Gemini pre-perihelion detection of CN/C3/C2 gas emission and dust fans in interstellar comet 3I/ATLAS

ATel #17503; Bryce Bolin (Eureka Scientific), Ian Wong (STScI), Brian Lemaux (Gemini Observatory), Laura-May Abron (Griffith Observatory), Matthew Belyakov (Caltech), Pablo Candia (Gemini Observatory), Kristin Chiboucas (Gemini Observatory), Rodrigo Carrasco (Gemini Observatory), Veronica Firpo (Gemini Observatory), Christoffer Fremling (Caltech), Josef Hanus (Charles University), Jeong-Eun Heo (Gemini Observatory), Piera King (Gemini Observatory), Keith Noll (GSFC), Karleyne Silva (Gemini Observatory)
on 20 Nov 2025; 02:37 UT
Credential Certification: Bryce Bolin (bolin.astro@gmail.com)

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We report the detection of CN, C3, and C2 gas emission features in the pre-perihelion visible spectrum of interstellar comet 3I/ATLAS taken with the Gemini South 8.1-m telescope at Cerro Pachon (observatory code I11, program GS-2025B-DD-102/PI: Bolin). We used the Gemini Multi-Object Spectrograph (GMOS) to observe 3I/ATLAS on 2025 September 14 23:54 UTC (see attached spectrum and g- and r-band images). The spectrum shows clear detections of CN, C3, and C2 gas emission features at 388 nm, 402 nm, and 517 nm, respectively. Additional CN and C2 gas emission features are seen at 421 nm and 474 nm, as seen in panel A of the attached image. Panel B shows g and r images of the comet, where its detection is divided by the azimuthal average. The enhanced images show evidence of a dust fan ~15 arcsec long, pointing in the anti-solar (southeast) direction, more extended than the compact appearance seen in the initial imaging (Bolin et al. 2025, MNRAS:L, 542, 1, pp. L139-L143). The appearance of the dust fans and morphology is comparable to the features seen in other images of the comet taken before perihelion (Ivanova et al. 2025, ATel #17372, Jewit & Luu, ApJL, 994, 1, id.L3, 13 pp.) and after perihelion (Jewitt & Luu 2025, ATel #17490). This study is based on observations obtained at the international Gemini Observatory, a program of NSF's NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation on behalf of the Gemini Observatory partnership.

Visible spectrum and images of 3I/ATLAS taken on 2025 September 14 UTC