Detailed abundances from integrated-light spectroscopy: Milky Way globular clusters ^⋆,⋆⋆

¹ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
e-mail: s.larsen@astro.ru.nl
² UCO/Lick Observatory, University of California, Santa Cruz, CA 95064, USA
³ Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA

Received: 25 November 2016
Accepted: 21 February 2017

Abstract

Context. Integrated-light spectroscopy at high spectral resolution is rapidly maturing as a powerful way to measure detailed chemical abundances for extragalactic globular clusters (GCs).

Aims. We test the performance of our analysis technique for integrated-light spectra by applying it to seven well-studied Galactic GCs that span a wide range of metallicities.

Methods. Integrated-light spectra were obtained by scanning the slit of the UVES spectrograph on the ESO Very Large Telescope across the half-light diameters of the clusters. We modelled the spectra using resolved Hubble Space Telescope colour–magnitude diagrams (CMDs), as well as theoretical isochrones, in combination with standard stellar atmosphere and spectral synthesis codes. The abundances of Fe, Na, Mg, Ca, Ti, Cr, and Ba were compared with literature data for individual stars in the clusters.

Results. The typical differences between iron abundances derived from our integrated-light spectra and those compiled from the literature are less than ~0.1 dex. A larger difference is found for one cluster (NGC 6752), and is most likely caused primarily by stochastic fluctuations in the numbers of bright red giants within the scanned area. As expected, the α-elements (Ca, Ti) are enhanced by about 0.3 dex compared to the Solar-scaled composition, while the [Cr/Fe] ratios are close to Solar. When using up-to-date line lists, our [Mg/Fe] ratios also agree well with literature data. Our [Na/Fe] ratios are, on average, 0.08–0.14 dex lower than average values quoted in the literature, and our [Ba/Fe] ratios may be overestimated by 0.20–0.35 dex at the lowest metallicities. We find that analyses based on theoretical isochrones give very similar results to those based on resolved CMDs.

Conclusions. Overall, the agreement between our integrated-light abundance measurements and the literature data is satisfactory. Refinements of the modelling procedure, such as corrections for stellar evolutionary and non-LTE effects, might further reduce some of the remaining offsets.