A spectroscopic study of the open cluster NGC 6475 (M 7)

Abstract

Aims. Clusters of stars are key objects for studying the dynamical and chemical evolution of the Galaxy and its neighbors, and are the most important laboratories to test the theory of stellar evolution. In particular, chemical composition is obtained from different kinds of stars (hot main-sequence stars, cool main-sequence stars, horizontal-branch stars, RGB stars) using different methodologies. Our first aim is to apply these methodologies to the stars of the open cluster NGC 6475 and, by obtaining a census of the most important elements, we will be able to test their consistency. Our second aim is to study the evolution of the surface chemical abundances as a function of the evolutionary phase of a star. We finally want to establish more robust fundamental parameters for this cluster. Methods. We selected high S/N high resolution spectra of 7 stars of the open cluster NGC 6475 from the ESO database covering the T eff range 4500-10 000 K and of luminosity class V (dwarf) and III (giants). We determined the chemical abundances of several elements. For hot stars (T eff > 9000 K), we applied the Balmer line fitting method to obtain atmospheric parameters, while for cool stars (T eff < 6500 K), the abundance equilibrium of FeI/II lines. For the two groups of stars, the use of different line-lists was mandatory. LTE approximation together with NLTE correction for some elements (C, N, O, Na, Mg) were applied. The abundances of many elements were obtained by measuring of the equivalent width of spectral lines. For those ones having only blended lines (O, He), real spectra were compared to synthetic ones. Hyperfine structure was taken into account for V and Ba. Results. First of all, we showed that the two methodologies we used give abundances that agree within the errors. This implies that no appreciable relative systematic effects are present for the derived chemical content of cool and hot stars. On the other hand, giants stars show clear chemical peculiarities with respect to the dwarfs affecting light elements (up to Si) and maybe Ba. This can be explained as an evolutionary effect. Then, once we had a new estimation of the metallicity for the cluster ([Fe/H] = +0.03 ± 0.02, [α/Fe] = −0.06 ± 0.02), we fitted suitable isochrones to the CMD of the cluster to obtain the basic parameters (E(B − V) = 0.08 ± 0.02, (m − M) 0 = 7.65 ± 0.05, Age = 200 ± 50).

On the fine structure of the Cepheid metallicity gradient in the Galactic thin disk

“…From left to right the columns give the cluster ID, the Galactocentric distance, the iron abundance available in the literature, the metallicity rescaled to current solar abundance, the references for the metallicity and the references for ages and distances. The acronyms given in Col. 7 refers to SF: (Salaris et al 2004;Friel 1995), C (Carraro group): (Carraro et al 2002(Carraro et al , 2005aMoitinho et al 2006;Villanova et al 2007Villanova et al , 2009), B (BOCCE): (Gratton & Contarini 1994;Bragaglia et al 2001;Kalirai & Tosi 2004;Bragaglia et al 2006;Carretta et al 2004Carretta et al , 2005Carretta et al , 2007, W: (WEBDA), and YON: (Yong et al 2012). Notes.…”

Bimodality of light and s-elements in M4 (NGC 6121)

“…The linelists for the chemical analysis were obtained from many sources (Gratton et al 2003, VALD & NIST 1 ;McWilliam & Rich 1994;McWilliam 1998, SPECTRUM 2 , and SCAN 3 ), and calibrated using the Solar-inverse technique by the spectral synthesis method (see Villanova et al 2009, for more details). For this purpose we used the high resolution, high S /N NOAO Solar spectrum (Kurucz et al 1984).…”

Abundances and physical parameters for stars in the open clusters NGC 5822 and IC 4756

“…Sodium and aluminium have been found by several authors to be overabundant in giants belonging to several open clusters, for instance IC 4756, NGC 6939 and NGC 714 (Jacobson et al 2007), NGC 6475 (Villanova et al 2009), Collinder 261 (Friel et al 2003), Berkeley 17 (Friel et al 2005), and Saurer 1 and Berkeley 29 (Carraro et al 2004). For some other clusters, only sodium overabundances were inferred in giants: IC 4651 , NGC 7789 and M 67 (Tautvaišienė et al 2000(Tautvaišienė et al , 2005Randich et al 2006), and NGC 1817 and NGC 2141 (Jacobson et al 2009).…”