GIANO-TNG spectroscopy of red supergiants in the young star cluster RSGC2^⋆

¹ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: livia.origlia@oabo.inaf.it
² INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
³ University of Cambridge, Cavendish Lab., JJ Thomson Av., Cambridge CB3 0HE, UK
⁴ University of Bologna, Physics & Astronomy Dept., Viale Berti Pichat 6-2, 40127 Bologna, Italy
⁵ INAF – Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
⁶ INAF – TNG, ORM Astronomical Observatory, 38787 Garafia, TF, Spain
⁷ University of Catania, Physics & Astronomy Dept. via S. Sofia 78, 95123 Catania, Italy
⁸ Lawrence Berkeley National Laboratory, 1 Cyclotron Road, MS 2-400, Berkeley CA 94720, USA

Received: 2 September 2013
Accepted: 4 November 2013

Abstract

Aims. The inner disk of the Galaxy has a number of young star clusters dominated by red supergiants that are heavily obscured by dust extinction and observable only at infrared wavelengths. These clusters are important tracers of the recent star formation and chemical enrichment history in the inner Galaxy.

Methods. During the technical commissioning and as a first science verification of the GIANO spectrograph at the Telescopio Nazionale Galileo, we secured high-resolution (R ≃ 50 000) near-infrared spectra of three red supergiants in the young Scutum cluster RSGC2.

Results. Taking advantage of the full YJHK spectral coverage of GIANO in a single exposure, we were able to identify several tens of atomic and molecular lines suitable for chemical abundance determinations. By means of spectral synthesis and line equivalent width measurements, we obtained abundances of Fe and other iron-peak elements such as V, Cr, Ni, of alpha (O, Mg, Si, Ca and Ti) and other light elements (C, N, Na, Al, K, Sc), and of some s-process elements (Y, Sr). We found iron abundances between half and one third solar and solar-scaled [X/Fe] abundance patterns of iron-peak, alpha and most of the light elements, consistent with a thin-disk chemistry. We found a depletion of [C/Fe] and enhancement of [N/Fe], consistent with CN burning, and low ¹²C/¹³C abundance ratios (between 9 and 11), requiring extra-mixing processes in the stellar interiors during the post-main-sequence evolution. Finally, we found a slight [Sr/Fe] enhancement and a slight [Y/Fe] depletion (by a factor of ≤2), with respect to solar.