ . Our upper limit on the C _2 -to-CN ratio ( ${\mathrm{log}}\,Q({{\rm{C}}}_{2})/Q({\rm{CN}})\lesssim -0.8$ ) indicates that 3I/ATLAS is a strongly carbon-chain-depleted comet. Further observations of 3I/ATLAS are required to verify the apparent carbon-chain depletion and to explore whether such composition represents a recurring trait of the interstellar comet population.