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From: atel@astronomerstelegram.org
Subject: ATel 17502, 17503
Message-Id: <20251120071214.8947E288082A@server.astronomerstelegram.org>
Date: Thu, 20 Nov 2025 02:12:14 -0500 (EST)

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Posted: Within the last 24 hours
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ATEL #17502							     ATEL #17502

Title:	Comet 3I/ATLAS: Post-perihelion spectroscopic results from
		PRL Mt Abu Observatory
Author:	Shashikiran Ganesh (PRL), Goldy Ahuja (PRL), Arvind B.(PRL,),
		Anil Bhardwaj(PRL)
Queries:	shashi@prl.res.in
Posted:	19 Nov 2025; 16:03 UT
Subjects:Optical, Comet

Observations of the comet 3I/ATLAS, also known as C/2025 N1 (ATLAS), were
obtained using the instruments on PRL&apos;s 1.2m telescope at the Mount
Abu Observatory during November 12 to 17 in the late hours of the night,
just before dawn. Initially, when the comet was visible for only a short
time, observations were carried out using an imager in the R filter (see
false colour image in the online link below).

On the later nights, we carried out spectroscopic observations on the PRL
1.2m telescope. The observations were conducted using the LISA instrument,
which provides spectral coverage from 3800 to 6800 &#8491;. At the time
of observation, the comet was at a heliocentric distance of around 1.51-1.53
AU and a geocentric distance of 2.08 - 2.05 AU. The slit was placed on
the photocenter of the coma of the rare interstellar object, and exposures
of 15 minutes were obtained with the telescope following the fast-moving
object in non-sidereal tracking mode. The spectrum was extracted using
a rectangular aperture of 32.60 &times; 1.76 arcseconds, corresponding
to approximately 48707 km &times; 2617 km on the coma. The spectrum was
flux-calibrated and solar continuum corrected to remove the dust contribution.
The spectra show typical emission lines and bands from CN, C2
and C3, as are also usually seen in Solar system comets. The
spectra are available in the link below.

To our knowledge, these are the first post-perihelion spectra of Comet
3I/ATLAS. The spectra show it to be a comet similar to those belonging
to the Solar System. Our value for the log(C2 / CN) production
rate ratio is approximately -0.73 (observation of November 16, 2025), which
indicates the comet to be similar to the depleted class of comets of the
solar system in terms of carbon composition. Observations with longer exposure
times will help confirm the production rates and ratios.

We are conducting further observations with PRL&apos;s Mount Abu telescopes
and encourage continued monitoring of 3I in the days to come.

Work at the Physical Research Laboratory is supported by the Department
of Space, Govt. of India.

Observations of the comet 3I/ATLAS from PRL Mt Abu Observatory: https://docs.google.com/document/d/1l0JIrdSlZfZzWJYB_yPvqq4DeZsfgC2QOByk-Qyd3VY/edit?usp=sharing
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Password Certification: Shashikiran Ganesh (shashi@prl.res.in)
https://www.astronomerstelegram.org/?read=17502
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ATEL #17503							     ATEL #17503

Title:	Gemini pre-perihelion detection of CN/C3/C2 gas emission and
		dust fans in interstellar comet 3I/ATLAS
Author:	Bryce Bolin (Eureka Scientific), Ian Wong (STScI),  Brian
		Lemaux (Gemini Observatory), Laura-May Abron (Griffith Observatory),
		Matthew Belyakov (Caltech), Pablo Candia (Gemini Observatory), Kristin
		Chiboucas (Gemini Observatory), Rodrigo Carrasco (Gemini Observatory),
		Veronica Firpo (Gemini Observatory), Christoffer Fremling (Caltech),
		Josef Hanus (Charles University), Jeong-Eun Heo (Gemini Observatory),
		Piera King (Gemini Observatory), Keith Noll (GSFC), Karleyne Silva
		(Gemini Observatory)
Queries:	bolin.astro@gmail.com
Posted:	20 Nov 2025; 02:37 UT
Subjects:Optical, Comet, Solar System Object

We report the detection of CN, C3, and C2 gas emission features in the
pre-perihelion visible spectrum of interstellar comet 3I/ATLAS taken with
the Gemini South 8.1-m telescope at Cerro Pachon (observatory code I11,
program GS-2025B-DD-102/PI: Bolin). We used the Gemini Multi-Object Spectrograph
(GMOS) to observe 3I/ATLAS on 2025 September 14 23:54 UTC (see attached
spectrum and g- and r-band images). The spectrum shows clear detections
of CN, C3, and C2 gas emission features at 388 nm, 402 nm, and 517 nm,
respectively. Additional CN and C2 gas emission features are seen at 421
nm and 474 nm, as seen in panel A of the attached image.

Panel B shows g and r images of the comet, where its detection is divided
by the azimuthal average. The enhanced images show evidence of a dust fan
~15 arcsec long, pointing in the anti-solar (southeast) direction, more
extended than the compact appearance seen in the initial imaging (Bolin
et al. 2025, MNRAS:L, 542, 1, pp. L139-L143). The appearance of the dust
fans and morphology is comparable to the features seen in other images
of the comet taken before perihelion (Ivanova et al. 2025, ATel #17372,
Jewit & Luu, ApJL, 994, 1, id.L3, 13 pp.) and after perihelion (Jewitt
& Luu 2025, ATel #17490).

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.

Visible spectrum and images of 3I/ATLAS taken on 2025 September 14 UTC:
https://static.wixstatic.com/media/184fc4_4b6ec8181530457ea8847c663e787e30~mv2.png/v1/fill/w_912,h_884,al_c,q_90,usm_0.66_1.00_0.01,enc_avif,quality_auto/184fc4_4b6ec8181530457ea8847c663e787e30~mv2.png
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Password Certification: Bryce Bolin (bolin.astro@gmail.com)
https://www.astronomerstelegram.org/?read=17503
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