On emission-line spectra obtained from evolutionary synthesis models I. Dispersion in the ionising flux and Lowest Luminosity Limits

TL;DR

This paper investigates the variability in ionising fluxes from stellar clusters due to incomplete sampling of stellar populations, establishing limits for the applicability of synthesis models and highlighting the impact on spectral diagnostics.

Contribution

It quantifies the dispersion in ionising spectra caused by sampling effects and defines the Lowest Luminosity Limit for applying synthesis models to stellar clusters.

Findings

Intensities at different ionisation edges are strongly correlated.

He II lines are heavily affected by sampling effects.

The Lowest Luminosity Limit varies with metallicity and age, ranging from 10^3 to over 10^6 solar masses.

Abstract

(abriged) Stellar clusters with the same general physical properties (e.g., total mass, age, and star-formation mode) may have very different stellar mass spectra due to the incomplete sampling of the underlying mass function; such differences are especially relevant in the high-mass tail due to the smaller absolute number of massive stars. The dispersion in the number of massive stars also produces a dispersion in the properties of the corresponding ionising spectra. In this paper, we lay the bases for the future analysis of this effect by evaluating the dispersion in the ionising fluxes of synthetic spectra. As an important consequence, we found that the intensities of synthetic fluxes at different ionisation edges are strongly correlated, a fact suggesting that no additional dispersion will result from the inclusion of sampling effects in the analysis of diagnostic diagrams; this is true for HII regions on all scales. Additionally, we find convincing suggestions that the He II lines are strongly affected by sampling, and so cannot be used to constrain the evolutionary status of stellar clusters. We also establish the range of applicability of synthesis models set by the Lowest Luminosity Limit for the ionising flux, that is the lowest limit in cluster mass for which synthesis models can be applied to predict ionising spectra. This limit marks the boundary between the situations in which the ionising flux is better modeled with a single star as opposed to a star cluster; this boundary depends on the metallicity and age, ranging from 10^3 to more than 10^6 Mo. As a consequence, synthesis models should not be used to try to account for the properties of clusters with smaller masses.