Electronic Telegram No. 5274 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/2023 H5 (LEMMON) [Editor's note: this text replaces that on CBET 5272; A/2023 H4] An apparently asteroidal object discovered on Apr. 19 (discovery observations tabulated below) on CCD images obtained with the Mt. Lemmon Survey's 1.5-m reflector has been found to show cometary activity by astrometric observers elsewhere after it was posted to the Minor Planet Center's PCCP webpage. 2023 UT R.A. (2000) Decl. Mag. Apr. 19.18324 9 56 38.92 -12 53 18.3 20.2 19.18840 9 56 38.86 -12 53 15.9 20.0 19.19355 9 56 38.80 -12 53 13.4 20.2 19.19872 9 56 38.73 -12 53 11.3 20.3 R. Weryk, Department of Physics and Astronomy, University of Western Ontario, reports his identification of pre-discovery images of this object on 2022 Dec. 1 obtained with the Pan-STARRS1 1.8-m Ritchey-Chretien reflector at Haleakala (at magnitude 22.1-22.4) and on 2023 Jan. 20 with the Pan-STARRS2 1.8-m Ritchey-Chretien reflector at Haleakala (at mag 20.9-21.3). The MPC's "isolated tracklet file" (which is accessible publicly) had already contained Pan-STARRS1 observations from Mar. 27.25-27.28 UT (at mag 20.5-20.6), with Weryk noting that four 45-s w-band survey images taken on that night showing a very condensed head of size 1".5 (full-width-at-half-maximum) in 1".2 seeing, with no tail. Weryk adds that three follow-up 60-s gri-band images obtained on June 12.26 with the 3.6-m Canada-France-Hawaii Telescope (R. Wainscoat's time; queue observer C. Cunningham; queue coordinator T. Burdullis) show the object to be clearly a comet, with a very condensed head of size 1".2 (FWHM) in 0".7 seeing and a straight tail about 2" long toward p.a. 140 degrees. Mt. Lemmon observations of the comet (again, apparently asteroidal in appearance) were later identified from Jan. 28.3 UT (mag 20.6-21.3). Ten stacked 120-s CCD exposures taken remotely on Apr. 21.5 by H. Sato (Bunkyo-ku, Tokyo, Japan) with an iTelescope 0.43-m f/6.8 astrograph at Siding Spring, NSW, showed only a stellar appearance; the magnitude was 20.0 as measured within a circular aperture of radius 3".8. This comet was given the designation C/2023 H5 because an apparently asteroidal object with a comet-like orbit that was found with the Pan-STARRS2 telescope on Apr. 23 was given the designation A/2023 H4 (cf. MPEC 2023-L26). The available astrometry for C/2023 H5 appears on MPEC 2023-M44. The following orbital elements by S. Nakano (Central Bureau) are from 60 observations spanning 2022 Dec. 1-2023 June 15 (mean residual 0".3), with corresponding original and future values of 1/a being +0.000001 and -0.000229 AU**-1, respectively (+/- 0.000020 AU**-1). There is no close approach to any major planets. Epoch = 2025 June 14.0 TT T = 2025 June 30.25572 TT Peri. = 60.09836 e = 1.0005160 Node = 159.47812 2000.0 q = 4.3125800 AU Incl. = 97.85601 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 8.0 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2023 05 16 09 53.83 -09 42.5 7.202 7.428 99.0 7.7 19.3 2023 05 26 09 54.40 -08 41.2 7.304 7.370 89.8 7.9 19.3 2023 06 05 09 55.79 -07 46.3 7.407 7.312 80.7 7.9 19.3 2023 06 15 09 57.93 -06 58.3 7.507 7.254 71.8 7.6 19.3 2023 06 25 10 00.74 -06 17.0 7.600 7.197 63.1 7.2 19.3 2023 07 05 10 04.13 -05 42.4 7.682 7.140 54.5 6.7 19.3 2023 07 15 10 08.03 -05 14.0 7.749 7.082 46.1 5.9 19.2 2023 07 25 10 12.35 -04 51.3 7.798 7.025 38.0 5.1 19.2 2023 08 04 10 17.01 -04 33.9 7.828 6.968 30.1 4.2 19.2 2023 10 23 10 56.77 -03 58.1 7.212 6.518 42.9 6.0 18.8 2023 11 02 11 00.76 -03 53.8 7.031 6.463 51.7 6.9 18.7 2023 11 12 11 04.23 -03 46.0 6.833 6.408 60.8 7.7 18.6 2023 11 22 11 07.03 -03 33.5 6.621 6.353 70.1 8.4 18.5 2023 12 02 11 09.07 -03 15.0 6.399 6.299 79.7 8.9 18.4 2023 12 12 11 10.24 -02 49.0 6.171 6.244 89.7 9.1 18.3 2023 12 22 11 10.41 -02 14.1 5.943 6.190 100.0 9.0 18.2 2024 01 01 11 09.50 -01 29.1 5.721 6.136 110.6 8.6 18.1 2024 01 11 11 07.45 -00 33.0 5.510 6.083 121.6 7.9 18.0 2024 01 21 11 04.25 +00 34.9 5.317 6.030 132.9 6.9 17.9 2024 01 31 10 59.95 +01 54.4 5.147 5.977 144.6 5.5 17.8 2024 02 10 10 54.66 +03 24.5 5.007 5.925 156.5 3.8 17.7 2024 02 20 10 48.59 +05 03.2 4.901 5.873 168.7 1.9 17.6 2024 03 01 10 42.05 +06 47.6 4.831 5.821 178.3 0.3 17.5 2024 03 11 10 35.34 +08 34.2 4.799 5.770 166.7 2.3 17.5 2024 03 21 10 28.86 +10 19.3 4.804 5.720 154.6 4.3 17.5 2024 03 31 10 22.92 +11 59.4 4.843 5.669 142.7 6.1 17.5 2024 04 10 10 17.83 +13 32.0 4.910 5.620 131.1 7.7 17.5 2024 04 20 10 13.80 +14 55.3 5.002 5.570 119.9 9.0 17.5 2024 04 30 10 10.96 +16 08.5 5.110 5.522 109.1 9.9 17.5 2024 05 10 10 09.38 +17 11.6 5.230 5.474 98.7 10.5 17.5 2024 05 20 10 09.04 +18 05.1 5.354 5.426 88.7 10.7 17.5 2024 05 30 10 09.90 +18 49.8 5.478 5.379 79.1 10.7 17.5 2024 06 09 10 11.88 +19 26.8 5.597 5.333 69.8 10.3 17.6 2024 06 19 10 14.88 +19 57.2 5.706 5.287 60.9 9.7 17.6 2024 06 29 10 18.78 +20 22.2 5.801 5.242 52.3 8.8 17.6 2024 07 09 10 23.51 +20 42.8 5.880 5.197 44.0 7.8 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 1 (CBET 5274) Daniel W. E. Green