Synthetic photometry of globular clusters: Uncertainties on synthetic colors

F Martins

doi:10.1051/0004-6361/201833083

Abstract

Context. Synthetic photometry is a great tool for studying globular clusters, especially for understanding the nature of their multiple populations. Aims. Our goal is to quantify the errors on synthetic photometry that are caused by uncertainties on stellar and observational/calibration parameters. These errors can be taken into account when building synthetic color-magnitude diagrams (CMDs) that are to be compared to observed CMDs. Methods. We have computed atmosphere models and synthetic spectra for two stars, Pollux and Procyon, that have stellar parameters typical of turn-off and bottom red giant branch stars in globular clusters. We then varied the effective temperature, surface gravity, microturbulence, the carbon, nitrogen, and oxygen abundances, and [Fe/H]. We quantified the effect on synthetic photometry in the following filters: Johnson UBVRI and HST F275W, F336W, F410M, F438W, F555W, F606W, and F814W. We also estimated the effects of extinction, atmospheric correction, and of the Vega reference spectrum on the resulting photometry. In addition, we tested the ability of our models to reproduce the observed spectral energy distribution and observed photometry of the two stars. Results. We show that variations are generally stronger in blue filters, especially those below 4500 Å. Dispersions on synthetic colors due to uncertainties on stellar parameters vary between less than 0.01 and to 0.04 magnitude, depending on the choice of filters. Uncertainties on the zero points, the extinction law, or the atmospheric correction affect the resulting colors at a level of a few 0.01 magnitudes in a systematic way. The models reproduce the flux-calibrated spectral energy distribution of both stars well. Comparison between synthetic and observed UBVRI photometry shows a variable degree of (dis)agreement. The observed differences indicate that different reduction and calibration processes are performed to obtain respectively observed and synthetic photometry, and they call for publication of all the details of the reduction process to produce synthetic photometry at a 0.01 mag level, which is required to interpret observed CMDs.

Highlights

Globular clusters were once known to be simple structures made of stars formed at the same time with the same initial chemical composition
The observed differences indicate that different reduction and calibration processes are performed to obtain respectively observed and synthetic photometry, and they call for publication of all the details of the reduction process to produce synthetic photometry at a 0.01 mag level, which is required to interpret observed color-magnitude diagrams (CMDs)
We have presented a study of uncertainties on synthetic photometry in the context of the understanding the properties of globular clusters

Summary

Introduction

Globular clusters were once known to be simple structures made of stars formed at the same time with the same initial chemical composition This picture has been deeply revised since various sub-groups of stars have been discovered in the vast majority of them. Determinations of surface chemical abundances indicate that some stars are enriched in nitrogen, sodium, and aluminum, while being at the same time depleted in carbon, oxygen, and magnesium (e.g., Sneden et al 1992; Kraft et al 1997; Carretta et al 2010). A wide range of enrichment or depletion is usually observed, leading to so-called anticorrelations between nitrogen and carbon, sodium and oxygen, and aluminum and magnesium (Yong et al 2006; Carretta et al 2006, 2009b; Gratton et al 2007; Marino et al 2011; Carretta 2015). The Hubble Space Telescope has pioneered the identification of such sequences (Bedin et al 2004; Piotto et al 2007, 2015; Milone et al 2010, 2012; Soto et al 2017), but they are observed with any high spatial resolution photometric facilities (Han et al 2009; Gruyters et al 2017)

Methods

Results

Conclusion