Journal Article

Chemical abundance ratios of galactic globular clusters from modelling integrated light spectroscopy

Daniel Thomas, Jonas Johansson and Claudia Maraston

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 412, issue 4, pages 2199-2210
Published in print April 2011 | ISSN: 0035-8711
Published online April 2011 | e-ISSN: 1365-2966 | DOI:
Chemical abundance ratios of galactic globular clusters from modelling integrated light spectroscopy

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In a companion paper we present new, flux-calibrated stellar population models of Lick absorption-line indices with variable element abundance ratios. The model includes a large variety of individual element variations, which allows the derivation of the abundances for the elements C, N, O, Mg, Ca, Ti and Fe besides total metallicity and age. We use this model to obtain estimates of these quantities from integrated light spectroscopy of galactic globular clusters. We show that the model fits to a number of indices improve considerably when various variable element ratios are considered. The ages we derive agree well with the literature and are all consistent with the age of the Universe within the measurement errors. There is a considerable scatter in the ages, though, and we overestimate the ages preferentially for the metal-rich globular clusters. Our derived total metallicities agree generally very well with literature values on the Zinn & West scale once corrected for α enhancement, in particular for those cluster where the ages agree with the colour–magnitude diagram ages. We tend to slightly underestimate the metallicity for those clusters where we overestimate the age, in line with the age–metallicity degeneracy. It turns out that the derivation of individual element abundance ratios is not reliable at an iron abundance [Fe/H] < −1 dex where line strengths become weaker, while the [α/Fe] ratio is robust at all metallicities. The discussion of individual element ratios focuses therefore on globular clusters with [Fe/H] > −1 dex. We find general enhancement of light and α elements, as expected, with significant variations for some elements. The elements O and Mg follow the same general enhancement with almost identical distributions of [O/Fe] and [Mg/Fe]. We obtain slightly lower [C/Fe] and very high [N/Fe] ratios, instead. This chemical anomaly, commonly attributed to self-enrichment, is well known in globular clusters from individual stellar spectroscopy. It is the first time that this pattern is obtained also from the integrated light. The α elements follow a pattern such that the heavier elements Ca and Ti are less enhanced. More specifically, the [Ca/Fe] and [Ti/Fe] ratios are lower than [O/Fe] and [Mg/Fe] by about 0.2 dex. Most interestingly this trend of element abundance with atomic number is also seen in recent determinations of element abundances in globular cluster and field stars of the Milky Way. This suggests that Type Ia supernovae contribute significantly to the enrichment of the heavier α elements as predicted by nucleosynthesis calculations and galactic chemical evolution models.

Keywords: stars: abundances; Galaxy: abundances; globular clusters: general; galaxies: formation; galaxies: stellar content

Journal Article.  8646 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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