The Visual Lightcurve of Comet C/1995 O1 (Hale-Bopp) from 1995 to 1999

The great comet C/1995 O1 (Hale-Bopp) presented a remarkable opportunity to study its long-term brightness over four years. We used 2240 observations published in the International Comet Quarterly from 17 observers during 1995 July to 1999 September to create a secular lightcurve. In order to account for observer differences, we present a novel algorithm to reduce scatter and increase precision in a lightcurve compiled from many sources. It is implemented in a publicly available code, ICQSPLITTER, which uses a self-consistent statistical approach. To first order, the comet's lightcurve approximates an r^-4 response for both pre- and postperihelion distances. The preperihelion data are better fit with a fifth-order polynomial with inflection points at 4.0, 2.6, 2.1, and 1.1 au, some of which are associated with physical changes in the coma. Outbursts may have occurred a few days before perihelion and at ∼2.2 and 7.4 au postperihelion. The Afρ values derived from the final magnitudes are consistent with an r^-1.5 dependence on heliocentric distance and are within a factor of 2-4 of those derived from spectroscopy and narrowband photometry. We present correlation equations for visual magnitudes and CO and H₂O production rates that are consistent with the preperihelion brightness increasing due to CO outgassing until about 2.6-3.0 au from the Sun and then are strongly correlated with H₂O production rates. We also present two generalized correlation equations that may be useful for observation planning and data analysis with the James Webb Space Telescope and other observatories.