Electronic Telegram No. 5445
Central Bureau for Astronomical Telegrams
Mailing address:  Hoffman Lab 209; Harvard University;
 20 Oxford St.; Cambridge, MA  02138; U.S.A.
e-mail:  cbatiau@eps.harvard.edu (alternate cbat@iau.org)
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Prepared using the Tamkin Foundation Computer Network


COMET C/2023 A3 (TSUCHINSHAN-ATLAS)
     Further to CBET 5404, J. N. Marcus clarifies and expands on his work
mentioned therein:  "The coma light-scattering model (Marcus 2007a, ICQ 29,
39; Marcus 2007b, ICQ 29, 119; URL http://www.icq.eps.harvard.edu/publ.html)
predicts strong forward-scattering enhancement of the brightness of comet
C/2023 A3 around the time of minimum scattering angle (7.0 deg) on Oct. 9.4
UT.  Based on an assumed coma dust-to-gas light ratio of 10 for a 'dusty'
comet (based on the tail morphology and low C_2 relative to dust-production
rates reported earlier by D. Schleicher), the modeled brightness
increase in visual magnitude, relative to that viewed from (normalized to) a
90-degree scattering angle, is as follows for the listed dates and
corresponding scattering angles:  2024 Sept. 29.0 UT (79.5 deg, -0.3 mag);
Oct. 2.0 (58.7, -1.1); 5.0 (36.7, -2.5); 6.0 (29.1, -3.2); 7.0 (21.4, -4.1);
8.0 (13.9, -5.3); 9.0 (7.9, -6.7); 9.4 (7.0, -6.9); 10.0 (8.5, -6.5); 11.0
(15.1, -5.1); 12.0 (23.0, -3.9); 13.0 (31.1, -3.0); 14.0 (39.1, -2.3); 15.0
(46.8, -1.7); 18.0 (67.1, -0.7); and 21.0 (83.1, -0.2)."  If a current
baseline set of total-visual-magnitude parameters of H = 6.0 and 2.5n = 8
is maintained to perihelion and beyond, Marcus predicts that the comet's
brightness could exceed mag 0 during Oct. 5-13, mag -2 during Oct. 7-11,
and mag -3 during Oct. 8-10, peaking around Oct. 9.4 at possibly brighter
than mag -4.
     Recent digital-camera total magnitudes and coma diameters for C/2023 A3
reported by K. Yoshimoto, Yamaguchi, Japan, using a 135-mm f/3.5 lens + CMOS
camera remotely at "Deep Sky Chile":  July 21.99 UT, 9.4, 3'.0; Aug. 9.00,
8.7, 2'.6.  Selected visual total-magnitude and coma-diameter estimates:
July 24.90, 9.3, 4' (V. Gonano, Udine, Italy, 20x80 binoculars); 25.91, 9.5,
3' (J. G. de S. Aguiar, Campinas, Brazil, 0.27-m reflector); Aug. 10.90, 8.6,
3' (Aguiar).


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      superseded by text appearing later in the printed IAU Circulars.

                         (C) Copyright 2024 CBAT
2024 September 10                (CBET 5445)              Daniel W. E. Green