Electronic Telegram No. 4104
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
CBAT Director:  Daniel W. E. Green; 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/2015 G2 (MASTER)
[Editor's note:  This text replaces that on CBET 4092.]
     Vladimir M. Lipunov, Sternberg Astronomical Institute, Lomonosov Moscow
State University; David Buckley, South African Astronomical Observatory
(SAAO); and Denis Denisenko, Sternberg Astronomical Institute, Moscow State
University, report the discovery of a comet on 60-s unfiltered survey images
taken with the "Mobile Astronomical System of the Telescope-Robots" (MASTER)
auto-detection system (double 0.40-m f/2.5 reflector) at the SAAO (discovery
observations tabulated below).  Denisenko first noticed the object as being
cometary, with a coma diameter of about 1'.5, and elongated toward the west,
on R-band images taken by P. Balanutsa et al.

     2015 UT             R.A. (2000) Decl.       Mag.
     Apr.  7.12551   21 56 37.54   -15 36 56.3   10.7
           7.13007   21 56 38.03   -15 37 00.7   10.6
           7.13464   21 56 38.56   -15 37 05.6   10.7

After the comet was posted on the Minor Planet Center's PCCP webpage, other
CCD astrometrists have commented on the object's cometary apparance.  E. Guido
and N. Howes (remotely using an iTelescope 0.50-m f/6.8 astrograph at Siding
Spring; Apr. 8.77 UT) write that ten stacked 30-s unfiltered exposures show a
very bright coma of red mag 13.7 that is nearly 3' in diameter with a tail
about 15' long in p.a. 253 deg.  H. Sato, Tokyo, Japan, notes that sixteen
stacked 10-s exposures taken with an iTelescope 0.51-m f/6.8 astrograph (+
luminance filter) at Siding Spring on Apr. 8.78 show the object to be strongly
condensed with an elongated coma of size 2'.9 and a tail longer than 15'
toward p.a. 252 degrees; the w-band magnitude was 10.3, as measured within a
circular aperture of radius 87".6.  C. Jacques (Belo Horizonte, MG, Brazil)
reports that six stacked CCD images obtained on Apr. 10.34 by E. Pimentel, J.
Barros, and himself with a 0.28-m f/2.2 astrograph at the SONEAR Observatory,
Oliveira, show a coma 2'35" in diameter with a strong central condensation
and a tail 2'40" long in p.a. 252 deg.
     R. Kaufman (Bright, Victoria, Australia) reports that four pre-discovery
images taken on Mar. 30.802 UT with a Canon 650D camera (+ 200-mm-f.l. lens)
in fairly poor sky conditions (and subject to lens aberrations around the
image edges) shows a bright, diffuse greenish object visible on the bottom
edge of the frame in the predicted position of this comet; M. Mattiazzo (Swan
Hill, Victoria, Australia) provided astrometric measures for the comet on
Kaufman's images, yielding magnitude 11.4-11.9.  Mattiazzo adds that he
obtained a 5-min CCD exposure of the comet remotely with an iTelescope 0.50-m
f/6.8 astrograph (field-of-view 30' wide) at Siding Spring on Apr. 8.80 that
shows a coma of diameter 2'.5 and an ion tail at least 15' long in p.a. 250
degrees.
     Visual total-magnitude and coma-diameter estimates:  Apr. 8.80 UT, 9.7,
2'.5 (Mattiazzo, 20-cm reflector; moonlight; comet appears moderately
condensed and is more condensed but slightly smaller and fainter than comet
88P, which lies 2.5 deg to the northeast); 9.34, 10.0, 1'.5 (A. Amorim,
Florianopolis, Brazil, 0.18-m reflector; moonlight); 9.48, 10.2:, 1'.5 (A.
Hale, Cloudcroft, NM, USA, 0.41-m reflector; low altitude; bright moonlight,
twilight).
     The available astrometry (including pre-discovery MASTER observations
from Mar. 30.1 UT, showing the comet at mag 11.5-11.6), the following
preliminary parabolic orbital elements by G. V. Williams, and an ephemeris
appear on MPEC 2015-G28.

     T = 2015 May  23.8022 TT         Peri. = 257.4779
                                      Node  = 110.0566  2000.0
     q = 0.779770 AU                  Incl. = 147.5512


NOTE: These 'Central Bureau Electronic Telegrams' are sometimes
      superseded by text appearing later in the printed IAU Circulars.

                         (C) Copyright 2015 CBAT
2015 May 23                      (CBET 4104)              Daniel W. E. Green