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From: atel@astronomerstelegram.org
Subject: ATel  17841
Message-Id: <20260612070317.A7F2E2880811@server.astronomerstelegram.org>
Date: Fri, 12 Jun 2026 03:03:17 -0400 (EDT)

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ATEL #17841							     ATEL #17841

Title:	2026 January Gemini imaging and spectroscopic monitoring of
		3I/ATLAS
Author:	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)
Queries:	bolin.astro@gmail.com
Posted:	12 Jun 2026; 05:44 UT
Subjects:Optical, Comet, Solar System Object

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: https://www.bolinastro.com/2026-january-3i-atlas-update
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Password Certification: Bryce Bolin (bolin.astro@gmail.com)
https://www.astronomerstelegram.org/?read=17841
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