Paper detail

Sloan-r' photometry of Comet 17P/Holmes beyond 3.8 AU: An observing methodology for short-period comets far from perihelion

A photometric method is described for accurately quantifying the brightness of short-period comets far from perihelion. The method utilizes the Sloan Digital Sky Survey Catalog (Data Release 7) as a homogeneous source of reference star magnitudes. Results are based on SDSS-r' filtered images taken using 2.0-m aperture telescopes for which the exposure time was adjusted to achieve a constant motion-blur of 2.0 pixels (0.56 arcsec) on the CCD chip. Aperture photometry using circular and tilted elliptical apertures was performed on images, which were stacked to increase signal to noise. Magnitude dependence on 'seeing' was determined, and this calibration was used to normalize photometry to constant seeing thereby maximizing photometric accuracy. From observations of comet 17P/Holmes between 2008 October and 2009 March, a very significant outburst of 17P was found to have occurred on 2009 Jan 4.7 (\pm0.5 day). Night-to-night measurements of the brightness of the inner coma (3000-km radius) exhibited a scatter of only 0.015-0.019 mag. No short time-scale (<36 hr) periodicity was found in the fading lightcurve. From literature data, it was estimated that reflected light from the nucleus contributed 7-11% of the signal within the inner coma and it is concluded that either the nucleus of 17P must be relatively spherical (projected axial ratio of <1.25), or, if its shape is more typical of other comet nuclei, it has a rotational period in excess of 10 days (assuming the observations were not made with the nucleus 'pole-on' to the Earth). Evidence from intermittent activity displayed by the nucleus is indicative of a possible 44-day rotation period.