Onset of CN Emission in 3I/ATLAS: Evidence for Strong Carbon-Chain Depletion (2509.01647v1)

Abstract: 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 $\sim$8$\times$10$^{23}$ 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 $Af\rho$ with values in the order of $\sim300$ cm. Our upper limit on the C$_2$-to-CN ratio ($\log Q(\mathrm{C}_2)/Q(\mathrm{CN})<-1.05$) places 3I/ATLAS among the most carbon-chain depleted comets known. The derived gas-to-dust production ratio of $\log Q (\mathrm{CN})/Af\rho<21.49$ 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.

• The paper presents a detailed spectroscopic campaign that detected the onset of CN emission in 3I/ATLAS with production rates rising from 7.52×10^23 to 9.49×10^23 s⁻¹.
• It uses low‐resolution spectroscopy paired with photometry to correlate molecular activation with increasing dust production, evidenced by Afρ values from 260 to 315 cm.
• The study establishes tight upper limits on C₂ and C₃ emissions, identifying 3I/ATLAS as one of the most carbon-chain depleted comets known.

Spectroscopic Onset of CN Emission and Carbon-Chain Depletion in Interstellar Comet 3I/ATLAS

Introduction

The paper presents a comprehensive spectroscopic and photometric monitoring campaign of 3I/ATLAS, the third confirmed interstellar comet, during its inbound passage at heliocentric distances of 3.2–2.9 au. The primary objective is to characterize the onset of optical gas-driven activity, focusing on the detection and quantification of CN emission and constraints on carbon-chain molecules (C$_2$, C$_3$). The work situates 3I/ATLAS within the broader context of interstellar and Solar System cometary populations, leveraging comparative analyses to infer compositional trends and evolutionary implications.

Observational Methodology

The campaign utilized the 2.4 m Hiltner telescope for low-resolution spectroscopy and the 1.3 m McGraw-Hill telescope for simultaneous photometry at the MDM Observatory. Spectroscopic data were acquired using the OSMOS instrument in triple-prism mode, providing broad wavelength coverage (3500–10,000 Å) at $\lambda/\Delta\lambda \sim 400$ in the blue. Photometric monitoring employed SDSS $g$, $r$, and Johnson $U$ filters. Data reduction pipelines implemented bias subtraction, wavelength calibration via planetary nebulae, extinction correction, and flux calibration against spectrophotometric standards. Extraction apertures were fixed to a projected distance of 15,000 km at the comet, and cosmic-ray rejection was performed on 1D spectra.

Reflectance Spectrum and Taxonomic Classification

The mean reflectance spectrum, co-added from four nights, exhibits a short-wavelength slope of 27.9%/kÅ (3900–6000 Å) and a shallower slope of 12.6%/kÅ (5500–7000 Å), consistent with D-type asteroid taxonomy and previous measurements of 3I/ATLAS.

Figure 1: Mean reflectance spectrum of 3I/ATLAS compared to previous observations and D-type taxonomy; vertical lines indicate positions of CN, C$_3$, and C$_2$ emission features.

Photometric colors ($U-g = 1.7 \pm 0.1$, $g-r = 0.62 \pm 0.02$) corroborate the spectroscopic reflectance, with a noted deficit at longer wavelengths relative to July spectra.

Detection and Evolution of CN Emission

A clear CN emission feature at 3870 Å was detected, absent in early August but emerging and strengthening from August 13 onward. The CN production rate was measured at $Q$(CN) $\sim 8 \times 10^{23}$ s$^{-1}$, which is at the lower end of Solar System comet activity at comparable heliocentric distances. The temporal evolution of CN flux suggests a steep increase, with the earliest detection preceding independent confirmations by VLT and Lowell Observatory.

Figure 2: CN emission feature in 3I/ATLAS spectra; top panels show comparison with 2I/Borisov and nightly evolution, bottom panels display continuum-subtracted CN flux for each night.

The CN production rates were derived using standard fluorescence efficiencies, yielding values of $7.52 \times 10^{23}$, $8.85 \times 10^{23}$, and $9.49 \times 10^{23}$ s$^{-1}$ for August 12, 16, and 17, respectively. These rates are a factor of $\sim$4 above VLT/X-shooter measurements but lower than some other contemporaneous reports.

Constraints on Carbon-Chain Molecules

No confident detections of C$_2$ or C$_3$ emission were obtained; upper limits were established using analogous procedures to CN. The derived upper limit for the C$_2$-to-CN ratio, $\log Q(\mathrm{C}_2)/Q(\mathrm{CN}) < -1.05$, places 3I/ATLAS among the most carbon-chain depleted comets known. C$_3$ constraints do not indicate significant depletion relative to Solar System comets.

Photometric Evolution and Dust Production

Radial brightness profiles reveal a measurable enhancement of the coma coincident with the onset of CN emission. The dust production parameter $Af\rho$ increased from $260 \pm 20$ cm (Aug 9) to $315 \pm 2$ cm (Aug 17) in the $r$-band, with similar trends in $g$ and $U$ bands. The evolution of $Af\rho$ is correlated with CN activation, supporting the interpretation of gas-driven activity.

Comparative Analysis: Interstellar vs. Solar System Comets

The CN production rate and activation distance for 3I/ATLAS are atypical compared to Solar System comets, with activation occurring farther from the Sun and at lower rates. The C$_2$-to-CN ratio is significantly below the threshold for carbon-chain depletion, aligning with early observations of 2I/Borisov but remaining more extreme. The gas-to-dust production ratio, $\log(Q(\mathrm{CN})/Af\rho) = 21.49$, is also at the low end of the Solar System distribution.

Figure 3: Comparison of C$_2$/CN ratio and gas-to-dust ratio for interstellar and Solar System comets; 3I/ATLAS is marked as strongly carbon-chain depleted and gas-poor relative to dust.

These findings suggest a possible compositional trend among interstellar comets toward carbon-chain depletion, though the small sample size precludes definitive conclusions. The rapid evolution of C$_2$/CN in 2I/Borisov, attributed to compositional heterogeneity, underscores the need for continued monitoring of 3I/ATLAS as it approaches perihelion.

Implications and Future Directions

The strong carbon-chain depletion and low gas-to-dust ratio observed in 3I/ATLAS have implications for models of comet formation and evolutionary processing in extrasolar environments. If such depletion is a recurring trait among interstellar comets, it may reflect primordial compositional differences or distinct evolutionary pathways compared to Solar System comets. The activation of CN at large heliocentric distances further constrains the volatile inventory and thermal history of the nucleus.

Continued spectroscopic and photometric monitoring, particularly as 3I/ATLAS approaches the Sun, will be essential to track the evolution of molecular emission features and refine compositional classifications. Comparative studies with future interstellar comet discoveries will enable statistical assessments of compositional trends and inform models of protoplanetary disk chemistry and dynamical evolution.

Conclusion

The monitoring campaign of 3I/ATLAS captured the onset of CN emission and provided stringent constraints on carbon-chain molecule abundances, establishing the comet as one of the most carbon-chain depleted objects observed to date. The low CN production rate and gas-to-dust ratio further distinguish 3I/ATLAS from typical Solar System comets. These results contribute to the emerging picture of interstellar cometary composition and highlight the importance of continued multi-wavelength observations to elucidate the formation and evolutionary histories of extrasolar small bodies.