Electronic Telegram No. 5004 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 O1 (NISHIMURA) Hideo Nishimura, Gansho-ji, Kakegawa (near Yuto), Japan, reported that he found a comet on three 15-s exosures taken on July 21.8 UT with a Canon EOS 6D digital camera + 200-mm-f.l. f/3.2 lens, the comet showing a diffuse 2'.5 coma with central condensation. Nishimura suspected that this might be an outburst of comet 8P/Tuttle (though the latter comet is located +1.96 degrees in R.A. and -0.25 degree in Decl. from Nishimura's comet). His discovery observations are tabulated below, together with confirming observations that he obtained (as above) on July 22.8. Nishimura was a co-discoverer of comet C/1994 N1. 2021 UT R.A. (2000) Decl. Mag. Observer July 21.78306 6 31 33.5 +30 37 52 10.7 Nishimura 21.78328 6 31 33.5 +30 37 55 " 21.78350 6 31 33.8 +30 37 54 " 22.77352 6 37 45.0 +30 48 30 10.2 " 22.77448 6 37 45.3 +30 48 30 " 22.77491 6 37 45.4 +30 48 29 " 22.77522 6 37 45.1 +30 48 32 " 22.77576 6 37 46.3 +30 48 34 " 22.78293 6 37 48.4 +30 48 30 " 22.78337 6 37 48.2 +30 48 44 " S. Kaneko, Kakegawa, Japan, has also confirmed Nishimura's comet on two 15-s exposures taken on July 21.78 UT using a 200-mm-f.l. f/3.0 lens with a Canon EOS 6D digital camera, noting the comet to have a green color; his astrometry is tabulated below. 2021 UT R.A. (2000) Decl. Observer July 22.77791 6 37 46.7 +30 48 28 Kaneko 22.77879 6 37 46.6 +30 48 33 22.78050 6 37 47.5 +30 48 36 22.78197 6 37 48.4 +30 48 44 22.78344 6 37 48.8 +30 48 32 22.78397 6 37 48.9 +30 48 33 T. Noguchi, Katori, Japan, provides the following astrometry from CCD images taken with a 0.23-m f/10 Schmidt-Cassegrain reflector. 2021 UT R.A. (2000) Decl. Mag. Observer July 23.76476 6 44 00.49 +30 57 58.4 9.9 Noguchi 23.76767 6 44 01.69 +30 57 59.0 " 23.77082 6 44 02.93 +30 58 00.7 " 23.77447 6 44 04.23 +30 58 03.2 " 23.77672 6 44 05.09 +30 58 04.4 " H. Abe, Yatsuka, Japan, provides the following astrometry from CCD images taken with a 0.26-m f/6.0 reflector. 2021 UT R.A. (2000) Decl. Mag. Observer July 23.78872 6 44 09.60 +30 58 11.5 10.2 Abe 23.78912 6 44 09.74 +30 58 12.4 " 23.79682 6 44 12.82 +30 58 14.9 " 23.79763 6 44 13.15 +30 58 15.5 " K. Itagaki, Okayama, Japan, provides the following astrometry from CCD images taken with a 0.50-m f/6.9 reflector, measured by H. Kaneda. 2021 UT R.A. (2000) Decl. Mag. Observer July 23.79353 6 44 11.45 +30 58 13.2 11.9 Itagaki 23.79380 6 44 11.55 +30 58 14.3 " 23.79431 6 44 11.75 +30 58 14.1 " 23.79458 6 44 11.92 +30 58 15.3 " M. Jaeger, Vienna, Austria, reports the following astrometry that he measured from CCD frames taken by E. Prosperi, S. Prosperi, and himself with a 0.28-m f/2.2 reflector at Stixendorf. Jaeger reports that twenty stacked 30-s exposures taken with this Schmidt telescope using a green filter show a coma of size 2'10" and total mag 9.2, with a possible tail 5' long in p.a. 287 degrees. 2021 UT R.A. (2000) Decl. Mag. Observer July 24.07144 6 45 57.58 +31 00 37.8 11.2 Jaeger 24.07719 6 45 59.68 +31 00 42.1 11.4 " 24.07896 6 46 00.42 +31 00 42.3 11.3 " 24.08116 6 46 01.19 +31 00 43.6 11.4 " 24.08379 6 46 02.19 +31 00 45.4 11.3 " 24.08627 6 46 03.32 +31 00 45.0 11.7 " CCD images taken on July 24.76 UT by K. Kadota at Ageo, Japan, with a 0.25-m f/5.0 reflector show a strong central condensation and a coma of diameter 3'.0 (total magnitude 9.8), with a faint tail 2'.0 long in p.a. 285 deg; his astrometry is tabulated below. 