Electronic Telegram No. 4388
Central Bureau for Astronomical Telegrams
Mailing address:  Hoffman Lab 209; Harvard University;
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Prepared using the Tamkin Foundation Computer Network


COMET 259P/GARRADD
     H. H. Hsieh, Planetary Science Institute and Academia Sinica Institute
of Astronomy and Astrophysics, reports that r'-band observations taken on
Apr. 26 and 29 with the Gemini South Observatory (seven 100-s exposures per
night; queue observer J. Chavez) show that comet 259P (cf. IAUC 8969; CBET
3115), which has been previously identified as an active minor planet and
main-asteroid-belt comet candidate, is currently active.  The object has
similar point-spread functions with respect to nearby stellar sources (as
measured in the direction perpendicular to the non-sidereal motion of the
object), with both the object and nearby stars having FWHM = 0".9 on Apr. 26
and 0".65 on Apr. 29.  In the Apr. 29 data, the object exhibits a faint,
tapering tail extending approximately 2" from the nucleus at p.a. about 270
degrees, roughly coinciding with the projected direction in the sky of the
anti-solar vector (p.a. 266 degrees).  During the period covered by these
observations, comet 259P ranged in heliocentric distance from 1.95 to 1.94
AU, and in true anomaly from 315 to 317 degrees.  The predicted mean R-band
magnitude of the inactive nucleus over this period is 24.0, based on
photometric parameters measured by MacLennan and Hsieh (2012, Ap.J. 758, L3).
Photometric measurements (calibrated using Pan-STARRS1 photometry of field
stars) indicate, however, that the comet's nuclear condensation has average
equivalent R-band magnitudes of 23.1 +/- 0.1 on Apr. 26 and 22.9 +/- 0.1 on
Apr. 29.  The apparent photometric excesses of about 1.0 magnitude implied by
these measurements are well beyond the expected range of possible rotational
variations and therefore indicate the likely presence of an unresolved dust
coma during these observations.
     Including its previously observed active apparition in 2008 (IAUC 8969),
these observations mark the second time that comet 259P has been seen to be
active.  The fact that 259P 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 initial identification as an ice-bearing main-asteroid-belt comet.
     The current observing window for 259P extends until approximately Dec.
2017, by which time it will have reached a true anomaly of about 60 degrees,
passing perihelion on Aug. 4.  However, it will only be easily accessible by
telescopes in the southern hemisphere for much of this time.  When the object
was first discovered to be active in 2008, it was monitored over a true
anomaly range of about 30-50 degrees by Jewitt et al. (2009, A.J. 137, 4313).
Observational monitoring during the current observing window, particularly by
telescopes in the southern hemisphere, is highly encouraged to characterize
the evolution of 259P's current active episode for comparison to its previous
active episode 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 2017 CBAT
2017 May 2                       (CBET 4388)              Daniel W. E. Green