Interstellar

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

By Keith Cowing

Status Report

astro-ph.EP

September 4, 2025

Filed under 3I/ATLAS, astro-ph.EP, astrochemistry, Comet, icy world, Interstellar, Spectroscopy

Onset of CN Emission in 3I/ATLAS: Evidence for Strong Carbon-Chain Depletion — Mean reflectance spectrum of 3I/ATLAS, co-added from the nights of August 10, 12, 16, and 17 with the 2.4 m MDM telescope (black). For comparison, we show the spectra of 3I/ATLAS reported by D. Z. Seligman et al. (2025) (yellow), obtained on July 4 with the 2.2 m UH telescope, and the D-type spectral taxonomy from F. E. DeMeo et al. (2009) (blue). Squares mark photometric measurements obtained with the 1.3 m MDM telescope. Vertical lines mark the positions of typical cometary emission features: CN at 3870 ˚A, C3 at 4050 ˚A, C2(∆ν = 1) at 4750 ˚A, and C2(∆ν = 0) at 5150 ˚A. — astro-ph.EP

Interstellar objects provide a direct window into the environmental conditions around stars other than the Sun.

The recent discovery of 3I/ATLAS, the second interstellar comet ever observed, 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-driven 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 ∼8×1023 s−1, towards 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.

Our upper limit on the C2-to-CN ratio (logQ(C2)/Q(CN)<−1.05) places 3I/ATLAS among the most carbon-chain depleted comets known. The derived gas-to-dust production ratio of logQ(CN)/Afρ<21.49 is likewise at the low end of the Solar System comet distribution.

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.

Luis E. Salazar Manzano, Hsing Wen Lin, Aster G. Taylor, Darryl Z. Seligman, Fred C. Adams, David W. Gerdes, Thomas Ruch, Tessa T. Frincke, Kevin J. Napier

Comments: Submitted to The Astrophysical Journal Letters (ApJL). 12 pages, 5 figures, and 3 tables
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2509.01647 [astro-ph.EP] (or arXiv:2509.01647v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2509.01647
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Submission history
From: Luis Salazar Manzano
[v1] Mon, 1 Sep 2025 17:46:53 UTC (1,990 KB)
https://arxiv.org/abs/2509.01647

Astrobiology

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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