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From: atel@astronomerstelegram.org
Subject: ATel  17363
Message-Id: <20250829070644.AE8FC28807BF@server.astronomerstelegram.org>
Date: Fri, 29 Aug 2025 03:06:44 -0400 (EDT)

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Posted: Within the last 24 hours
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ATEL #17363							     ATEL #17363

Title:	The Anti-Solar Tail of Comet 3I/ATLAS Seen in Deep Multi-band
		Imaging Taken at Gemini South
Author:	Bryce Bolin (Eureka Scientific), Laura-May Abron (Griffith
		Observatory), Matthew Belyakov (CIT), Juan Pablo Carvajal (PUCC),
		Christoffer Fremling  (CIT), Josef Hanus (Charles University), Carl
		Ingebretsen (JHU), Piera King (Gemini Observatory), Baltasar Luco
		(PUCC), Keith Noll (NASA GSFC), Thomas H. Puzia (PUCC), Rohan Rahatgaonkar
		(PUCC), Karleyne Silva (Gemini Observatory), Ian Wong (STScI)
Queries:	bolin.astro@gmail.com
Posted:	29 Aug 2025; 04:10 UT
Subjects:Optical, Comet, Solar System Object

We report deep imaging and u, g, r, i color measurements of interstellar
comet 3I/ATLAS taken with the Gemini South 8.2-m telescope at Cerro Pachon
(observatory code I11). We used the Gemini Multi-Object Spectrograph (GMOS)
to observe 3I/ATLAS on 2025 August 27 23:53 UTC (see attached u, g, r,
and i composite images). The comet has an extended appearance with a tail
~30 arcsec long, pointing towards the anti-solar direction in the South
East. Additionally, the comet had a coma ~10 arcsec wide, significantly
more extended than its compact appearance seen in initial imaging (Bolin
et al. 2025, MNRAS:L, 542, 1, pp. L139-L143). The seeing measured using
nearby stars was ~0.5 arcsec.

Using our multi-band images, we measured the following color indices using
an aperture with a 5.3 arcsec, equivalent to 10,000 km at the 2.59 au geocentric
distance of the comet: u-g = 1.45 +/- 0.04,  g-r = 0.68 +/- 0.01, r-i =
0.32 +/- 0.01, and g-i = 1.00 +/- 0.01, equivalent to a spectral slope
of 16.1 +/- 0.4 % / 100 nm. The g-r and r-i color and overall color slope
measured with the GMOS  appear to be in line with the spectral slope measurements
for the comet reported in other recent results (e.g., Puzia et al. 2025,
arXiv: 2508.02777, Santana-Ros et al. 2025, arXiv: 2508.00808, Beniyama
2025, arXiv:2508.08829, Rahatgaonkar et al. 2025, arXiv: 2508.18382).

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.

Images of 3I/ATLAS taken in u, g, r, and i bands on 2025 August 27 UTC:
https://static.wixstatic.com/media/184fc4_79eee34678dc4150a62596b8874ecb94~mv2.png/v1/fill/w_954,h_954,al_c,q_90,usm_0.66_1.00_0.01,enc_avif,quality_auto/3I_ATLAS_GMOS_S_2025_Aug_28_u_120_s_g_60_s_r_60_s_i_60_s.png
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Password Certification: Bryce Bolin (bolin.astro@gmail.com)
https://www.astronomerstelegram.org/?read=17363
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