Abstract
The optical integrated spectra of three LMC young stellar clusters (NGC 1984, NGC 1994 and NGC 2011) exhibit concave continua and prominent molecular bands which deviate significantly from the predictions of single stellar population (SSP) models. In order to understand the appearance of these spectra, we create a set of young stellar population (MILES) models, which we make available to the community. We use archival International Ultraviolet Explorer integrated UV spectra to independently constrain the cluster masses and extinction, and rule out strong stochastic effects in the optical spectra. In addition, we also analyze deep colour-magnitude diagrams of the clusters to provide independent age determinations based on isochrone fitting. We explore hypotheses including age-spreads in the clusters, a top-heavy initial mass function, different SSP models and the role of red supergiant stars (RSG). We find that the strong molecular features in the optical spectra can only be reproduced by modeling an increased fraction of about 20 per cent by luminosity of RSG above what is predicted by canonical stellar evolution models. Given the uncertainties in stellar evolution at Myr ages, we cannot presently rule-out the presence of Myr age-spreads in these clusters. Our work combines different wavelengths as well as different approaches (resolved data as well as integrated spectra for the same sample) in order to reveal the complete picture. We show that each approach provides important information but in combination can we better understand the cluster stellar populations.
Highlights
One of the most significant results of the past decade in the field of star clusters is the discovery of extended mainsequence turn offs of intermediate-age star clusters (100 Myr < age < 10 Gyr) in the Large Magellanic Cloud (LMC)
Results for 10 Myr are shown in Table 4, where we have defined as Post-MS stars all those stars with log g < 3; from this group, we define BSG those with log Teff > 4, YSG those with log Teff ∈ [3.628, 4] where log Teff = 3.628 correspond to Teff = 4250K, and red supergiant stars (RSG) as stars with log Teff < 3.628
We produced a series of single stellar population (SSP) models based on the MILES spectral library which extend to young (6.3 Myr) ages
Summary
One of the most significant results of the past decade in the field of star clusters is the discovery of extended mainsequence turn offs (eMSTO) of intermediate-age star clusters (100 Myr < age < 10 Gyr) in the Large Magellanic Cloud (LMC). The strong contrast in the spectral shape of the blue and red supergiants (BSGs, RSGs) determines the shape of the spectrum of the stellar population, which will be determined by the relative weight between these stars as a function of wavelength. This is relevant for ages around ∼10 Myr where the net contribution of the RSGs to the total light peaks, affecting dramatically the spectrum shape and some molecular band features This ratio, which depends on mass loss, convection and mixing processes, is key for testing both the stellar evolutionary models and the stellar population models in this age regime
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