Confidence limits of evolutionary synthesis models III. On time-integrated quantities

TL;DR

This paper examines the limitations of current evolutionary synthesis models, identifies issues with interpolation methods and calculation techniques, and proposes solutions to improve the accuracy of time-integrated stellar population predictions.

Contribution

It highlights unphysical results from common interpolation methods and introduces alternative approaches, also quantifying stochastic effects on model dispersion.

Findings

Linear interpolation in log M - log t_k plane produces unphysical results.

Some methods for calculating time-integrated quantities may yield unrealistic outcomes.

Dispersion in the 14N/12C ratio increases over time due to stochastic effects.

Abstract

Evolutionary synthesis models are a fundamental tool to interpret the properties of observed stellar systems. In order to achieve a meaningful comparison between models and real data, it is necessary to calibrate the models themselves, i.e. to evaluate the dispersion due to the discreteness of star formation as well as the possible model errors. In this paper we show that linear interpolations in the log M - log t_k plane, that are customary in the evaluation of isochrones in evolutionary synthesis codes, produce unphysical results. We also show that some of the methods used in the calculation of time-integrated quantities (kinetic energy, and total ejected masses of different elements) may produce unrealistic results. We propose alternative solutions to solve both problems. Moreover, we have quantified the expected dispersion of these quantities due to stochastic effects in stellar populations. As a particular result, we show that the dispersion in the 14N/12C ratio increases with time.