Electronic Telegram No. 3564
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 133P/ELST-PIZARRO
     H. H. Hsieh, University of Hawaii, reports that r'-band observations
taken on June 4 (total exposure time 1800 sec; queue observer M. Pohlen)
and June 14 (total exposure time 1800 sec; queue observer A. Matulonis) with
the 8.1-m Gemini North Observatory, and R-band observations (total exposure
time 360 sec) taken on June 12 with the 10-m Keck I telescope, show that comet
133P is currently active.  The object's nucleus appears point-source-like in
all composite images from each night with point-spread functions having FWHMs
of 1".1 on June 4, 0".8 on June 12, and 0".8 on June 14, identical to the
typical FWHMs of nearby stellar sources on those nights.  However, a thin
linear dust tail, similar in morphology to the dust tails observed in the
past for this object, extending as far as 50 arcsec from the nucleus in the
projected direction (p.a. about 250 degrees) of the anti-solar vector on the
sky (which also coincides with the projected direction of the negative
heliocentric-velocity vector on the sky) is visible in all images.
     During the period covered by these observations, the object ranged in
heliocentric distance from 2.69 to 2.70 AU, in geocentric distance from 2.47
to 2.35 AU, and in true anomaly from 28.2 to 30.5 degrees.  The predicted mean
R-band magnitude of the inactive nucleus over this period ranges from 20.8 to
20.7, based on photometric parameters measured by Hsieh, Jewitt, and Fernandez
(2009, Ap.J. 694, L111).  Photometric measurements (calibrated using SDSS
photometry of field stars) of the Gemini and Keck data indicate, however, that
the comet's nucleus has equivalent R-band magnitudes of 20.63 +/- 0.02 on June
4, 20.66 +/- 0.04 on June 12, and 20.58 +/- 0.02 on June 14.  The apparent
photometric excesses of 0.1-0.2 mag implied by these measurements are within
the expected range of rotational variation (+/- 0.3 mag) of the nucleus, but
are also consistent with photometric excesses measured for the object during
its active period in 2002 (Hsieh et al. 2010, MNRAS 403, 363).
    Including previously observed active apparitions in 1996, 2002, and 2007,
these observations now mark the fourth time that 133 has been seen to be
active.  The fact that 133P exhibits regularly repeated periods of activity,
interspersed with periods of quiescence, is a strong indicator that its
activity is likely driven by the sublimation of volatile material and is
unlikely to be due to other mechanisms such as impact-driven dust ejection or
electrostatic dust levitation -- consistent with its identification as an
ice-bearing main-belt comet.  The current observing window for 133P extends
until approximately January 2014, by which time it will have reached a true
anomaly of about 80 degrees.  Previous observations of 133P have shown it to
be active over a true anomaly range of -10 to +110 degrees -- and, as such,
observational monitoring of this object over the current observing window is
highly encouraged to characterize the evolution of its current activity for
comparison to previous active episodes.


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

                         (C) Copyright 2013 CBAT
2013 June 22                     (CBET 3564)              Daniel W. E. Green