[Iaude] CBET 5087: COMET C/2021 A1

quai at eps.harvard.edu quai at eps.harvard.edu
Wed Dec 22 13:34:13 EST 2021


                                                  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 at eps.harvard.edu (alternate cbat at 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



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