Stochasticity, a variable stellar upper-mass limit, binaries and star-formation rate indicators

TL;DR

This study uses synthetic spectra to analyze how stochastic sampling, binary stars, and a variable upper-mass limit affect star-formation rate indicators, revealing that stochastic sampling is favored for small clusters and binaries influence the observed properties.

Contribution

It introduces a comprehensive model incorporating stochastic sampling, binary evolution, and variable upper-mass limits to better interpret star-formation indicators in clusters and galaxies.

Findings

Observations favor stochastic sampling for clusters under 100M(Sun).

Binary stars obscure some effects of a mass-dependent upper limit.

Binary evolution leads to more massive stars, affecting star-formation rate indicators.

Abstract

Using our Binary Population And Spectral Synthesis (BPASS) code we explore the effects on star-formation rate indicators of stochastically sampling the stellar initial mass function, adding a cluster mass dependent stellar upper-mass limit and including binary stars. We create synthetic spectra of young clusters and star-forming galaxies and compare these to observations of H(alpha) emission from isolated clusters and the relation between H(alpha) and FUV emission from nearby galaxies. We find that observations of clusters tend to favour a purely stochastic sampling of the initial mass function for clusters less than 100M(Sun), rather than the maximum stellar mass being dependant on the total cluster mass. It is more difficult to determine whether the same is true for more massive clusters. We also find that binary stars blur some of the observational differences that occur when a cluster-mass dependent stellar upper-mass limit is imposed when filling the IMF. The effect is greatest when modelling the observed H(alpha) and FUV star-formation rate ratios in galaxies. This is because mass transfer and merging of stars owing to binary evolution creates more massive stars and stars that have greater mass than the initial maximum imposed on the stellar population.