The range of validity of cluster masses and ages derived from broad-band photometry

TL;DR

This paper investigates the limitations of broad-band photometry in accurately determining stellar cluster masses and ages, highlighting the impact of stochastic sampling effects and proposing improvements with extended filter sets.

Contribution

It demonstrates the unreliability of classical UBV photometry for older clusters and shows that including longer-wavelength filters and weighting improves measurement accuracy.

Findings

Classical UBV photometry is unreliable for clusters older than 30 Ma.

Adding longer-wavelength filters enhances measurement accuracy.

Sampling effects dominate the observed mass uncertainties.

Abstract

I analyze the stochastic effects introduced by the sampling of the stellar initial mass function (SIMF) in the derivation of the individual masses and the cluster mass function (CMF) from broad-band visible-NIR unresolved photometry. The classical method of using unweighted UBV photometry to simultaneously establish ages and extinctions of stellar clusters is found to be unreliable for clusters older than approx. 30 Ma, even for relatively large cluster masses. On the other hand, augmenting the filter set to include longer-wavelength filters and using weights for each filter increases the range of masses and ages that can be accurately measured with unresolved photometry. Nevertheless, a relatively large range of masses and ages is found to be dominated by SIMF sampling effects that render the observed masses useless, even when using UBVRIJHK photometry.