A New Empirical Brightening Model for Distantly Active Long-Period Comets

Modern sky surveys are now discovering comets at distances beyond 5 to 10 au from the Sun, a range expected to increase with the upcoming Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST). However, the mechanisms driving comet activity at such distances remain poorly understood, as temperatures are too low for efficient water ice sublimation, the usual driver closer to the Sun (within 3–5 au). At these larger distances, activity is likely driven by the sublimation of more volatile ices such as CO and CO₂, or by phase transitions in amorphous water ice. But the relative importance of these processes remains uncertain. Understanding the drivers of distant activity is essential not only for modeling the physical evolution of cometary nuclei, but also for interpreting future discoveries and planning future observations, including spacecraft missions like ESA’s Comet Interceptor (Jones et al., 2024). One way to gain insight is through the characterization of the observed brightening behavior of comets as they approach the Sun using well-calibrated photometry over a wide range of heliocentric distances.

As part of the Las Cumbres Observatory (LCO) Outbursting Objects Key (LOOK) Project, we have been monitoring more than 45 long-period comets discovered inbound beyond 5 au using LCO’s global network of 1-meter telescopes since 2020 (Lister et al. 2022; Holt et al. 2024). The LOOK dataset provides frequent, uniform photometric coverage from shortly after discovery through perihelion. Using this dataset, we compare the performance of two empirical models for comet brightening: the traditional power-law model with a constant slope, and a more flexible model in which the brightening rate varies linearly with heliocentric distance. This new model reflects the observed trend that many comets brighten more rapidly at large heliocentric distances, with the rate of brightening decreasing as they approach the Sun (Holt et al., 2024).

We fit both models and perform a statistical comparison on the pre-perihelion lightcurves of a high-quality subset of comets with well-sampled photometric coverage across large heliocentric distance ranges. Model performance is evaluated using residual metrics and standard statistical tools for model selection. Across this sample, we find that the variable-slope model consistently provides a better fit to the data beyond 3 au from the Sun. We will present a comparison of model performance across the sample, highlighting where the variable-slope model is favored. In addition to these sample-wide trends, we will show representative fits to individual comets that illustrate how the flexible model captures early brightening more effectively and is better at predicting future brightness at smaller heliocentric distances based on distant observations (e.g., Figure 1). These examples demonstrate how the model improves both the fit to existing data and the reliability of extrapolated predictions, which is critical for follow-up coordination and mission planning.

We will present the range of fit coefficients and their correlation with other comet parameters (such as absolute magnitude or dynamical type). We will also explore the limitations of both models, especially near the Sun, where increased activity due to water-ice sublimation, outbursts, and seasonal effects introduces additional complexity. Finally, we consider the implications for LSST predictions and how this empirical model can inform target selection for the ESA Comet Interceptor mission.

Figure 1. Example lightcurve of C/2023 A3 (Tsuchinshan–ATLAS), showing observed pre-perihelion heliocentric magnitudes versus heliocentric distance. Overplotted are the fits from the constant-slope model (blue line) and the variable-slope model (orange line). The variable model better captures the brightening behavior at large heliocentric distances.

 

References: 

Holt, C. et al., 2024, PSJ, 5, 12

Jones, G. et al., 2024, Space Science Reviews, 22, 9

Lister, T. et al., 2022, PSJ, 3, 7