High resolution spectroscopic analysis of seven giants in the bulge globular cluster NGC 6723

¹ Laboratoire Lagrange (UMR 7293), Université Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, CS 34229, 06304 Nice Cedex 04, France
e-mail: arojas@oca.eu
² Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago, Chile
³ Millennium Institute of Astrophysics, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
⁴ Museo Interactivo Mirador, Dirección de Educación, Av. Punta Arenas, 6711 La Granja, Santiago, Chile
⁵ INAF-Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Naples, Italy

Received: 12 September 2015
Accepted: 17 December 2015

Abstract

Context. Globular clusters associated with the Galactic bulge are important tracers of stellar populations in the inner Galaxy. High resolution analysis of stars in these clusters allows us to characterize them in terms of kinematics, metallicity, and individual abundances, and to compare these fingerprints with those characterizing field populations.

Aims. We present iron and element ratios for seven red giant stars in the globular cluster NGC 6723, based on high resolution spectroscopy.

Methods. High resolution spectra (R ~ 48 000) of seven K giants belonging to NGC 6723 were obtained with the FEROS spectrograph at the MPG/ESO 2.2 m telescope. Photospheric parameters were derived from ~130 Fe i and Fe ii transitions. Abundance ratios were obtained from line-to-line spectrum synthesis calculations on clean selected features.

Results. An intermediate metallicity of [Fe/H] = −0.98 ± 0.08 dex and a heliocentric radial velocity of v_hel = −96.6 ± 1.3 km s^-1 were found for NGC 6723. Alpha-element abundances present enhancements of [O/Fe] = 0.29 ± 0.18 dex, [Mg/Fe] = 0.23 ± 0.10 dex, [Si/Fe] = 0.36 ± 0.05 dex, and [Ca/Fe] = 0.30 ± 0.07 dex. Similar overabundance is found for the iron-peak Ti with [Ti/Fe] = 0.24 ± 0.09 dex. Odd-Z elements Na and Al present abundances of [Na/Fe] = 0.00 ± 0.21 dex and [Al/Fe] = 0.31 ± 0.21 dex, respectively. Finally, the s-element Ba is also enhanced by [Ba/Fe] = 0.22 ± 0.21 dex.

Conclusions. The enhancement levels of NGC 6723 are comparable to those of other metal-intermediate bulge globular clusters. In turn, these enhancement levels are compatible with the abundance profiles displayed by bulge field stars at that metallicity. This hints at a possible similar chemical evolution with globular clusters and the metal-poor of the bulge going through an early prompt chemical enrichment.