Electronic Telegram No. 4397
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/2017 K4 (ATLAS)
     Larry Denneau, University of Hawaii, on behalf of the "Asteroid
Terrestrial-impact Last Alert System" (ATLAS) Team, reports the discovery of
a comet on CCD exposures taken with the ATLAS 0.5-m f/2.0 Schmidt telescope
at Haleakala (discovery observations tabulated below), noting that John
Tonry and Aren Heinze found a modest 4" coma and a tail to the northwest.

     2017 UT             R.A. (2000) Decl.       Mag.
     May  26.51169   17 12 16.32   -39 08 52.2   17.0
          26.52853   17 12 15.69   -39 08 50.1   17.0
          26.56167   17 12 14.48   -39 08 46.6   17.0
          26.57849   17 12 13.86   -39 08 44.9   17.0

     After the comet was posted on the Minor Planet Center's NEOCP and PCCP
webpages, other CCD astrometrists also have commented on the cometary
appearance.  Three 40-s gri-filtered exposures taken by R. J. Wainscoat and
D. Woodworth with the 3.6-m Canada-France-Hawaii Telescope at Mauna Kea on
May 27.5 UT show that the object is clearly cometary, with a coma having FWHM
1".2 in 0".6 seeing; there is also a broad tail extending about 20" long to
the west-northwest (measured by M. Micheli and Wainscoat).  G-band exposures
taken by W. H. Ryan and E. V. Ryan with the Magdalena Ridge Observatory 2.4-m
f/8.9 reflector on May 28.3 show a distinct coma of magnitude 16.1-16.2 and
a short tail toward p.a. about 270 degrees.  H. Sato, Tokyo, Japan, writes
that six stacked 60-s exposures taken on May 28.5 with an iTelescope 0.43-m
f/6.8 astrograph (+ luminance filter) at Siding Spring show a strongly
condensed, disk-like coma 10" in diameter and no tail; the w-band magnitude
was 15.8 as measured within a circular aperture of radius 5".2.  E. Guido,
Castellammare di Stabia, Italy, notes that ten stacked 120-s unfiltered
exposures taken remotely on May 29.4 with an iTelescope 0.43-m f/6.8
astrograph at Siding Spring show a diffuse coma about 10" in diameter; the
red magnitude was given as 16.4-17.0.  Six stacked images obtained by S.
Foglia, L. Buzzi, P. Concari, C. Cremaschini, G. Galli, and M. Tombelli
with an iTelescope 0.18-m f/2.8 Newtonian reflector at Siding Spring on
May 29.4 show a soft appearance with a coma 15" in diameter and a 1' tail
in p.a. 180 deg.
     The available astrometry, the following preliminary parabolic orbital
elements by G. V. Williams (from 105 observations spanning May 26-June 1),
and an ephemeris appear on MPEC 2017-L04.

     T = 2018 Jan. 13.3271 TT         Peri. =  20.4242
                                      Node  = 297.6505  2000.0
     q = 2.540595 AU                  Incl. =  17.1907


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

                         (C) Copyright 2017 CBAT
2017 June 1                      (CBET 4397)              Daniel W. E. Green