Electronic Telegram No. 5281 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 P/2023 M4 (ATLAS) An apparently asteroidal object discovered on CCD images taken on June 21 UT with a 0.5-m f/2 Schmidt reflector at Sutherland, South Africa, in the course of the "Asteroid Terrestrial-Impact Last Alert System" (ATLAS) search program has been found to show cometary appearance by CCD astrometrists elsewhere after it was posted on the Minor Planet Center's PCCP webpage. The discovery observations are tabulated below: 2023 UT R.A. (2000) Decl. Mag. June 21.06121 21 46 58.50 -10 26 42.7 16.8 21.06392 21 46 58.46 -10 26 42.5 16.8 21.06799 21 46 58.42 -10 26 42.2 16.8 21.07298 21 46 58.38 -10 26 42.0 16.8 R. Weryk, Department of Physics and Astronomy, University of Western Ontario, reports that four 45-s pre-discovery w-band survey images taken on June 18.6 UT with the Pan-STARRS1 1.8-m Ritchey-Chretien reflector at Haleakala, HI, show a very condensed head of size 1".3 (full-width-at-half-maximum) and mag 17.1 in 1".1 seeing with no tail. Six stacked 120-s exposures taken on June 22.39 by Hidetaka Sato (Bunkyo-ku, Tokyo, Japan) with a 0.43-m f/6.8 astrograph located at the Utah Desert Remote Observatory (near Beryl Junction, UT, USA) show a strongly condensed round coma 15" in diameter of mag 16.9 with no tail; the magnitude was 16.9 as measured within a circular aperture of radius 7".6. T. Prystavski (Lyiv, Ukraine) writes that the object appeared stellar and of total mag 16.2 on a stacked image obtained on June 30.4 with a "Deep Sky Chile" 0.51-m f/6.8 corrected Dall-Kirkham telescope located in the Rio Hurtado valley; additional observations by Prystavski with a 0.32-m f/9 Ritchey-Chretien reflector at Siding Spring, NSW, Australia, on July 12.64- 12.65 suggest a coma of total mag 16.5 with size 4".2 (FWHM) in 3".3 seeing. Thirty-six stacked 60-s exposures taken by T. Ikemura at Shinshiro, Japan, on July 16.6 with a 0.38-m f/4.2 reflector (and measured/reported by Hirohisa Sato) show only a stellar appearance, with red magnitude 16.1. K. Ly (El Monte, CA, USA) writes that he has found this comet to be active in archival DECam images taken with the 4-m reflector at Cerro Tololo during March-June 2021. A single 90-s i-band exposure taken by observer K. Vivas on 2021 Mar. 17.35 UT showed the comet with a very condensed head of size 1".2 (FWHM) in 1".0 seeong; the comet exhibited a broad, 6"-long tail spanning p.a. 277-317 degrees and had an i-band magnitude of 21.7 as measured in a 5" diameter aperture. Another 90-s i-band exposure taken by J. Carballo on 2021 Mar. 23.17 showed the comet again with a very condensed, essentially asteroidal head in 0".9 seeing; there was a faint, 11"-long straight tail pointing toward p.a. 299 degrees, and it had an i-band magnitude of 21.6 as measured in a 5" diameter aperture. A pair of 50-s g- and r-band exposures taken by A. Eltvedt and T. Shanks on 2021 May 12.11 showed the comet appearing condensed with size 1".4-1".6 (FWHM) in 1".2-1".4 seeing, but with no apparent tail (likely because any tail was obstructed by a nearby star); the comet's g- and r-band magnitudes were 21.7 and 21.4, respectively. Lastly, a single 30-s g-band exposure taken by A. Zenteno on 2021 June 1.17 showed the comet with a very condensed head of size 1".3 (FWHM) in 1.2" seeing, along with a faint, 5"-long straight tail in p.a. 315 degrees; the comet's g-band magnitude was 21.6 as measured in a 5" diameter aperture. Ly adds that he searched additional archival DECam images from 2019 and archival Pan-STARRS images from 2009-2011, but he was unable to find any conclusive detections of the comet in those. A pair of 120-s i-band exposures taken with the DECam on 2019 Jan. 27 did not show obvious signs of the comet down to apparent magnitude 24. Two single-night sets of observations (with the comet presumably then stellar in appearance) that were obtained with the Pan-STARRS1 telescope on 2021 Apr. 9 (mag 21.6-21.9) and 2021 May 3 (mag 21.6-21.7) were found in the MPC's "isolated tracklet file". Subsequent to the reporting of the ATLAS discovery