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, at the time, Lunar and Planetary Laboratory 1.4-m at the Catalina Station and the Steward Observatory 2.3-m at Kitt Peak (see, for instance, Roemer 1976).
As Roemer was carrying out her photometric work, progress was made in another observational front: space-based ultraviolet observations allowed the observation of the Lyman-alpha line of hydrogen at 1216 Å and thereby an estimate of the water production rate. Other species were also observed in the UV, and a very important data base for inferring H2O production rates came from IUE observations of the OH bands near 309 nm. In parallel, radio observations of the 18-cm lines of OH allowed an independent determination of the water production rate. A'Hearn & Millis (1980) introduced narrow band filters to isolate different molecular species (e.g. C2, C3, CN, OH) in their ground-based photometric observations of comets with the aim of determining their production rates. The study of gas production rates of different species thus gave a confirmation that H2O controls the cometary outgassing near the Sun and should be the most abundant volatile in comets.
In the presence of data on H2O production rates, cometary nuclear magnitudes took on a new meaning, viz., in terms of the sizes and activity levels of the nuclei. However, the determination of the nuclear size is not straightforward, since it also depends on the albedo of the nuclear surface. Delsemme & Rud (1973) attempted for the first time to derive both the nuclear radii and albedos of comets C/1969 T1 (Tago-Sato-Kosaka), C/1969 Y1 (Bennett) and 2P/Encke (1971 perihelion passage) by combining their measured water production rates close to perihelia with Roemer's nuclear magnitudes obtained at large heliocentric distances. Even though the derived albedos for C/1969 T1 Tago-Sato-Kosaka and C/1969 Y1 Bennett were too high for what one would expect of dirty ice surfaces, theirs was nevertheless a pioneering and influential approach to this problem.
Meanwhile, 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
8.2 km
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. We restrict our sample to the comets of the Jupiter family (JF) that we define following Valsecchi (1992) as those with Tisserand constants T > 2 and periods P < 20 yr (there are so far only four comets with P < 20 yr that have T < 2). We choose this population for two reasons: (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). Admittedly, we do not know to what extent this presumption is correct, since the JF population may be contaminated with comets coming from other sources as, for instance, the Oort cloud (Bailey 1986) or the Trojans (Rabe 1972), it is likely that such contamination represent only a minority of the whole population.
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).
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