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

TL;DR

This study reports the onset of optical gas emission in the interstellar comet 3I/ATLAS as it inbound at roughly 3.2–2.9 au. A 10-night spectroscopic/photometric campaign with the MDM Observatory detected CN emission and constrained its production rate to about $Q(\mathrm{CN}) \sim (4.8-7.2) \times 10^{24}$ s$^{-1}$, with dust activity indicated by $A f\rho$ values near $3\times10^2$ cm and a gas-to-dust ratio of $\log Q(\mathrm{CN})/A f\rho \approx 22.37$. The analysis yields an upper limit on carbon-chain species relative to CN, $\log Q(\mathrm{C}_2)/Q(\mathrm{CN}) < -0.77 \pm 0.15$, indicating strong carbon-chain depletion; C3 remains unconstrained in the same regime. Compared with 2I/Borisov and Solar System comets, 3I/ATLAS appears to share depletion traits, suggesting either primordial differences in interstellar formation environments or rapid surface processing during interstellar travel. The results establish a benchmark for the composition of interstellar comets and highlight the importance of prompt optical monitoring to capture volatile inventories before solar heating alters surface layers.

Abstract

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)$\sim6\times$10$^{24}$ 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 $\sim300$ cm. We derived a gas-to-dust production ratio of $\log Q (\mathrm{CN})/Afρ\sim22.4$. Our upper limit on the C$_2$-to-CN ratio ($\log Q(\mathrm{C}_2)/Q(\mathrm{CN})\lesssim-0.8$) 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.

Figures (5)

• Figure 1: 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 Seligman2025 (yellow), obtained on July 4 with the 2.2 m UH telescope, and the D-type spectral taxonomy from 2009Icar..202..160D (blue). Squares mark photometric measurements obtained with the 1.3 m MDM telescope. Vertical bands mark the positions of typical cometary emission features: CN at 3870 $\mathrm{\AA}$, C$_3$ at 4050 $\mathrm{\AA}$, C$_2$($\Delta\nu=1$) at 4750 $\mathrm{\AA}$, and C$_2$($\Delta\nu=0$) at 5150 $\mathrm{\AA}$.
• Figure 2: CN emission feature of 3I/ATLAS detected with the 2.4 m MDM telescope. The vertical green band mark the expected position of CN emission near 3870 $\mathrm{\AA}$. Top left: Zoom-in on the CN region of the reflectance spectrum. In addition to the comparison spectra shown in Figure \ref{['fig:reflectance']}, we include a Gaussian-smoothed curve to improve visibility (blue). Top right: Co-added, continuum-subtracted flux of 3I/ATLAS from this work (gray), compared with the CN emission flux detected in 2I/Borisov by Lin2020 (brown). A Gaussian smoothing applied to our data is also shown (black). Bottom: Continuum-subtracted CN flux measured individually on August 10, 12, 16, and 17.
• Figure 3: Radial profiles of 3I/ATLAS on three different nights. Left: linear scale. Right: logarithmic scale. A common PSF, convolved with the images, is shown for comparison (solid black).
• Figure 4: Measured brightness and expected brightness evolution of 3I/ATLAS. The expected curve shows the estimated development of the comet’s brightness based on previous observations, fitted to data from the Comet Observations Database (COBS) and the Minor Planet Center (MPC).
• Figure 5: Comparison of carbon-chain to CN ratio and gas-to-dust ratio constraints for interstellar and Solar System comets. Left: Logarithmic C$_2$/CN ratio as a function of heliocentric distance. The white and black circle with a downward-pointing arrow marks our upper limit for 3I/ATLAS, while the orange squares indicate, in order of decreasing distance, the measurements for 2I/Borisov reported by Lin2020, Bannister2020, and Aravind2021. Points show measurements of Solar System comets from the 2010PDSS.8133E....O database, with red points corresponding to Jupiter Family Comets (JFC), green points to Halley-type comets (HT), and blue points to Long-Period Comets (LPC). The horizontal dotted line separates typical from carbon-chain depleted comets. Middle: Logarithmic C$_3$/CN ratio versus heliocentric distance. Right: Logarithmic gas-to-dust ratio versus heliocentric distance. Formatting in the middle and right panels follows the left panel, with only the Aravind2021 measurement for 2I/Borisov shown. The white and black circle in the right panel represents our measured value for 3I/ATLAS (not an upper limit).