observations to the MPC, pre-discovery observations were reported to the MPC from the Catalina Sky Survey's 0.7-m Schmidt telescope on 2023 June 13 (mag 17.2-17.4), June 14 (mag 17.3-17.4), June 16 (mag 17.2-17.5), and June 19 (mag 17.1-17.2). The astrometry appears on MPEC 2023-O51. The following elliptical orbital elements by S. Nakano (Central Bureau) are from 448 observations spanning 2021 Mar. 17-2023 July 21 (mean residual 0".3). The comet passed 0.80 AU from Jupiter on 1998 June 7 and 2.43 AU from Saturn on 2004 Apr. 1 UT. Epoch = 2009 June 18.0 TT T = 2009 June 13.05539 TT Peri. = 319.50734 e = 0.2792283 Node = 296.43184 2000.0 q = 3.9441602 AU Incl. = 7.58952 a = 5.4721350 AU n = 0.07699613 P = 12.80 years Epoch = 2022 Apr. 11.0 TT T = 2022 Apr. 14.95546 TT Peri. = 320.65578 e = 0.2799831 Node = 296.29874 2000.0 q = 3.9290498 AU Incl. = 7.59123 a = 5.4568855 AU n = 0.07731910 P = 12.75 years Epoch = 2034 Dec. 24.0 TT T = 2034 Dec. 13.11613 TT Peri. = 320.82253 e = 0.2812679 Node = 296.14342 2000.0 q = 3.8867476 AU Incl. = 7.61213 a = 5.4077833 AU n = 0.07837457 P = 12.58 years Epoch = 2047 June 19.0 TT T = 2047 June 12.64862 TT Peri. = 320.79348 e = 0.2819286 Node = 296.00909 2000.0 q = 3.8528417 AU Incl. = 7.63100 a = 5.3655410 AU n = 0.07930194 P = 12.43 years The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 7.0 and 2.5n = 10, noting that these do not represent the brightness of the comet in the pre-perihelion 2021 observations (which are fainter by 4-5 magnitudes). While there may be a huge asymmetry in brightness with respect to perihelion, it seems more likely that there has been an extended outburst the the past month. Monitoring of this comet's brightness is encouraged. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2023 06 25 21 46.47 -10 22.8 3.683 4.377 127.5 10.6 16.2 2023 07 05 21 44.23 -10 17.8 3.592 4.395 137.5 9.0 16.2 2023 07 15 21 40.73 -10 19.6 3.520 4.414 147.8 7.0 16.2 2023 07 25 21 36.18 -10 27.5 3.473 4.433 158.3 4.9 16.2 2023 08 04 21 30.92 -10 40.2 3.452 4.451 168.8 2.5 16.2 2023 08 14 21 25.34 -10 55.9 3.459 4.470 176.0 0.9 16.2 2023 08 24 21 19.87 -11 12.9 3.496 4.490 167.9 2.7 16.2 2023 09 03 21 14.95 -11 29.2 3.562 4.509 157.3 5.0 16.3 2023 09 13 21 10.93 -11 43.2 3.654 4.529 146.7 7.0 16.4 2023 09 23 21 08.09 -11 53.8 3.770 4.549 136.3 8.8 16.5 2023 10 03 21 06.61 -11 59.7 3.906 4.569 126.2 10.2 16.6 2023 10 13 21 06.55 -12 00.6 4.058 4.589 116.3 11.2 16.7 2023 10 23 21 07.91 -11 55.9 4.222 4.609 106.8 11.9 16.8 2023 11 02 21 10.61 -11 45.6 4.393 4.629 97.5 12.3 16.9 2023 11 12 21 14.55 -11 29.6 4.568 4.650 88.5 12.3 17.0 2023 11 22 21 19.59 -11 08.0 4.743 4.670 79.8 12.0 17.1 2023 12 02 21 25.59 -10 41.0 4.914 4.691 71.2 11.5 17.2 2023 12 12 21 32.40 -10 08.8 5.079 4.712 62.9 10.7 17.3 2023 12 22 21 39.89 -09 31.8 5.233 4.732 54.7 9.8 17.3 2024 01 01 21 47.93 -08 50.3 5.375 4.753 46.6 8.6 17.4 2024 01 11 21 56.39 -08 04.7 5.502 4.774 38.7 7.4 17.5 2024 04 20 23 23.58 +01 29.1 5.738 4.987 38.1 7.1 17.8 2024 04 30 23 30.82 +02 26.2 5.659 5.008 45.8 8.3 17.8 2024 05 10 23 37.49 +03 21.0 5.564 5.030 53.5 9.3 17.7 2024 05 20 23 43.50 +04 12.8 5.457 5.051 61.4 10.1 17.7 2024 05 30 23 48.76 +05 00.9 5.339 5.073 69.4 10.8 17.7 2024 06 09 23 53.19 +05 44.7 5.213 5.094 77.7 11.2 17.7 2024 06 19 23 56.67 +06 23.6 5.083 5.115 86.1 11.4 17.6 2024 06 29 23 59.15 +06 56.7 4.951 5.137 94.8 11.4 17.6 2024 07 09 00 00.52 +07 23.3 4.820 5.158 103.8 11.0 17.5 2024 07 19 00 00.75 +07 42.7 4.696 5.179 113.1 10.4 17.5 2024 07 29 23 59.81 +07 54.4 4.582 5.201 122.6 9.5 17.5 2024 08 08 23 57.73 +07 57.9 4.483 5.222 132.5 8.2 17.4 2024 08 18 23 54.63 +07 53.1 4.403 5.243 142.6 6.7 17.4 2024 08 28 23 50.66 +07 40.5 4.345 5.264 152.8 5.0 17.4 2024 09 07 23 46.06 +07 20.7 4.314 5.285 162.8 3.2 17.4 2024 09 17 23 41.16 +06 55.2 4.312 5.306 170.8 1.7 17.4 2024 09 27 23 36.27 +06 26.1 4.339 5.327 169.2 2.0 17.5 2024 10 07 23 31.75 +05 55.5 4.397 5.348 160.3 3.6 17.5 2024 10 17 23 27.89 +05 25.7 4.483 5.369 150.0 5.3 17.6 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2023 CBAT 2023 July 22 (CBET 5281) Daniel W. E. Green