[1] Astrophys Observ Turin, INAF, Str Osservatorio 20, I-10025 Pino Torinese, Turin, Italy
[2] Univ Turin, Dipartimento Fis, I-10125 Turin, Italy
[3] Univ Hertfordshire, Sch Phys Astron & Math, Ctr Astrophys Res, Hatfield AL10 9AB, Herts, England
[4] Univ Basel, Dept Phys, CH-4056 Basel, Switzerland
[5] UK Network Bridging Disciplines Galact Chem Evolu, Basel, Switzerland
[6] Univ Wurzburg, D-97074 Wurzburg, Germany
[7] Max Planck Inst Astrophys, D-85748 Garching, Germany