Electronic Telegram No. 5487 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) URL http://www.cbat.eps.harvard.edu/index.html Prepared using the Tamkin Foundation Computer Network COMET C/2024 Y1 (MASEK) Martin Masek [in TeX format, Ma\v sek], of Liberec, Czech Republic, and a member of the Institute of Physics of the Czech Academy of Sciences, reports his discovery of a diffuse comet of mag about 14-15 on CMOS (C4-16000) images obtained on Dec. 25 and 26 using a 300-mm-f.l. f/2.8 telephoto lens [one of the F/(Ph)otometric Robotic Atmospheric Monitor (FRAM) robotic instruments] at the Pierre Auger Observatory in Argentina with the aid of Tycho-Tracker software. His astrometry has large residuals due to the low resolution of the lens (approximately 6" per pixel), with discovery positions provided below. 2024 UT R.A. (2000) Decl. Mag. Observer Dec. 25.07643 21 52 00.47 -68 53 14.5 15.3 Masek 25.10221 21 52 23.04 -68 53 20.4 15.2 " 26.11537 22 07 13.90 -68 56 00.7 14.8 " 26.12943 22 07 26.10 -68 55 49.7 14.1 " 26.13605 22 07 31.94 -68 55 54.8 14.6 " Masek confirmed the comet as diffuse with mag around 14.5 in CCD images obtained on Dec. 27.1 UT with another FRAM instrument (135-mm-f.l. telephoto lens) located near Paranal, Chile. Masek elaborates: "The main observational program of the FRAM telescopes is measuring atmospheric extinction above the Pierre Auger Observatory (Argentina) and the Cherenkov Telescope Array (Chile and La Palma). During the remaining observing time, the telescopes are used for various interesting astronomical targets (photometry of variable stars, astrometry of near-earth asteroids and objects from the NEOCP/PCCP [webpages], and comet photometry). In my leisure time, I used one of the FRAM telescopes to search for comets. Comet hunting is not an official observing program of the Institute of Physics; it was my personal initiative. I made the discovery using images taken with a Canon 300mm f/2.8 telephoto lens + CMOS C4-16000 camera from Moravian Instruments. The field of view of a single image is 7 degrees x 7 degrees. I then analyzed the images using the Tycho-Tracker software by Daniel Parrott (Synthetic Tracking tool). On the night of Dec. 24/25, I selected a region above the western horizon at the start of the night, where I attempted to search areas not covered by large surveys. I captured a mosaic of seven fields, with the entire mosaic completed in five minutes. I repeated the full mosaic fifteen times, so each field was captured in fifteen images, each with a 30-s exposure, which I later analyzed using Tycho-Tracker. I discovered this as my first comet after about 43 observing sessions (which might have taken approximately 50 hours of total observation time)." After the comet was posted on the Minor Planet Center's PCCP webpage, other astrometrists have commented on the cometary appearance. CMOS images taken by A. Schnabel and D. Ruhela, and measured/reported by S. Deen (Simi Valley, CA, USA), using a 0.43-m reflector at Rio Hurtado, Chile, on Dec. 28.1 UT in 1" seeing show a very condensed coma of size about 2" (full-width-at- half-maximum) with a notable tail pointing roughly away from the earth (and thus vaguely resembling a coma) that extends 30" from the nuclear condensation (but mostly pointing towards p.a. 105-150 degrees). A. Hale (Cloudcroft, NM, USA) took 120-s CCD images remotely on Dec. 28.43 with a 0.35-m f/3 Cassegrain reflector located at Siding Spring, NSW, Australia, that show a condensed-but- quite-diffuse comet of mag 17.7-18.1 with a 24" coma and no tail. Twelve stacked 30-s CMOS exposures taken by H. Sato (Bunkyo-ku, Tokyo, Japan) with a 0.25-m f/3.8 astrograph located at Rio Hurtado, Chile, on Dec. 28.1 UT also show a strongly condensed comet with an outer coma of 1'.4 in diameter and a semi-circular tail 1'.6 long toward p.a. 120-300 degrees; the magnitude was 15.3 as measured within a circular aperture of radius 41".8. Twenty stacked 30-s CCD exposures taken remotely on Dec. 28.4 by Sato using a 0.51-m f/6.8 astrograph located at Siding Spring show a strongly condensed comet with an outer coma 1'.0 in diameter and no obvious tail; the magnitude was 16.2 as measured within a circular aperture of radius 32".8. Ten stacked 30-s CCD exposures taken on Dec. 28.43 UT by K. Yoshimoto (Kumage, Yamaguchi, Japan) with a 0.51-m f/6.8 reflector located at Siding Spring show a diffuse coma 60" in diameter "spreading to the south", with total mag 15.5. Twenty stacked 20-s CCD images taken remotely by F. D. Romanov (Yuzhno-Morskoy, Nakhodka, Russia) using an iTelescope 0.51-m f/6.8 reflector (+ Luminance filter) at Siding Spring on Dec. 29.43 show a very condensed coma 11" in diameter with total mag 17.8 (using G-band comparison- star magnitudes from the Gaia DR2 catalogue) and no tail. The available astrometry appears on MPEC 2024-Y250. The following parabolic orbital elements by S. Nakano (Central Bureau) are from 58 observations spanning Dec. 25-31 (mean residual 0".6). The comet will pass 2.81 AU from Jupiter on 2025 May 6 UT. T = 2024 Nov. 26.99610 TT Peri. = 238.29363 Node = 112.36748 2000.0 q = 0.8258269 AU Incl. = 64.87964 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 15.0 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2024 11 26 18 02.53 -50 55.9 1.350 0.826 37.5 46.7 15.0 2024 12 06 18 46.45 -58 44.0 1.266 0.843 41.7 51.0 14.9 2024 12 16 20 00.48 -65 35.0 1.175 0.900 48.3 54.7 15.0 2024 12 26 22 05.53 -68 56.0 1.091 0.987 56.5 56.3 15.1 2025 01 05 00 26.69 -64 56.2 1.031 1.094 65.8 55.0 15.4 2025 01 15 02 00.08 -54 26.0 1.016 1.215 74.8 51.4 15.7 2025 01 25 02 54.40 -41 23.4 1.058 1.343 82.1 46.6 16.1 2025 02 04 03 29.69 -28 44.7 1.161 1.474 86.4 41.9 16.7 2025 02 14 03 55.79 -17 57.7 1.315 1.608 87.4 37.8 17.2 2025 02 24 04 17.08 -09 20.1 1.509 1.742 85.7 34.5 17.8 2025 03 06 04 35.70 -02 35.9 1.731 1.875 82.2 31.6 18.4 2025 03 16 04 52.73 +02 38.4 1.970 2.007 77.6 28.9 18.9 2025 03 26 05 08.74 +06 44.4 2.220 2.138 72.3 26.4 19.4 2025 04 05 05 24.07 +09 58.7 2.473 2.268 66.5 23.9 19.8 2025 04 15 05 38.89 +12 33.3 2.727 2.396 60.4 21.4 20.2 2025 04 25 05 53.30 +14 37.1 2.976 2.522 54.2 18.9 20.6 2025 05 05 06 07.36 +16 16.7 3.217 2.647 47.8 16.4 20.9 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2024 CBAT 2024 December 31 (CBET 5487) Daniel W. E. Green