2026 January Gemini imaging and spectroscopic monitoring of 3I/ATLAS

ATel #17841; 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), Erin Figura (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)
on 12 Jun 2026; 05:44 UT
Credential Certification: Bryce Bolin (bolin.astro@gmail.com)

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We report multi-wavelength imaging and spectroscopic observations of the interstellar comet 3I/ATLAS obtained on 2026 January 20-21 with the Gemini Multi-Object Spectrograph (GMOS) on the Frederick C. Gillett Gemini North 8.1-m telescope (observatory code T15) atop Maunakea. Imaging was acquired in the g, r, i, and Z bands (effective wavelengths of 465, 622, 771, and 880 nm), together with long-slit visible spectroscopy. The comet exhibits a tail approximately 152 arcsec long (~250,000 km at the comet's geocentric distance of 2.3 au) extending toward the southeast, and a coma approximately 45 arcsec wide. From r-band imaging, we measured a phase-angle-corrected A(0)f(rho) = 952+/-5 cm using a projected aperture radius of 10,000 km (6 arcsec given the comet's geocentric distance of 2.3 au). Photometry within the same projected aperture yields color indices of g-r = 0.71 +/- 0.01, r-i = 0.25 +/- 0.02, and i-z = 0.08 +/- 0.02. The measured g-r color is slightly bluer than values reported from observations obtained on 2025 November 26 (Bolin et al. 2025a, ATel #17561) and 2025 July 2 (Bolin et al. 2025b, MNRAS, 542, L139-L143), but similar to color values measured in 2025 December (Bolin et al. 2025c, ATel #17719). Long-slit GMOS spectroscopy reveals emission from CN (388 nm), C3 (402 nm), and C2 (517 nm). Gas emissions from all three species were weaker than those observed in 2025 December (ATel #17719). After continuum removal following the methods of Bolin et al. (2021, AJ, 161, 116) and Bolin et al. (2024, MNRAS, 527, L42-L46), gas production rates were derived using the Haser model (Haser, 1957) with parent and daughter photolysis scale lengths and fluorescence efficiencies from Cochran (1985) and Cochran et al. (1992). Assuming a gas outflow velocity of 0.5 km s-1 at 1 au, scaled to the comet's heliocentric distance of 3.28 au, we obtain production rates of Q(CN) = (4.54 +/- 0.05) E24 s-1, Q(C3) = (5.79 +/- 0.06) E23 s-1, and Q(C2) = (5.85 +/- 0.05) E24 s-1. The GMOS g, r, i, and Z images and 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.

GMOS multiband images and spectrum of 3I/ATLAS taken on 2026 January 20-21 UTC