Pre-perihelion Emergence of the CN Gas Coma in 3I/ATLAS Temporally and Spatially Resolved by the 7-Dimensional Telescope

TL;DR

This paper investigates the pre-perihelion activation of CN gas in the interstellar object 3I/ATLAS using time-series, spatially resolved photometry with the 7-Dimensional Telescope. By decomposing the coma into CN-dominated outer gas and inner dust components via two-dimensional surface-brightness modeling, the authors show a CN-onset at $r_h\approx 2.97\,\text{au}$ accompanied by rapid gas expansion and increasing $Q_{\rm CN}/A f\rho$ toward perihelion. The CN emission is traced primarily by the m400 band, with a line-proxy methodology based on a Haser framework to derive $Q_{\rm CN}$, and the outer CN morphology remains self-similar while brightening, suggesting a distributed CN source from dust grains. Relative to Solar System comets, 3I/ATLAS is dust-rich with carbon-chain depletion, consistent with formation in a cold, UV-irradiated, or metal-poor environment and indicating that ISOs can exhibit Solar-System-like volatile processing prior to perihelion.

Abstract

We present time-series medium-band (R~20-40) observations of the third interstellar object 3I/ATLAS (C/2025 N1) obtained with the 7-Dimensional Telescope (7DT), enabling spatially resolved monitoring of its gas and dust activity from 2025 July to September. The m400-band image (lambda_c = 400 nm, Delta lambda approx 25 nm) reveals the emergence of pronounced and spatially extended CN emission at heliocentric distances r_h < 3 au. This onset is consistently identified across multiple diagnostics, including a break in the light-curve evolution, excess reflectance, inward expansion of annular excess beyond 10,000-20,000 km, growth of the coma half-light radius from ~11,000 to ~19,000 km, and a rapid rise in the CN production rate Q_CN relative to Af rho. We further separate the CN-emitting and dust-scattered components through two-dimensional surface-brightness fitting into inner (dust) and outer (gas) components. The outer component preserves a nearly constant profile shape, varying only in normalization, implying relatively fast expansion of CN-bearing molecules. Together, these results reveal a transition in the optical from dust-dominated scattering at large heliocentric distances to volatile-driven, gas-dominated activity as 3I/ATLAS enters the inner Solar System. The timing and characteristics of the CN activation resemble the volatile enhancement observed in 2I/Borisov, suggesting that both known active interstellar objects exhibit comparable activation behavior at heliocentric distances of ~2-3 au.

Figures (8)

• Figure 1: $2\times2$-binned stacked images of 3I/ATLAS obtained with 7DT on 2025 August 11 and 24 and September 18 (UT), centered on the target. The top panel shows the m400 image, and the bottom panel shows the $r$-band image. Concentric apertures corresponding to projected physical radius of $5{,}000$ km, $15{,}000$ km, and $25{,}000$ km are overplotted with dotted, dashed and solid lines for each; these were used to extract photometry at fixed physical scales given the changing geocentric distance. White arrows indicate north and east (all panels share the same orientation), and yellow and cyan arrows mark the directions toward the Sun and of the object's motion, respectively.
• Figure 2: Phase-- and distance--corrected absolute magnitudes $H$ of 3I/ATLAS as a function of heliocentric distance $r_h$ and MJD, measured within a projected $15{,}000$ km radius aperture. The top axis shows the corresponding MJD. For clarity, each filter sequence is vertically offset. Filled circles (diamonds) denote detections ($S/N\ge5$) in medium bands (broad bands) with $1\sigma$ photometric uncertainties, while inverted triangles mark measurements with $S/N<5$ that are excluded from the fits. Dashed lines show weighted linear fits of $H$ versus $r_h$ for each filter, with slopes $s$ (mag au$^{-1}$) printed next to the sequences. For the m400 band, we additionally overplot a continuous broken--linear fit (solid line) with the break constrained to $2.5 \le x_b \le 3.5$ au and optimized from the data, yielding best--fit $x_b \simeq 2.97$ au, corresponding to the $\sim$MJD 60904.3 (vertical grey dashed line) and pre/post slopes ($s_{\rm pre}$ at larger $r_h$ and $s_{\rm post}$ at smaller $r_h$) as annotated.
• Figure 3: Time-series reflectance spectra of the interstellar object 3I/ATLAS obtained with 7DT medium-band (blue squares) and broadband (orange diamonds) photometry within circular apertures of radius $15{,}000$ km centered on the nucleus, obtained between 2025 July 13 and September 18. Each panel corresponds to one observing night, with medium-band points normalized at the m550 filter. Downward triangles indicate $5\sigma$ upper limits for non-detections. The shaded gray region represents the reflectance template of D-type asteroids from 2009Icar..202..160D for reference. Vertical colored-bands mark expected gas emission features: CN (violet and red), C$_2$ (green), C$_3$ (blue) and [O I] (yellow).
• Figure 4: Evolution of optical colors of 3I/ATLAS as a function of heliocentric distance. Blue and red circles represent the $g-r$ and $r-i$ colors measured in this study, respectively, while open squares indicate early measurements from 2025arXiv251209020H. For comparison, color measurements of the inactive ISO 1I 2017ApJ...851L..38B2018ApJ...852L...2B and the active ISO 2I, 2019ApJ...886L..29J are also shown.
• Figure 5: Time evolution of CN-related gas activity and dust production in 3I/ATLAS as a function of heliocentric distance $r_h$ (au); the upper abscissa shows the corresponding MJD based on our observing geometry. (Top) CN production rate $Q_{\rm CN}$ (molecules s$^{-1}$); (Middle) $Af\rho$ (cm) as a proxy for dust production; (Bottom) the ratio $Q_{\rm CN}/Af\rho$ (molecules s$^{-1}$,cm$^{-1}$). Our measurements are shown with $1\sigma$ uncertainties using fixed projected circular apertures of physical radius $1.0\times10^{4}$ km (blue upward arrows) and $1.5\times10^{4}$ km (orange upward arrows); the arrows indicate lower limits, while downward triangles mark $3\sigma$ upper limits. Dashed lines indicate representative power-law scalings of the form $Q_{\rm CN}\propto r_h^{\gamma}$ (see legend), including our fit for the $1.5\times10^{4}$ km aperture and literature-reported slopes for comparison. Black plus symbols show Solar System comets from 1995Icar..118..223A, evaluated at their reported $r_h$; these values were measured within a projected circular aperture of radius 10,000 km. Literature measurements for ISOs are distinguished by symbol shape. Gray squares show 3I/ATLAS from the MDM campaign 2025ApJ...993L..23S, where CN was extracted within a projected circular aperture of radius 15,000 km and $Af\rho$ was measured within 10,000 km. Red squares show 3I/ATLAS from VLT spectroscopy 2025ApJ...995L..34R, including X-shooter and UVES slit observations with effective projected slit lengths of order $(5$--$10)\times10^{3}$ km at the comet. Green and cyan squares indicate IFU-based extractions of 3I/ATLAS obtained with Keck-II/KCWI 2025arXiv251011779H and UH88/SNIFS 2025arXiv251209020H, respectively, using circular apertures of 2$^{\prime\prime}$ and 3$^{\prime\prime}$ radius, corresponding to projected physical radii of $\sim4\times10^{3}$ and $\sim6\times10^{3}$ km at $\Delta\sim2.5$--2.7 au. The purple cross denotes 2I from 2019ApJ...885L...9F, where CN was measured from long-slit spectroscopy and $Af\rho$ is commonly reported for a 10,000 km circular aperture.