Electronic Telegram No. 5087 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 A1 (LEONARD) J. Crovisier, N. Biver, and D. Bockelee-Morvan, LESIA, Observatoire de Paris, report: "While monitoring the 18-cm lines of OH in comet C/2021 A1 at the Nancay radio telescope, we observed a sudden increase of the line intensities starting on Dec. 13, followed by a sustained enhancement of activity. The 1667-MHz line area, which was around 460 mJy*km/s on Dec. 9-12, increased to 875 +/- 40 on Dec. 13.50 UT, to 2160 +/- 17 on Dec. 15.54, then dropped to 1160 +/-16 (Dec. 16.5), 830 +/- 20 (Dec. 17.57), 1140 +/- 23 (Dec. 18.58), and 1000 +/- 17 (Dec. 19.59). The corresponding modelled OH production rates are, respectively, 2.6, 6.3, 22, 10, 9, 18, and 27 x 10**28 molecules/s. (However, these rates should be taken with some caution since they are derived using a steady-state Haser model including collisional quenching, which might not be valid for such a transient phenomenon.)" E. Jehin, Y. Moulane, and J. Manfroid, STAR Institute, University of Liege, report that they obtained three sets of images with the cometary HB narrowband filters (Farnham et al. 2000, Icarus 147, 180) and with broad-band filters B, V, R_c, and I_c with the 0.6-m "TRAPPIST-South" telescope at La Silla, Chile, of comet C/2021 A1 at high airmass under photometric conditions when the comet was at r = 0.70 AU and Delta = 0.36 AU. Preliminary production rates at 10000 km using a Haser model with V_p = V_d = 1 km/s were computed as well as the dust-production-rate proxy A(0)f[rho], which was estimated by profile fitting at 10000 km (A'Hearn et al. 1984, AJ 89, 579) and corrected for the phase angle (Schleicher 2007, Icarus 191, 322). The three sets of measurements were in very good agreement, and the average value for each quantity is as follows for Dec. 19: Q(OH) = (9.31 +/- 0.33) x 10**28 s-1; Q(NH) = (5.90 +/- 0.20) x 10**26 s-1; Q(CN) = (2.21 +/- 0.10) x 10**26 s-1; Q(C_2) = (2.61 +/- 0.08) x 10**26 s-1; Q(C_3) = (2.60 +/- 0.05) x 10**25 s-1. From these measurements, C/2021 A1 has the composition of a typical comet with log[Q(C_2)/Q(CN))] = 0.07, and it has a typical dust/gas ratio with log[Afp/Q(OH)] = -25.98 (A'Hearn et al. 1995, Icarus 118, 223). The bright false nucleus is sharp and of size 5".5 (full-width-at-half-maximum) in 2".0 seeing in I and z' images at highest resolution (0".60/pixel), with no sign of a fragment or of disruption even in a 4-min-long integration in R band. Both CCD and visual observations of comet C/2021 A1 have noted an apparent outburst in brightness on Dec. 20 UT, after relatively few available magnitudes are available when the comet was closest to the sun on Dec. 14. Further to CBET 5077, the following selected CCD magnitudes and coma diameters have been reported: Dec. 3.83 UT, 6.8, 14'.2 (K. Yoshimoto, Yamaguchi, Japan, 0.08-m refractor + CMOS camera; tail over 2.3 deg long in p.a. 320 deg); 14.36, 3.7, 1'.5 (S. Yoneyama and K. Yoshimoto, Ogaki, Gifu, Japan, 200-mm f/2.8 lens + Canon EOS Kiss X6i camera); 17.69, 5.3, 1'.04 (T. Scarmato, Calabria, Italy, 25-cm reflector); 18.70, 4.3, 1'.04 (Scarmato). K. Kadota, Ageo, Japan, supplies the following total unfiltered CCD magnitudes: Dec. 15.36, 3.0; 16.36, 3.5; 17.36, 3.8; 18.36, 4.0; 19.37, 4.3; 20.37, 3.5; 21.37, 3.8. C. Hergenrother, Tucson, AZ, USA, reports the following CCD Cousins R magnitudes and coma diameters for the comet using a 7.2-cm f/5 refractor: Dec. 19.51 UT, 4.4, 7'.7; 20.05, 4.1, 7'.7; 21.07, 3.3, 11'.5. With a green (V) filter, Hergenrother got these values: Dec. 9.53, 5.4, 28'.9; 17.05, 4.3, 10'.6; 19.05, 3.7, 12'.5. Selected visual total-magnitude and coma-diameter estimates (an asterisk denoting correction for atmospheric extinction, usually using the ICQ tables): Nov. 26.85 UT, 8.1, 6' (K. Yoshimoto, Yamaguchi, Japan, 20x100 binoculars); 28.84, 7.4, 10' (Yoshimoto, 8x42 binoculars); 30.47, 7.1, 5' (C. Hergenrother, Tucson, AZ, U.S.A., 10x50 binoculars); Dec. 1.46, 7.0, 6' (Hergenrother); 2.48, 6.7, 8' (Hergenrother); 3.25, 6.4, 12' (J. J. Gonzalez, Leon, Spain, 10x50 binoculars; 2.4-deg tail in p.a. 310 deg); 4.50, 6.3, > 10' (K. Wierzchos, Mt. Lemmon, AZ, USA, 7x50 binoculars); 5.56, 5.8, 17' (C. S. Morris, Fillmore, CA, U.S.A., 10x50 binoculars); 6.52, 6.0, > 10' (Wierzchos); 8.56, 5.4, 16' (Morris); 9.50, 5.0, 12' (Hergenrother); 11.54, 4.8, 7' (Hergenrother); 15.75, 3.3, 7' (Gonzalez, Asturias, Spain); 16.72, 4.6:*, 4' (L. C. da Silva, Toulouse, France, 7x50 binoculars; altitude 5 degrees); 16.74, 4.1, 5' (C. Labordena, Castellon, Spain, 11x80 binoculars); 17.06, 4.1, 5' (Hergenrother); 17.75, 4.3, 5' (Gonzalez); 18.08, 4.2, 10' (Morris, Fillmore, CA, U.S.A., 16x70 binoculars; altitude about 8 deg); 18.47, 4.2, 5' (M. Mattiazzo, Swan Hill, Vic., Australia, 8x40 binoculars); 19.08, 3.9, 10' (Morris); 19.96, 4.6, 7' (M. Goiato, Aracatuba, Brazil, 7x50 binoculars; moonlight); 20.06, 3.5, 6' (Hergenrother); 20.38, 3.3, -- (J. Drummond, near Gisborne, New Zealand, naked eye; remarked as "quite a lot brighter than" the previous night); 20.47, 2.9*, -- (Mattiazzo, naked eye); 20.73, 2.5*, 4' (da Silva; altitude 5 deg); 20.93, 3.0*, 10' (W. Souza, Sao Paulo, Brazil, 15x70 binoculars); 20.96, 3.5*, - (A. Amorim, Florianopolis, Brazil, naked eye; clouds); 20.97, 2.8*, -- (Goiato, naked eye; "comet significantly brighter"; altitude about 8 deg); 21.07, 3.7*, 6' (Hergenrother); 21.35, 3.7:, 6' (Y. Nagai, Saitama, Japan, 20x100 binoculars); 21.96, 4.9, 10' (J. G. de S. Aguiar, Campinas, Brazil, 11x80 binoculars); 21.97, 4.0*, -- (Goiato). NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2021 CBAT 2021 December 22 (CBET 5087) Daniel W. E. Green