Electronic Telegram No. 5608
Central Bureau for Astronomical Telegrams
Mailing address:  Hoffman Lab 209; Harvard University;
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Prepared using the Tamkin Foundation Computer Network


COMET 3I/ATLAS
     Y.-J. Kuan, National Taiwan Normal University (NTNU); I. M. Coulson, East
Asian Observatory; M. A. Cordiner, Goddard Space Flight Center, NASA (GSFC),
and Catholic University; S. B. Charnley and S. N. Milam, GSFC; N. X. Roth,
GSFC and American University; Y.-L. Chuang and M.-K. Lin, Academia Sinica
Institute of Astronomy and Astrophysics; Y.-N. Lee, NTNU; and B. Pimpanuwat,
National Astronomical Research Institute of Thailand, report the detection of
HCN:J=3D3-2 (265 GHz) emission from comet 3I/ATLAS at heliocentric distance r
= 2.13 AU.  Observations were made on Sept. 14 UT with the James Clerk Maxwell
Telescope on Mauna Kea, using the 'U'u (A-band) heterodyne receiver and the
ACSIS spectroscopic backend.  The integrated line intensity was 37.0 (+/- 6.5)
mK*km/s.  Non-local-thermodynamic-equilibrium analysis using the SUBLIME code
(Cordiner et al 2022, Ap.J. 929, 38) yields an expansion velocity of 0.45 (+/-
0.14) km/s and a production rate of HCN molecules, Q(HCN), of 4.5 (+/- 1.9) x
10**25 molecules/s (2.0 kg/s) assuming a kinetic gas temperature of 30K and
Q(H_2O) = 1 x 10**28 molecules/s.  A week earlier, on Sept. 7 when r = 2.33
AU, the authors measured Q(HCN) to be 1.5 (+/- 0.5) x 10**25 molecules/s, and
in the month prior, on a roughly weekly cadence during which r = 3.29-2.45 UT,
they obtained 3-sigma upper limits of between 0.7 and 1.2 x 10**25**
molecules/s.

     Further to CBET 5596, Twenty-one stacked 60-s CCD exposures taken by
Hirohisa Sato at Sukagawa, Japan, with a 0.25-m f/4 reflector during July
28.55-28.56 UT showed a condensed coma about 11".5 in diameter of total mag
16.6.  CCD exposures taken remotely on Aug. 9.45 by Hidetaka Sato (Tokyo,
Japan) with a 0.51-m f/6.8 astrograph located at Siding Spring, NSW, show a
strongly condensed coma 10" in diameter; the magnitude was 16.1 as measured
within a circular aperture of radius 5".4.  CCD exposures taken remotely by
K. Yoshimoto (Yamaguchi, Japan) on Sept. 12.4 with a 0.51-m f/6.8 reflector
located at Siding Spring show a diffuse outer coma 2'.2 in diameter and a
condensed inner coma with a tail 1'.0 long in p.a. 103 deg; the magnitude
was given as 14.1.  CCD exposures taken by K. Kadota at Ageo, Japan, with a
0.25-m f/5 reflector yield the following total magnitudes:  Aug. 19.44,
15.8; Sept. 1.43, 15.3; 14.42, 14.2.


NOTE: These 'Central Bureau Electronic Telegrams' are sometimes
      superseded by text appearing later in the printed IAU Circulars.

                         (C) Copyright 2025 CBAT
2025 September 18                (CBET 5608)              Daniel W. E. Green