Brightness Behavior of Distant Oort Cloud Comets

Dynamically new comets provide important insights into the conditions of the presolar nebula and its evolution, as they are believed to have experienced minimal solar heating before their discovery. Since 2020 August, the Las Cumbres Observatory (LCO) Outbursting Objects Key Project has utilized LCO's network of 1 m telescopes to consistently monitor long-period comets (LPCs) discovered inbound beyond 5 au from the Sun approximately every 3 days in order to characterize the brightness evolution, colors, and coma morphology of distant comets over a range of heliocentric distances. We report the long-term time-series photometry of 21 objects in our sample and coma morphology analyses for six comets with heliocentric distances less than 3 au. We find that LPCs rapidly brighten further from the Sun, and the brightening rate decreases as heliocentric distance decreases. We do not observe a clear difference in brightening rate for returning versus dynamically new comets. When LPCs are within 3 au of the Sun, they consistently exhibit a color change in a 20,000 km aperture consistent with an increase in gas production driven by water-ice sublimation. We find that returning comets experience the color change closer to the Sun than dynamically new comets, likely because their volatiles are more deeply buried or they have a higher dust-to-gas ratio after the upper layers are depleted of volatiles during a previous solar passage.