Detailed abundance analysis from integrated high-dispersion spectroscopy: globular clusters in the Fornax dwarf spheroidal^⋆,⋆⋆

¹ Department of Astrophysics / IMAPP, Radboud University Nijmegen, 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 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁴ Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA

Received: 26 June 2012
Accepted: 3 September 2012

Abstract

Aims. We describe our newly developed approach to detailed abundance analysis from integrated-light high-dispersion spectra of star clusters. As a pilot project, we measure abundances of several Fe-peak, α- and neutron capture elements from spectra of three globular clusters (GCs) in the Fornax dwarf spheroidal galaxy, obtained with UVES on the ESO Very Large Telescope.

Methods. We divided the cluster colour − magnitude diagrams into about 100 bins and used the Kurucz ATLAS9 and SYNTHE codes to compute synthetic spectra for each bin. Stellar parameters were derived empirically from Hubble Space Telescope data for the brighter stars, while theoretical isochrones were used for extrapolation below the detection limit. The individual model spectra were co-added and the abundances were iteratively adjusted until the best match to the observed spectra was achieved.

Results. We find [Fe/H] = −2.3, −1.4 and −2.1 for Fornax 3, 4 and 5, with estimated ± 0.1 dex uncertainties. Fornax 3 and 5 are thus similar in metallicity to the most metal-poor Milky Way GCs and fall near the extreme metal-poor end of the field star metallicity distribution in Fornax. The [α/Fe] ratios, as traced by Ca and Ti, are enhanced with respect to the solar composition at the level of ~+0.25 dex for Fornax 3 and 5, and possibly slightly less (~+0.13 dex) for Fornax 4. For all three clusters the [Mg/Fe] ratio is significantly less elevated than [Ca/Fe] and [Ti/Fe], possibly an effect of the abundance anomalies that are well-known in Galactic GCs. We thus confirm that Mg may be a poor proxy for the overall α-element abundances for GCs. The abundance patterns of heavy elements (Y, Ba and Eu) indicate a dominant contribution to nucleosynthesis from the r-process in all three clusters, with a mean [Ba/Eu] ~ −0.7, suggesting rapid formation of the GCs.

Conclusions. Combining our results with literature data for Fornax 1 and 2, it is now clear that four of the five Fornax GCs fall in the range −2.5 < [Fe/H] < −2, while Fornax 4 is unambiguously and substantially more metal-rich than the others. The indications that abundance anomalies are detectable in integrated light are encouraging, particularly for the prospects of detecting such anomalies in young, massive star clusters of which few are close enough for individual stars to be observed in detail.