[IAUC] CBET 2512: 20101021 : COMET 103P/HARTLEY

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                                                  Electronic Telegram No. 2512
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 en eps.harvard.edu (alternate cbat en iau.org)
URL http://www.cbat.eps.harvard.edu/index.html
Prepared using the Tamkin Foundation Computer Network


COMET 103P/HARTLEY
     M. F. A'Hearn and L. M. Feaga, on behalf of the DIXI/EPOXI science team,
report that routine sampling with the Deep Impact Medium Resolution Imager
(MRI) enabled the discovery of an unusual cometary phenomenon on comet 103P
(sometimes known by its old name, "Hartley 2") that may recur.  Between Sept.
9 and 17, the outgassing of CN, as acquired with the CN filter (sensitive to
both the dust continuum and CN gas), increased by a factor of five and then
slowly decreased, returning to its prior trend line by Sept. 24.  There was
no apparent change in the reflected continuum, acquired with a clear filter,
other than a small, gradual increase consistent with the increases before
and after this period in both CN and dust due to the decreasing range from
the spacecraft to the comet and the increasing activity of the nucleus as it
approaches the sun.  This long-duration, gradual increase and decrease of
gaseous emission without any increase in the dust is very unlike typical
cometary outbursts, which have sudden onsets and are usually accompanied by
considerable dust.  It is dissimilar to the activity observed at comet 9P
(old-style name "Tempel 1") and not apparently associated with the dust-free
CN jets observed in this and other comets.  Thus it is not like anything
that the authors are aware of in any other comet.  Observers should be aware
of this type of activity when planning observations and interpreting their
data.  In addition, H_2O and CO_2 have been unambiguously detected in the
coma using the High Resolution Instrument infrared spectrometer (HRI-IR) on
Oct. 16.

     N. H. Samarasinha and B. E. A. Mueller, Planetary Science Institute; and
M. F. A'Hearn and T. L. Farnham, University of Maryland, obtained CN
narrowband images at the Kitt Peak National Observatory 2.1-m telescope from
Sept. 30 to Oct. 4 UT.  The enhanced images clearly indicate the rotation of
a jet feature centered around a position angle near 30 deg.  This is
presumably the same feature reported by Knight et al. (IAUC 9163).  This
feature moves from the northwest towards the earth's direction, almost towards
the line of sight, and then moves to the east-northeast and finally to the
far side before starting the cycle again; while on the far side from the
earth, its level of activity decreases significantly before starting to
increase again.  The repeatability of the CN morphology is consistent with a
periodicity around 17.6 hours.  Small differences in the morphology during
some cycles suggest that there may be a slight rotational excitation, probably
a low-excitation short-axis mode.  The CN coma morphology is compatible with
a nucleus having a high obliquity and a retrograde rotation and suggests a
low-to-mid-latitude active region in the negative hemisphere.  A preliminary
estimate for the rotational angular momentum vector is R.A. = 345 deg, Decl.
= -15 deg, and the uncertainty is as large as 20 deg in some directions.  The
continuum images from the same observing run show the dust tail, but no clear
jet features are discernible.

     M. Knight and D. Schleicher, Lowell Observatory, report new narrowband
imaging of comet 103P using the Hall 1.1-m telescope at Lowell Observatory.
Further to Knight et al. (IAUC 9163), the CN gas feature was centered at
a position angle near 355 deg on 2010 Aug. 13-17, near 350 deg on Sept. 9-13,
and near 350 deg on Oct. 16, 17, and 19.  Numerical modeling of these position
angles and the sense of rotation yields a rotation axis having an obliquity of
about 15 deg in the comet's orbital frame, corresponding to R.A. = 310 deg,
Decl. = +80 deg, assuming principal axis rotation.  Preliminary modeling
suggests that the CN jet originates at a latitude of +50 to +60 deg.  An
additional, fainter CN feature was seen towards the southeast in the October
1.1-m-telescope images and in additional images obtained Oct. 12-14 on the
0.8-m telescope at Lowell Observatory.  For this pole solution, the comet's
maximum sub-Earth latitude is attained in early October, resulting in an
overlap of the two CN features towards the east, as was observed in the Oct.
12-14 images and possibly explaining the morphology described by Samarasinha
et al. (above).  With this pole solution, the sub-earth latitude should be
near the comet's equator at the time of the EPOXI spacecraft encounter,
yielding side-on corkscrews.


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

                         (C) Copyright 2010 CBAT
2010 October 21                  (CBET 2512)              Daniel W. E. Green