Testing the accuracy of synthetic stellar libraries

Abstract

One of the main ingredients of stellar population synthesis models is a library of stellar spectra. Both empirical and theoretical libraries are used for this purpose, and the question about which one is preferable is still debated in the literature. Empirical and theoretical libraries are being improved significantly over the years, and many libraries have become available lately. However, it is not clear in the literature what are the advantages of using each of these new libraries, and how far behind models are compared to observations. Here we compare in detail some of the major theoretical libraries available in the literature with observations, aiming at detecting weaknesses and strengths from the stellar population modelling point of view. Our test is twofold: we compared model predictions and observations for broad‐band colours and for high‐resolution spectral features. Concerning the broad‐band colours, we measured the stellar colour given by three recent sets of model atmospheres and flux distributions, and compared them with a recent UBVRIJHK calibration which is mostly based on empirical data. We found that the models can reproduce with reasonable accuracy the stellar colours for a fair interval in effective temperatures and gravities. The exceptions are (1) the U−B colour, where the models are typically redder than the observations, and (2) the very cool stars in general (V−K≳ 3). Castelli & Kurucz is the set of models that best reproduce the bluest colours (U−B, B−V) while Gustafsson et al. and Brott & Hauschildt more accurately predict the visual colours. The three sets of models perform in a similar way for the infrared colours. Concerning the high‐resolution spectral features, we measured 35 spectral indices defined in the literature on three high‐resolution synthetic libraries, and compared them with the observed measurements given by three empirical libraries. The measured indices cover the wavelength range from ∼3500 to ∼8700 Å. We found that the direct comparison between models and observations is not a simple task, given the uncertainties in parameter determinations of empirical libraries. Taking that aside, we found that in general the three libraries present similar behaviours and systematic deviations. For stars with Teff≤ 7000 K, the library by Coelho et al.is the one with best average performance. We detect that lists of atomic and molecular line opacities still need improvement, specially in the blue region of the spectrum, and for the cool stars (Teff≲ 4500 K).

Evolutionary stellar population synthesis with MILES – II. Scaled-solar and α-enhanced models

“…McWilliam et al 2008). Martins & Coelho (2007) find for the C-dominated features that the difference between the theoretical and empirical stars is emphasised for stars cooler than ≃ 4500 K. These stars have a large impact on the old SSPs represented in Fig. 5.…”

“…The Coelho et al (2005) theoretical library covers effective temperatures, T eff , between 3500 and 7000 K, surface gravities, log g, between +0.0 and +5.0, iron metallicities, [Fe/H] The accuracy of this library has been assessed in Martins & Coelho (2007), who showed that this library had the highest success in reproducing spectral features of observed stars among the theoretical libraries analysed in that work. The spectra have higher spectral resolution and wavelength coverage than MILES, therefore they were convolved with a gaussian kernel and were resampled to match MILES spectral resolution, dispersion and wavelength coverage.…”

“…Cenarro et al (2001a,b); Cushing, Raynier & Vacca (2005) ;Raynier, Cushing & Vacca (2009), that are also implemented in these models Conroy & van Dokkum 2012a). Alternatively, theoretical stellar libraries at high spectral resolutions have also been developed for this purpose (e.g., Barbuy et al 2003;Murphy & Meiksin 2004;Zwitter, Castelli & Munari 2004;Coelho et al 2005;Martins et al 2005;Munari et al 2005;Rodríguez-Merino et al 2005;Frémaux et al 2006;Martins & Coelho 2007;Leitherer et al 2010;Palacios et al 2010;Kirby 2011;de Laverny et al 2012;Coelho 2014). Models that make use of these libraries are, e.g., Leitherer et al (1999), Schiavon, Barbuy & Bruzual (2000), González-Delgado et al (2005); Coelho et al (2007); Buzzoni et al (2009); Percival et al (2009) ;Maraston & Strömbäck (2011).…”

“…To our knowledge, the three self-consistent models for a fair range of ages and metallicities available in the literature are Coelho et al (2007); Percival et al (2009). However, both these models carry a caveat: the stellar spectral libraries employed are theoretical, and as such they carry wavelength-dependent systematic deviations when compared to observed spectra (Martins & Coelho 2007;Bertone et al 2008;Coelho 2014). Inspired by the work of Cervantes et al (2007) and Prugniel et al (2007), and in order to bring together the predictive power of fully-theoretical models and the accuracy of the base models, Walcher et al (2009) proposed a method to use the predictions of fully-theoretical SSP models only differentially.…”

Spectral Evolution Models for the Next Decade

“…The pros and cons of either approach are discussed extensively in the literature (e.g., Gustaffson et al 2007;Martins & Coelho 2007).…”