Secular brightness curves of 272 comets

Aims. We investigate the brightening behavior of long-period comets as a function of dynamical age, defined by the original reciprocal semimajor axis, 1/a0. Our goal is to test long-standing claims about comet behavior using a large number of available measurements. Methods. We used a large set of photometric observations to compute and analyze global and local brightening curves for 272 long-period comets. We fit the observed magnitudes with a linear model in log heliocentric distance, from which we derived brightening parameters for each comet. We categorized the sample into dynamically new, intermediate, and old comets, and we compared their brightening behavior. We also examined the relationships between dynamical age and other orbital and physical parameters. Results. Dynamically new comets are seen to brighten more slowly than old comets, particularly within 3 au from the Sun. The brightening rate of new comets appears to vary with heliocentric distance. New comets are intrinsically brighter than old comets and exhibit a tighter correlation between brightening parameters.