The influence of multiple stars on the high-mass stellar initial mass function and age-dating of young massive star clusters

TL;DR

This paper investigates how unresolved multiple star systems affect the observed stellar initial mass function and age estimates in young massive star clusters, finding minimal impact on the IMF slope but significant effects on age spread and mass estimation.

Contribution

It quantifies the influence of unresolved multiple systems on the IMF and age-dating, providing corrections for observational biases in young star clusters.

Findings

Unresolved multiples cause only small changes (~0.1) in the IMF slope.

Between 15% and 60% of cluster mass can be hidden in unresolved systems.

Unresolved binaries induce a significant age spread among pre-main-sequence stars.

Abstract

The study of young stellar populations has revealed that most stars are in binary or higher order multiple systems. In this study the influence on the stellar initial mass function (IMF) of large quantities of unresolved multiple massive stars is investigated by taking into account stellar evolution and photometrically determined system masses. The models where initial masses are derived from the luminosity and colour of unresolved multiple systems show that even under extreme circumstances (100% binaries or higher order multiples) the difference between the power-law index of the mass function of all stars and the observed mass function is small (~0.1). Thus, if the observed IMF has the Salpeter index alpha = 2.35 then the true stellar IMF has an index not flatter than alpha = 2.25. Additionally, unresolved multiple systems may hide between 15 and 60% of the underlying true mass of a star cluster. While already a known result, it is important to point out that the presence of a large number of unresolved binaries amongst pre-main-sequence (PMS) stars induces a significant spread in the measured ages of these stars even if there is none. Also, lower-mass stars in a single-age binary-rich cluster appear older than the massive stars by about 0.6 Myr.