Electronic Telegram No. 4307
Central Bureau for Astronomical Telegrams
Mailing address:  Hoffman Lab 209; Harvard University;
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Prepared using the Tamkin Foundation Computer Network


COMET 238P/READ
     H. H. Hsieh, Planetary Science Institute and Academia Sinica Institute of
Astronomy and Astrophysics, reports that r'-band observations taken on July
8 with the 3.6-m Canada-France-Hawaii Telescope (five 180-s exposures; queue
observer P. Forshay) and on August 6 with the 8.1-m "Gemini North" telescope
(five 180-s exposures; queue observer M. Schwamb) show that comet 238P (cf.
IAUCs 8624, 9158, 9164, 9183), which has been previously identified as a
main-asteroid-belt comet, is currently active.  The object appears slightly
more diffuse than nearby stellar sources, with point-spread functions having
FWHMs of 0".75 on July 8 (compared to nearby stellar sources with FWHMs of
0".70) and 0".83 on Aug. 6 (compared to nearby stellar sources with FWHMs of
0".73).  In the Gemini data, the object exhibits a clearly visible tapering
tail extending at least 20" from the nucleus at p.a. about 245 degrees,
roughly coinciding with the projected directions on the sky of both the
anti-solar vector and the object's negative heliocentric velocity vector
(i.e., the plane of its orbit).  During the period covered by these
observations, comet 238P ranged in heliocentric distance from 2.44 to 2.41
AU, and in true anomaly from 328 to 337 degrees.  The predicted mean R-band
magnitude of the inactive nucleus over this period ranges from 24.0 to 23.5,
based on photometric parameters measured by Hsieh et al. (2011, Ap.J. 736,
L18).  Photometric measurements (calibrated using SDSS photometry of field
stars) indicate, however, that the comet's nuclear condensation has average
equivalent R-band magnitudes of 22.5 on July 8 and 21.7 on Aug. 6.  The
apparent photometric excesses of 1.5-1.8 magnitudes implied by these
measurements are well beyond the expected range of possible rotational
variations, and therefore likely indicate the presence of a substantial dust
coma during these observations.
     Including previously observed active apparitions in 2005 and 2010-2011
(IAUC 8624, 9183), these observations mark the third time that 238P has been
seen to be active.  The fact that 238P exhibits regularly repeated periods of
activity, interspersed with periods of quiescence, is a strong indicator that
its activity is driven by the sublimation of volatile material, consistent
with its identification as an ice-bearing main-asteroid-belt comet (similar
to other recurrently active main-asteroid-belt objects such as comets
133P/Elst-Pizarro, 313P/Gibbs, and 324P/La Sagra, which have also been
identified as likely icy main-asteroid-belt comets).
     The current observing window for 238P extends until approximately Feb.
2017, by which time it will have reached a true anomaly of about 40 degrees.
The comet's next available observing window extends from approximately
Sept. 2017 to May 2018, over which it will cover a true anomaly range of
about 85 to 130 degrees.  Main-asteroid-belt comets 133P and 324P have been
observed to exhibit activity at true anomalies as large as 120 degrees (cf.
Hsieh and Sheppard 2015, MNRAS 454, L81), and so 238P may likewise still
exhibit activity during the latter observing window.  Observational
monitoring during both the current and next upcoming observing windows is
highly encouraged to characterize the evolution of 238P's current active
episode for comparison to its previous active episodes and to other
main-asteroid-belt comets.


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

                         (C) Copyright 2016 CBAT
2016 August 25                   (CBET 4307)              Daniel W. E. Green