December 2025 Gemini spectroscopic and photometric constraints on the activity and gas contents of 3I/ATLAS

ATel #17719; Bryce Bolin (Eureka Scientific), Ian Wong (STScI), Laura-May Abron (Griffith Observatory), Brian Lemaux (Gemini Observatory), Leila Alamos (Gemini Observatory), Matthew Belyakov (Caltech), Kristin Chiboucas (Gemini Observatory), Christoffer Fremling (Caltech), Josef Hanuš (Charles University), Oleksandra Ivanova (Astronomical Institute of Slovak Academy of Sciences), Eunchong Kim (Gemini Observatory), Ludmilla Kolokolova (University Of Maryland), Jen Miller (Gemini Observatory), Keith Noll (PSI)
on 12 Mar 2026; 00:11 UT
Credential Certification: Bryce Bolin (bolin.astro@gmail.com)

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We report photometry and spectroscopy of the interstellar comet 3I/ATLAS obtained on 2025 December 22 with the Gemini Multi-Object Spectrograph on the Frederick C. Gillett Gemini North 8.1-m telescope (observatory code T15) at the summit of Maunakea. Imaging was acquired in the g, r, i, and z bands (effective wavelengths of 465, 622, 771, and 880 nm), along with long-slit visible spectroscopy. The comet has a tail ~110 arcsec long, pointing to the southeast, and a coma ~45 arcsec wide. We measured an r-band, phase-angle-corrected A(0)f(rho) = 2464 +/- 26 cm using a projected aperture radius of 10,000 km (7.7 arcsec wide at the comet's 1.8 au geocentric distance). Color indices of g-r = 0.69 +/- 0.02, r-i = 0.23 +/- 0.02, and i-Z = 0.07 +/- 0.02 are calculated from photometry measured using a projected aperture radius of 10,000 km. The g-r color index measured from these observations is slightly bluer compared to that measured from g and r observations taken on 2025 November 26 by Bolin et al. (2025, ATel #17561), and in observations taken on 2025 July 2 reported by Bolin et al. (2025, MNRAS:L, 542, 1, pp. L139-L143). Using GMOS in long-slit spectroscopy mode, we detected CN (388 nm), C3 (402 nm), and C2 (474 nm/517 nm), which are weaker than in the 2025 November 26 observations described in ATel #17561. Assuming a gas outflow velocity of 0.5 km s-1 at 1 au, scaled to the heliocentric distance of 2.37 au, we estimate production rates using the Haser model (Haser 1957, Bull. Soc. R. Sci. Liege, 43, 740-750) with parent and daughter photolysis scale lengths and fluorescence efficiencies from Cochran (1985, AJ, 90, 2609-2614) and Cochran et al. (1992, Icarus, 918, 2, 151-162). Continuum removal follows the method described in Bolin et al. (2021, AJ, 161, 3, id.116, 15 pp.) and Bolin et al. (2024, MNRAS:L, 527, 1, p. L42-L46). The resulting production rates are: Q(CN) = 6.49 +/- 0.19 E26 S-1, Q(C3) = 6.98 +/- 0.14 E24 s-1, and Q(C2) = 2.36 +/- 0.02 E26 s-1. Images in the g, r, i, and Z bands and the GMOS spectrum of 3I/ATLAS are available at the link below. This work 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. This work was enabled by observations made from the Gemini North telescope, located within the Maunakea Science Reserve and adjacent to the summit of Maunakea. We are grateful for the privilege of observing the Universe from a place unique in both its astronomical quality and cultural significance. We appreciate the support of the Shadow the Scientists (StS) initiative in making these observations possible. StS is made possible through the Creating Equity in STEAM (CrEST) program at the University of California, Santa Cruz, and support from the Heising-Simons Foundation.

Visible images and spectrum of 3I/ATLAS taken on 2025 December 22 UTC