Electronic Telegram No. 5430 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 JN_16 (LEMMON) An apparently asteroidal object discovered on CCD images taken on 2023 May 10 with the Mt. Lemmon Survey's 1.5-m reflector (discovery observations tabulated below) has been found to show cometary appearance by observers elsewhere. The object was assigned the minor-planet designation 2023 JN_16 when announced on MPS 1890142 (dated 2023 June 15). 2023 UT R.A. (2000) Decl. Mag. May 10.23253 14 12 07.70 -16 07 04.2 19.6 10.23942 14 12 07.33 -16 07 02.5 19.2 10.24515 14 12 07.00 -16 07 01.1 19.1 10.25088 14 12 06.71 -16 07 00.3 K. Ly (El Monte, CA, USA) and Arndt Schnabel (Leipzig, Germany) found a fuzzy 2"- to 3"-long extension protruding in p.a. 295-300 degrees from 2023 JN_16 in twenty-one unfiltered 600-s exposures taken on 2023 June 15.96-16.11 UT with Schnabel's 0.43-m reflector located at Rio Hurtado, Coquimbo, Chile; comparing the object's stacked point-spread function to those of stacked nearby background stars, 2023 JN_16 then appeared to be slightly extended with a size of 1".3-1".5 (full-width-at-half-maximum), compared to the stars' sizes of 1".0-1".2 (FWHM) in the best-seeing frames. The mean apparent magnitude of 2023 JN_16 then was V = 20.4. Earlier observations by Schnabel of 2023 JN_16 on 2023 June 3, 5, and 15 (following a request by S. Deen of Simi Valley, CA, USA, who with the others noted a comment by P. VanWylen via the "Minor Planets Mailing List" on June 1 that it then showed an unusually bright absolute magnitude) show no conclusive evidence of activity. Additional stacks of 600-s exposures by Schnabel on following nights are less clear regarding cometary activity but seem to suggest a slight elongation of about 1"-2" toward p.a. 310-340 deg on 2023 June 17.1 and a slightly fuzzier head than the background stars on June 18.1. Ly and Deen performed the astrometry and photometry from Schnabel's June 2023 images. Ly adds that eighteen additional co-added 600-s images taken by Schnabel and Deen on 2023 July 15.03-15.16 show a very condensed coma 6" in diameter with no tail (V = 20.7-20.8). Ly further adds that a search of long-exposure (> 200 s) archival images from the OmegaCAM wide-field imager on the 2.6-m VLT Survey Telescope at Paranal, Chile, and from the DECam imager on the 4-m reflector at Cerro Tololo show nothing at the predicted positon of 2023 JN_16 down to mag 23. A pair of VR-band exposures (480-500 s) from DECam on 2018 Apr. 12-13 show no signs of the object down to magnitude 24.5 within its 90" (1-sigma positional uncertainty) region; the object's predicted magnitude at that time, based on the 2023 observations, would have been 20.3 -- a further indication of cometary activity in 2023. Following an alert by P. VanWylen (Memphis, TN, USA) and K. Ly that 2023 JN_16 may be a newly activated main-belt asteroid, Q.