Heliocentric Light Curves of Long-Period Comets

We investigate the heliocentric brightening behavior of long-period comets as a function of their dynamical age, parameterized by the original reciprocal semimajor axis, 1/a0. The analysis is designed to test longstanding assertions regarding cometary photometric evolution using a substantial dataset of magnitude measurements reported to the Minor Planet Center.

Our sample comprises 272 long-period comets, for which we derive brightening parameters by fitting observed magnitudes with a linear model in  logarithmic heliocentric distance of the form T − 5 logΔ = m + k log r, where T is total magnitude, and Δ and r are geocentric and heliocentric distance. The fitted brightening parameters, m and k, allow us to quantify both heliocentric and local brightening rates.

Adopting the dynamical classification scheme introduced by Oort, we separate the comets into three dynamical groups — new, intermediate, and old — and then systematically compare their photometric behavior.

The results indicate that:

• dynamically new comets brighten more gradually than older ones, particularly at heliocentric distances less than 3 AU;
• the brightening rates of new and intermediate comets exhibit a clear dependence on heliocentric distance; and
• new comets tend to be intrinsically brighter and display a tighter correlation between their derived brightening parameters.

This analysis is based on a study accepted for publication in A&A (https://doi.org/10.1051/0004-6361/202453565), and a preprint is available at
https://arxiv.org/abs/2504.00565.

Figure 1 - Number of comets and strands analyzed against reciprocal semi major axis (1/a0). Dynamically new, intermediate, and old comet ranges are indicated.

 

Figure 2 - Global heliocentric light curve of comet C/2019 U6 (right) and the same data grouped by observatory and filter (left). These grouped sets of data, called strands, allow more reliable relative photometry and assessment of uncertainties.

Figure 3 - Average brightening in magnitudes of long period comets relative to 1 AU, categorized by Oort dynamical group.

Figure 4 - Rate of brightening versus heliocentric distance for LPCs for each Oort group. Different colors split the LPCs into interior and exterior to 2, 3 and 4 au.

Figure 5 - Relation between the brightening parameters m and k for each Oort group. Best fit lines are fit to each group. Inset shows the cumulative distribution of residuals for each fit.