Open clusters as key tracers of Galactic chemical evolution

Abstract

Context. Galactic open clusters are since long recognized as one of the best tools for investigating the radial distribution of iron and other metals. Aims. We employed FLAMES at VLT to collect UVES spectra of bright giant stars in a large sample of open clusters, spanning a wide range of Galactocentric distances, ages, and metallicities. We present here the results for four clusters: Berkeley 20 and Berkeley 29, the two most distant clusters in the sample; Collinder 261, the oldest and the one with the minimum Galactocentric distance; Melotte 66. Methods. Equivalent width analysis was carried out using the spectral code MOOG and Kurucz model atmospheres to derive abundances of Fe, Al, Mg, Si, Ca, Ti, Cr, Ni, Ba; non-LTE Na abundances were derived by direct line-profile fitting. Results. We obtain subsolar metallicities for the two anticenter clusters Be 20 ([Fe/H] = −0.30, rms = 0.02) and Be 29 ([Fe/H] = −0.31, rms = 0.03), and for Mel 66 ([Fe/H] = −0.33, rms = 0.03), located in the third Galactic quadrant, while Cr 261, located toward the Galactic center, has higher metallicity ([Fe/H] = +0.13, rms = 0.05 dex). The α-elements Si, Ca and Ti, and the Fe-peak elements Cr and Ni are in general close to solar; the s-process element Ba is enhanced. Non-LTE computations of Na abundances indicate solar scaled values, suggesting that the enhancement in Na previously determined in giants in open clusters could be due to neglected non-LTE effects. Conclusions. Our results support the presence of a steep negative slope of the Fe radial gradient up to about 10-11 kpc from the Galactic center, while in the outer disk the [Fe/H] distribution seems flat. All the elemental ratios measured are in very good agreement with those found for disk stars of similar metallicity and no trend with Galactocentric distance seems to be present.

The Ca ii triplet in red giant spectra: [Fe/H] determinations and the role of [Ca/Fe]

“…It has not been considered further. The mean velocity of the 17 stars classified as members is 19.8 ± 0.9 km s −1 (std error of mean) which agrees well with the values of 20.7 ± 0.6 and 22.1 ± 0.9 km s −1 given by Sestito et al (2008) and Carlberg (2014), respectively. The ten M67 stars observed are all known to be cluster members and the current data support that classification.…”

“…As for the balance of the sample, eight of the nine stars also have velocities and line strengths consistent with membership. The final star is Mel66-W1580 (WEBDA id number), which Sestito et al (2008) indicate is a fast rotator. The radial velocity determined here is 50.9 ± 2.0 km s −1 while that given by Carlberg (2014) is -14 ± 17 km s −1 ; the star is probably a binary.…”

The evolution of the Milky Way: new insights from open clusters

“…However, not all the investigators have arrived at similar measures of the radial metallicity gradient which may be partly related to the different methods adopted (see for example, Magrini et al 2009;Heiter et al 2014). Our current understanding suggests a gradient of about −0.06 dex kpc −1 (Friel et al 2002;Pancino et al 2010) to −0.20 dex kpc −1 (Frinchaboy et al 2013) in the radial range 5 to 10 kpc with a nearly flat trend (−0.02 dex kpc −1 ) beyond 13 kpc (Carraro et al 2007;Sestito et al 2008;Pancino et al 2010;Yong et al 2012;Frinchaboy et al 2013;Cantat-Gaudin et al 2016).…”

“…In recent years, modern high-resolution spectrographs and large reflectors have provided high-quality spectra of stars in OCs for secure measures of abundances, and extended abundance estimates to distant OCs in the direction of Galactic anti-centre (Carraro et al 2007;Yong et al 2005Yong et al , 2012Sestito et al 2006Sestito et al , 2008. However, not all the investigators have arrived at similar measures of the radial metallicity gradient which may be partly related to the different methods adopted (see for example, Magrini et al 2009;Heiter et al 2014).…”

Open clusters towards the Galactic centre: chemistry and dynamics

“…More recently, with the advent of high-resolution spectrographs on 8m-class telescopes, it has become possible to derive more secure [Fe/H] estimates, and also to extend metallicity determinations to OCs in the outer disc, beyond 15 kpc from the Galactic centre (Carraro et al 2004;Carretta et al 2004;Yong et al 2005;Sestito et al 2006Sestito et al , 2008. These studies have confirmed the steep slope in the gradient for 7 kpc < R GC < 11 kpc and have shown that the distribution becomes flatter for Galactocentric distances (R GC ) above 11−12 kpc (cf.…”