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Introduction

The determination of comet brightness has been one of the important goals of comet observers for the last century or so. The challenge of making reliable estimates has been enormous since one is dealing with extended sources in motion with respect to background stars. Most estimates of comet magnitudes have been done by visual or photographic methods. However, CCD observations have become increasingly common in recent years.

Most of the determined comet magnitudes have referred classically to the magnitude of the gaseous coma surrounding the comet nucleus. These have been known as ''total'' magnitudes. Max Beyer was one of the pioneers in trying to determine visually magnitudes of the comet nucleus during the 1930's and -40's. His nuclear magnitudes were grossly underestimated (i.e. the nucleus brightness overestimated) due to coma contamination. The following most serious attempt at determining a homogeneous data set of nuclear magnitudes was carried out by Elizabeth Roemer during a time span of more than 25 years (from 1950 to the late seventies). Roemer used photographic plates taken with the 1-m Ritchey-Chrétien reflector at the Flagstaff Station of the U.S. Naval Observatory (see, for instance, Roemer [1976]).

Near the end of the ``Roemer era'' there was still serious doubts that a comet nucleus would have been resolved in any case (e.g. Sekanina [1976]), so most researchers tended to regard ``nuclear'' magnitudes as the magnitude of the solid nucleus plus an inner coma. During the 1980's, comet 1P/Halley of course became the main target of cometary research and the great opportunity to observe for the first time a bare nucleus by means of spacecraft fly-by. This goal was successfully accomplished by the Giotto and Vega missions. The 1P/Halley nucleus turned out to be an elongated body of 14.2 km x 8.2 km x 7.5 km of very low albedo and with an active surface area no greater than about 15% of the total (Keller et al. [1987]). The knowledge of the nucleus size allowed for the first time a direct comparison between comet size and the earlier estimates of the nuclear magnitude based on ground-based CCD observations of 1P/Halley at distances greater than 8 AU. The nucleus size derived by Jewitt & Danielson ([1984]) from these distant observations turned out to be a factor of two smaller than the size derived by the space missions. In any case, the new technology of CCD cameras attached to large telescopes proved to be very promising at observing distant comets - where they are presumably little active or inactive - with the scope of deriving nuclear magnitudes and sizes.

CCD photometry of comets became of widespread use in the post-Halley era, with observers like David Jewitt, Karen Meech, Tom Gehrels and James Scotti among the pioneers in the use of this new technology for the study of comets. The much higher sensitivity of CCD detectors and the use of medium-sized to large telescopes allowed the observation of a large number of comets beyond 3 AU and the early recovery of short-period comets when they had little activity or no activity at all. Of particular relevance is the work of James Scotti with the 91-cm Spacewatch telescope. Scotti has not only observed distant comets systematically, contributing to the early recovery of a large fraction of the short-period comets, but due to the high-resolution surface photometry capacity of its CCD exposures, he has also introduced a method of coma subtraction to derive an improved magnitude of the nucleus (see Sect. 3.2).

The latest improvement on the quality of the observations has been achieved with the data taken by the Hubble Space Telescope, obtained under the leadership of Philippe Lamy. The outstanding image quality obtained by the HST allows the application of a more refined coma subtraction technique even in highly active comets (Lamy & Toth [1995] and later references, see below).

In summary, the last decade of the century has seen a growing activity in photometric observations of distant comets. However, overall only a wealth of photometric data has been produced without detailed analysis of its physical meaning. There are just a few exceptions for some particular comets. We now deem that the time is ripe to undertake a broad analysis of the observed nuclear magnitudes. In the catalog we have included data for all the periodic comets, indistinticly of the dynamical class. Nevertheless we think that in an analysis of the data we have to separate Jupiter family comets from Halley-type or Centaurs comets. There are mainly two reason for such distinction: (1) a large fraction of the JF population has been extensively observed photometrically, whereas long-period comets and Halley-type comets show only scattered data; and (2) we would like to analyze a homogeneous population, presumably coming from the same source region (in this case the Edgeworth-Kuiper belt).

The present catalog is a continuation of a project started several years ago (see Fernández et al. [1992]), that includes our own observational program (Licandro et al. [1999a]). A theoretical analysis based on the information described here is presented separately (Fernández et al. [1999]). A previous version of the catalog was published in 2000 (Tancredi et al. [2000]), including data till 1999. In the present version we update the data base including observations till January 2003.

An analysis of the distribution of nuclear magnitudes of JF comets is presented in the following article: Tancredi et al.([2005], "Nuclear Magnitudes and the Size Distribution of Jupiter Family Comets").


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Gonzalo Tancredi

2005-11-01