Electronic Telegram No. 5063 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/2021 T4 (LEMMON) An apparently asteroidal object discovered on CCD survey images obtained with the Mount Lemmon 1.5-m reflector on Oct. 7 has been found to show cometary appearance by CCD astrometrists elsewhere after it was posted on the Minor Planet Center's PCCP webpage. The Mt. Lemmon astrometry is tabulated below and includes pre-discovery observations from Sept. 9 that were found and reported later. 2021 UT R.A. (2000) Decl. Mag. Sept. 9.45558 4 12 37.73 +13 19 36.3 9.46218 4 12 37.65 +13 19 34.4 9.46976 4 12 37.59 +13 19 34.3 21.6 Oct. 7.34914 4 02 44.34 +12 01 40.4 20.2 7.35429 4 02 44.16 +12 01 39.1 20.1 7.35944 4 02 44.03 +12 01 38.1 20.2 7.36460 4 02 43.86 +12 01 37.3 20.3 8.37113 4 02 12.64 +11 58 18.3 20.5 8.37842 4 02 12.43 +11 58 17.0 20.5 8.38608 4 02 12.19 +11 58 15.6 20.3 8.39316 4 02 11.97 +11 58 13.7 20.3 R. Weryk, Department of Physics and Astronomy, University of Western Ontario, reports on pre-discovery images obtained with the Pan-STARRS1 and the Pan-STARRS2 1.8-m Ritchey-Chretien reflectors at Haleakala, Hawaii, after the posting of the Mt. Lemmon discovery; the Pan-STARRS1 images from Aug. 7.6 (at mag 21.5-21.6) and Oct. 2.6 UT (at mag 20.0), and the Pan-STARRS2 images from Sept. 5.55-5.58 (at mag 20.2-20.7) and 15.52-15.56 (mag 20.3-20.6) were not good enough to discern cometary activity. But Weryk adds that there were Jan. and Feb. exposures that should have shown the object if it were a minor planet (i.e., "5 log r" light curve), and its invisibility thus suggests a steeper, cometary light curve. Weryk further notes that two 45-s w-band Pan-STARRS1 images taken on Oct. 27.6 show this to be a comet with a condensed coma of size 1".6-1".7 (full-width-at-half-maximum) in 1".1 seeing, with a broad 2" tail spanning p.a. 70-115 degrees. Twenty stacked 60-s exposures taken remotely by H. Sato, Tokyo, Japan, on Oct. 9.4 UT with a 0.43-m f/6.8 astrograph located near Mayhill, NM, USA, show only a stellar appearance; the magnitude was 19.9 as measured within a circular aperture of radius 5".7. E. Bryssinck, Kruibeke, Belgium, writes that twelved stacked, luminance-filtered 120-s exposures taken remotely on Oct. 11.3 by a large group of observers using a "Telescope Live" 0.6-m f/6.5 reflector in Rio Hurtado, Chile, show a condensed but slightly elongated coma of size 9" x 6", elongated toward p.a. 321 degrees, with no tail; the magnitude was 19.8 in an aperture of radius 5".6. CCD exposures taken by L. Buzzi, Varese, Italy, with a 0.84-m f/3.5 reflector on Oct. 13.15-13.17 (and measured by A. Aletti) in marginal seeing show the comet to be just slightly larger than stars nearby; images taken in better conditions on Oct. 15.1 show the head to have a size of 4".8 (FWHM) in 4".0-4".3 seeing with no tail. The available astrometry appears on MPEC 2021-U187. The following orbital elements by S. Nakano (CBAT) are from 65 observations spanning 2021 Aug. 7-Oct. 28 (mean residual 0".4), with corresponding original and future values of 1/a being -0.000103 and +0.000769 AU**-1, respectively (mean error +/- 0.000145), indicating that the comet will pass 2.19 AU from Jupiter on 2022 Dec. 17 and 0.54 AU from the earth on 2023 July 20 UT. Epoch = 2023 Aug. 4.0 TT T = 2023 July 31.41541 TT Peri. = 329.81110 e = 1.0000379 Node = 257.88447 2000.0 q = 1.4832480 AU Incl. = 160.77749 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 9.0 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2021 09 23 04 08.91 +12 45.3 6.774 7.284 116.9 7.1 20.1 2021 10 03 04 04.86 +12 15.6 6.549 7.203 127.6 6.3 19.9 2021 10 13 03 59.68 +11 42.7 6.344 7.123 138.6 5.3 19.8 2021 10 23 03 53.46 +11 06.9 6.164 7.041 149.7 4.1 19.7 2021 11 02 03 46.36 +10 29.1 6.016 6.960 160.5 2.7 19.6 2021 11 12 03 38.58 +09 50.4 5.902 6.878 169.3 1.5 19.6 2021 11 22 03 30.44 +09 12.2 5.825 6.795 168.2 1.7 19.5 2021 12 02 03 22.25 +08 35.9 5.785 6.713 158.5 3.1 19.4 2021 12 12 03 14.36 +08 03.0 5.780 6.630 147.2 4.6 19.4 2021 12 22 03 07.09 +07 34.6 5.807 6.546 135.6 6.0 19.3 2022 01 01 03 00.68 +07 11.6 5.860 6.462 124.1 7.2 19.3 2022 01 11 02 55.32 +06 54.6 5.934 6.378 112.7 8.2 19.3 2022 01 21 02 51.11 +06 43.5 6.021 6.294 101.6 8.8 19.3 2022 01 31 02 48.09 +06 38.2 6.115 6.209 90.9 9.1 19.3 2022 02 10 02 46.24 +06 38.0 6.210 6.123 80.4 9.1 19.3 2022 02 20 02 45.49 +06 42.3 6.299 6.037 70.3 8.9 19.2 2022 03 02 02 45.74 +06 50.3 6.377 5.951 60.4 8.3 19.2 2022 03 12 02 46.91 +07 01.1 6.440 5.865 50.9 7.6 19.2 2022 03 22 02 48.87 +07 13.9 6.483 5.778 41.7 6.6 19.2 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2021 CBAT 2021 October 29 (CBET 5063) Daniel W. E. Green