-z. Ye and M. S. P. Kelley (University of Maryland, MD) and D. Bodewits (Auburn University), on behalf of the Zwicky Transient Facility (ZTF) Partnership, examined the 2023 ZTF images for pre-discovery detections. The earliest detection in ZTF data is on 2023 Apr. 29.3 UT at magnitude r = 19.1 (PS1 photometric system). However, a single r-band image taken on 2023 Apr. 27.3 showed nothing at the predicted position of 2023 JN_16 to a limiting magnitude of 20.5 (5-sigma). Similarly, the object was not detected on any ZTF images taken before 2023 Apr. 27. Additional detections were made up to 2023 June 25, after which the object faded below the detection limit (as of their reporting on 2023 Aug. 28). The intrinsic brightness of the object remained constant since the first detection (i.e., no unusual brightening or fading of over 0.3 mag was seen), and the object appeared point-source-like in ZTF data and did not develop any cometary features (a coma and/or a tail). It can be concluded that something impulsive happened between 2023 Apr. 27 and 29, causing the object to brighten by at least 1.4 mag. Ye adds that follow-up observations on 2023 Sept. 5.11 using the 4.3-m Lowell Discovery Telescope revealed a faint tail 15" long in p.a. 295 deg. Deen recently reported that additional CMOS images taken (as above) by Schnabel and D. Ruhela on 2024 June 29.15-29.23 UT show this object to have an odd appearance -- "obviously highly diffuse" with no apparent condensation (magnitude V = 22.0) and with a tail at least 60" long. Deen has further identified archival images taken with the 8.3-m Subaru Telescope at Mauna Kea on 2018 Mar. 18.5 showing a stellar object of mag i = 24.7 at the predicted position. Deen's astrometry and all other available astrometry appear on MPEC 2024-Q04. The following orbital elements by S. Nakano (Central Bureau) are from 74 observations spanning 2018 Mar. 17-2024 June 29 (mean residual 0".3). There are no close approaches to any major planets, and Nakano was unable to identify any other observations in minor-planet astrometry archives. Epoch = 1998 May 27.0 TT T = 1998 June 4.79541 TT Peri. = 351.83289 e = 0.1500137 Node = 12.19323 2000.0 q = 2.2914980 AU Incl. = 3.72091 a = 2.6959236 AU n = 0.22266026 P = 4.43 years Epoch = 2002 Nov. 22.0 TT T = 2002 Nov. 7.11016 TT Peri. = 351.83691 e = 0.1483719 Node = 12.17831 2000.0 q = 2.2956083 AU Incl. = 3.71934 a = 2.6955526 AU n = 0.22270622 P = 4.43 years Epoch = 2007 Apr. 10.0 TT T = 2007 Apr. 18.55379 TT Peri. = 353.09629 e = 0.1478681 Node = 12.13544 2000.0 q = 2.2970622 AU Incl. = 3.71359 a = 2.6956649 AU n = 0.22269231 P = 4.43 years Epoch = 2011 Oct. 6.0 TT T = 2011 Sept.21.77645 TT Peri. = 353.23728 e = 0.1494104 Node = 12.12007 2000.0 q = 2.2931979 AU Incl. = 3.71337 a = 2.6960099 AU n = 0.22264957 P = 4.43 years Epoch = 2016 Feb. 22.0 TT T = 2016 Feb. 20.43259 TT Peri. = 352.54407 e = 0.1494644 Node = 12.04213 2000.0 q = 2.2923678 AU Incl. = 3.70988 a = 2.6952051 AU n = 0.22274930 P = 4.42 years Epoch = 2020 July 10.0 TT T = 2020 July 25.06877 TT Peri. = 352.62469 e = 0.1467868 Node = 11.89853 2000.0 q = 2.2997810 AU Incl. = 3.70315 a = 2.6954354 AU n = 0.22272075 P = 4.42 years Epoch = 2025 Jan. 5.0 TT T = 2024 Dec. 30.61037 TT Peri. = 353.15579 e = 0.1468015 Node = 11.87587 2000.0 q = 2.2996906 AU Incl. = 3.70261 a = 2.6953759 AU n = 0.22272813 P = 4.42 years Epoch = 2029 May 24.0 TT T = 2029 June 4.88444 TT Peri. = 353.37274 e = 0.1497106 Node = 11.72816 2000.0 q = 2.2915737 AU Incl. = 3.70576 a = 2.6950515 AU n = 0.22276835 P = 4.42 years Epoch = 2033 Nov. 19.0 TT T = 2033 Nov. 3.46999 TT Peri. = 352.67705 e = 0.1495524 Node = 11.66296 2000.0 q = 2.2923383 AU Incl. = 3.70655 a = 2.6954490 AU n = 0.22271906 P = 4.42 years NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2024 CBAT 2024 August 17 (CBET 5430) Daniel W. E. Green