Electronic Telegram No. 4074
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 D1 (SOHO)
     Z. Sekanina, Jet Propulsion Laboratory, reports that cursory dynamical
examination of the comet's image, taken with an 0.15-m f/2.8 refractor in
Spain on Feb. 27.807 UT and described by M. Masek et al. (CBET 4073; for the
image, cf. website URL http://gloria-project.eu/2015/03/comet-c2015-d1/),
suggests that the narrow headless tail was composed of large dust grains,
released from the comet during a period of time centered on 1 hr after
perihelion and subjected to the radiation pressure accelerations of less than
0.005 the sun's gravitational acceleration.  Such particles are typically
larger than 0.5 mm in diameter.  The peak velocity of the released dust,
estimated from the width of the tail, was about 65 m/s, derived on an
unlikely assumption that the event was an outburst of extremely short
duration.  The view at the time of this observation was nearly from the
comet's orbital pole, so we see the dust scattered in the orbit plane.
     This dust is related to the flare-up that began 0.6 hr before perihelion
and peaked 3.2 hr after perihelion (MPEC 2015-D73); if large particles were
continuously released during this period of time, the debris would be
distributed between the position angles of 85 and 78 deg, consistent with the
reported range of tail orientation.  There is no evidence for dust related to
the subsequent flare-up that started about 10 hr after perihelion.  This
debris would appear at position angles of less than 70 deg.  If dust
particles from this event were primarily micron-sized and/or smaller, they
would be scattered at the observation time many degrees away and could no
longer be detected.  No significant amounts of millimeter-sized and larger
grains were apparently released during this event.


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      superseded by text appearing later in the printed IAU Circulars.

                         (C) Copyright 2015 CBAT
2015 March 7                     (CBET 4074)              Daniel W. E. Green