2021 UT R.A. (2000) Decl. Mag. Observer July 24.76344 6 50 22.80 +31 06 22.4 9.8 Kadota 24.76568 6 50 23.63 +31 06 24.0 " 24.76794 6 50 24.52 +31 06 24.9 " 24.77193 6 50 26.07 +31 06 26.5 " 24.77420 6 50 26.91 +31 06 27.8 " Hirohisa Sato, Sukagawa, Fukushima-ken, Japan, reports that five stacked 30-s CCD exposures taken on July 24.8 UT with a 0.25-m f/4 reflector show a strongly condensed coma about 1'.4 in diameter and a tail about 1'.1 long toward p.a. about 289 degrees; his astrometry is tabulated below. 2021 UT R.A. (2000) Decl. Mag. Observer July 24.76397 6 50 23.03 +31 06 22.6 10.0 Sato 24.77008 6 50 25.40 +31 06 25.3 " 24.77497 6 50 27.28 +31 06 27.8 " Astrometry measured by M. Yoshimi, Fukuchiyama city, Kyoto, Japan, from images taken using a 0.20-m f/5 reflector (+ Canon EOS 6D camera); 2021 UT R.A. (2000) Decl. Mag. Observer July 24.78464 6 50 31.01 +31 06 33.4 11.8 Yoshimi 24.78589 6 50 31.44 +31 06 33.0 " 24.78660 6 50 31.84 +31 06 32.7 " Fourteen stacked 15-s CCD exposures taken by K. Yoshimoto (Kumage, Yamaguchi, Japan) with a 0.20-m f/8.0 Ritchey-Chretien reflector on July 24.8 UT show a condensed coma 1'.0 in diameter and no tail; his astrometry is provided below: 2021 UT R.A. (2000) Decl. Mag. Observer July 24.80515 6 50 38.84 +31 06 42.1 10.3 Yoshimoto 24.80677 6 50 39.41 +31 06 43.9 " 24.80959 6 50 40.52 +31 06 43.9 " K. Harikae, Funabashi, Japan, identified pre-discovery images of the comet on 40-s exposures taken with a 0.15-m f/2.5 reflector (+ Canon EOS 6D camera), with his astrometry tabulated below. 2021 UT R.A. (2000) Decl. Mag. Observer July 18.75417 6 13 04.5 +29 59 33 10.5 Harikae 19.75764 6 19 09.6 +30 13 38 " The following parabolic orbital elements by S. Nakano (CBAT) are from 50 observations spanning July 21-24 (mean residual 1".0). The Harikae observations were not used, as they have residuals of about half an arcmin. Nakano searched unsuccessfully for potential linkage to lost comets. T = 2021 Aug. 12.95714 TT Peri. = 72.67306 Node = 41.20143 2000.0 q = 0.7963497 AU Incl. = 27.33647 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 8.5 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2021 05 06 01 53.05 +03 16.8 2.799 1.879 19.5 10.3 12.9 2021 05 11 02 03.55 +04 51.5 2.718 1.811 21.0 11.5 12.7 2021 05 16 02 14.60 +06 29.6 2.635 1.743 22.4 12.8 12.5 2021 05 21 02 26.28 +08 11.3 2.552 1.675 23.7 14.0 12.3 2021 05 26 02 38.67 +09 56.8 2.470 1.607 24.8 15.3 12.1 2021 05 31 02 51.87 +11 46.0 2.387 1.539 25.8 16.7 11.9 2021 06 05 03 06.00 +13 39.1 2.306 1.471 26.6 18.0 11.7 2021 06 10 03 21.18 +15 35.6 2.227 1.403 27.2 19.3 11.4 2021 06 15 03 37.57 +17 35.0 2.150 1.336 27.6 20.6 11.2 2021 06 20 03 55.33 +19 36.5 2.077 1.270 27.8 21.9 10.9 2021 06 25 04 14.64 +21 38.7 2.007 1.204 27.7 23.1 10.7 2021 06 30 04 35.72 +23 39.5 1.943 1.141 27.4 24.2 10.4 2021 07 05 04 58.74 +25 36.0 1.884 1.080 26.9 25.2 10.1 2021 07 10 05 23.87 +27 24.3 1.832 1.021 26.0 25.9 9.9 2021 07 15 05 51.17 +28 59.6 1.788 0.967 25.0 26.4 9.6 2021 07 20 06 20.58 +30 16.2 1.752 0.918 23.7 26.5 9.4 2021 07 25 06 51.89 +31 08.2 1.725 0.875 22.3 26.1 9.2 2021 07 30 07 24.63 +31 30.1 1.709 0.840 20.8 25.4 9.1 2021 08 04 07 58.13 +31 17.8 1.702 0.815 19.2 24.2 8.9 2021 08 09 08 31.61 +30 29.8 1.705 0.800 17.7 22.6 8.9 2021 08 14 09 04.26 +29 07.4 1.717 0.797 16.2 20.8 8.9 2021 08 19 09 35.40 +27 15.0 1.739 0.805 14.9 18.9 8.9 2021 08 24 10 04.60 +24 58.6 1.769 0.824 13.9 17.1 9.1 2021 08 29 10 31.64 +22 25.1 1.807 0.854 13.0 15.4 9.2 2021 09 03 10 56.50 +19 41.2 1.852 0.892 12.2 13.8 9.4 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2021 CBAT 2021 July 25 (CBET 5004) Daniel W. E. Green