Gemini post-perihelion visible photometry and spectroscopy of interstellar comet 3I/ATLAS

ATel #17561; Bryce Bolin (Eureka Scientific), Ian Wong (STScI), Laura-May Abron (Griffith Observatory), Jameeka Marshall (UC, Santa Cruz), Davide Farnocchia (Jet Propulsion Laboratory), Marco Micheli (ESA Neo Coordination Centre), Brian Lemaux (Gemini Observatory), Emily Peavy (Gemini Observatory), Erin Figura (Gemini Observatory), Denise Hung (Gemini Observatory), Emily Peavy (Gemini Observatory), Matthew Belyakov (Caltech), Christoffer Fremling (Caltech), Ariel Graykowski (Seti Institute), Josef Hanuš (Charles University), Oleksandra Ivanova (Astronomical Institute of Slovak Academy of Sciences), Ludmilla Kolokolova (University Of Maryland), Teo Mocnik (Gemini Observatory), Keith Noll (PSI), Matias Rodriguez (Gemini Observatory), David Tholen (University Of Hawaii), Peter Veres (Minor Planet Center)
on 24 Dec 2025; 20:36 UT
Credential Certification: Bryce Bolin (bolin.astro@gmail.com)

Tweet

We report results from multi-band imaging (g = 465 nm, r = 622 nm, i = 771 nm, Z = 880 nm) and long-slit visible spectroscopy of interstellar comet 3I/ATLAS, observed at the summit of Maunakea on 2025 November 26 using Gemini North (observatory code T15). The comet appears circular in the g-band image, with a coma ~130 arcsec wide. In contrast, the r-band image has a tear-drop-shaped tail pointing in the anti-heliocentric velocity vector direction with a similar width as the coma in the g-band image. The comet has a similar appearance in all three r-, i-, and Z-band images. The colors measured using a fixed-volume aperture with a radius of 10,000 km (~7 arcsec at the comet's 1.92 geocentric distance) are g-r = 0.73 +/- 0.02, r-i = 0.07 +/- 0.02, and i-z = 0.00 +/- 0.02, significantly bluer than the red colors reported by Bolin et al. 2025, MNRAS:L, 542, 1, pp. L139-L143. Using the brightness of the comet measured in r-band with a 10,000 km wide aperture, we compute a phase angle-correct A(0)f(rho) = 6779.1 +/- 92.4 cm. Additionally, we detect the emission of CN (388 nm), C3 (402 nm), and C2 (474 nm/517 nm) in the longslit spectrum obtained nearly simultaneously with the multi-band images, similar to the spectra and photometry reported in ATels #17502, #17503, #17515, and #17538. The estimated production values of these gases in the comet using the Haser model (Haser 1957, Bull. Soc. R. Sci. Liege, 43, 740-750), the parent/daughter photolysis length scales and fluorescence efficiency factors from Cochran 1985, AJ, 90, 2609-2614 and Cochran et al. 1992, Icarus, 918, 2, 151-162, and the continuum removal technique of Bolin et al. 2021, AJ, 161, 3, id.116, 15 pp. Assuming an outflow velocity of the gas of 0.5 km/s at 1 au from the Sun, we obtain a CN production rate of 7.22 +/- 0.22 E26 s-1, 7.22 +/- 0.22 E26 s-1, a C3 production rate of 1.80 +/- 0.02 E25 s-1, and a C2 production rate of 4.27 +/- 0.04 E26 s-1. Plots of the g, r, i, and z images of the comet, as well as the long-slit spectrum, are available at the link below. 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. 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 November 26 UTC