Volume 572, December 2014
|Number of page(s)||28|
|Published online||02 December 2014|
1 Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, PO Box 94249, 1090 GE Amsterdam, The Netherlands
2 Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
3 Observatoire de la Côte d’Azur, CNRS UMR 7293, BP4229, 06304 Nice Cedex 4, France
4 Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
5 Laboratoire d’astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
6 GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France
7 Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Road, Victoria, BC, V8P 1A1, Canada
8 Instituto de Astrofisica de Canarias, calle via Lactea s/n, 38205 San Cristobal de La Laguna, Tenerife, Spain
9 Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain
10 McDonald Observatory, The University of Texas at Austin, 1 University Station, C1400, Austin, TX 78712-0259, USA
11 Dept of Physics, North-West University, Mafikeng Campus, Private Bag X2046, Mmabatho 2745, South Africa
12 UPJV, Université de Picardie Jules Verne, 33 rue St. Leu, 80080 Amiens, France
13 European Southern Observatory, Karl Schwarzschild Str. 2, 85748 Garching b. München, Germany
14 European Southern Observatory, Alonso de Cordova 3107, Santiago, Chile
Received: 31 March 2014
Accepted: 3 October 2014
Context. Fornax is one of the most massive dwarf spheroidal galaxies in the Local Group. The Fornax field star population is dominated by intermediate age stars but star formation was going on over almost its entire history. It has been proposed that Fornax experienced a minor merger event.
Aims. Despite recent progress, only the high metallicity end of Fornax field stars ([Fe/H] > –1.2 dex) has been sampled in larger number via high resolution spectroscopy. We want to better understand the full chemical evolution of this galaxy by better sampling the whole metallicity range, including more metal poor stars.
Methods. We use the VLT-FLAMES multi-fibre spectrograph in high-resolution mode to determine the abundances of several α, iron-peak and neutron-capture elements in a sample of 47 individual red giant branch stars in the Fornax dwarf spheroidal galaxy. We combine these abundances with accurate age estimates derived from the age probability distribution from the colour-magnitude diagram of Fornax.
Results. Similar to other dwarf spheroidal galaxies, the old, metal-poor stars of Fornax are typically α-rich while the young metal-rich stars are α-poor. In the classical scenario of the time delay between Type II (SNe II) and Type Ia Supernovae (SNe Ia), we confirm that SNe Ia started to contribute to the chemical enrichment at [Fe/H] between –2.0 and –1.8 dex. We find that the onset of SNe Ia took place between 12–10 Gyr ago. The high values of [Ba/Fe], [La/Fe] reflect the influence of SNe Ia and AGB stars in the abundance pattern of the younger stellar population of Fornax.
Conclusions. Our findings of low [α/Fe] and enhanced [Eu/Mg] are compatible with an initial mass function that lacks the most massive stars and with star formation that kept going on throughout the whole history of Fornax. We find that massive stars kept enriching the interstellar medium in α-elements, although they were not the main contributor to the iron enrichment.
Key words: stars: abundances / galaxies: individual: Fornax / galaxies: evolution
Based on FLAMES observations collected at the European Southern Observatory, proposal number 080.B-0784.
Table 7 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (22.214.171.124) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/572/A88
Appendices are available in electronic form at http://www.aanda.org
© ESO, 2014
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