Comprehensive abundance analysis of red giants in the open clusters Stock 2, NGC 2168, 6475, 6991, and 7762

Abstract

We have analysed high-dispersion echelle spectra (R = 60000) of red giant members of five open clusters to derive abundances for many elements from Na to Eu. The [Fe/H] values are −0.06 ± 0.03 for Stock 2, −0.11 ± 0.03 for NGC 2168, −0.01 ± 0.03 for NGC 6475, 0.00 ± 0.03 for NGC 6991 and −0.07 ± 0.03 for NGC 7662. Sodium is enriched in the giants relative to the abundance expected of main sequence stars of the same metallicity. This enrichment of [Na/Fe] by about +0.25 attributed to the first dredge-up is discussed in the light of theoretical predictions and recently published abundance determinations. Abundance ratios [El/Fe] for other elements are with very few exceptions equal to those of field giants and dwarfs, i.e., [El/Fe] ≃ 0.00 for [Fe/H] ∼ 0.0. An exception is the overabundance of La, Ce, Nd and Sm in NGC 6991 but this is consistent with our previous demonstration that the abundances of these s-process products vary by about ±0.2 among clusters of the same [Fe/H], a variation found also among field giants and dwarfs.

Stellar Population Astrophysics (SPA) with the TNG: Stock 2, a little-studied open cluster with an eMSTO

“…The former is a reddening-free diagnostic whereas in the latter, extinction has been taken into account when calculating the luminosity. The location of the stars in the HR diagram is better than in the Kiel diagram, where gravities lie away with respect to those of the isochrone around 0.2 dex, as already came out in the comparison with results from Reddy & Lambert (2019). Additionally, TO stars show a large dispersion in this diagram.…”

“…The RVs obtained by us are, in general, compatible within the errors with those found in the literature, as displayed in Table 5 for stars in common with Mermilliod et al (2008), who measured RVs for red giants in open clusters, and Reddy & Lambert (2019). Although Mermilliod et al (2008) claimed binarity for g3 and g9, we have not seen any feature in their spectra that might confirm it, as also Reddy & Lambert (2019) concluded. However, given the discrepancies for the latter, perharps it might be a long-period variable.…”

“…Regarding the abundances, we compared our results (separately for MS stars and giants) to those of Reddy & Lambert (2019), with which we have 17 chemical elements in common. For the comparison, the values from Reddy & Lambert (2019) have been scaled to our solar references. In Fig.…”

“…However, despite its proximity, physical parameters for this cluster such as age or chemical composition are not precisely known. According to the literature (Stock 1956;Krzeminski & Serkowski 1967;Robichon et al 1999;Spagna et al 2009) the distance to Stock 2 ranges between 300 and 350 pc, although the most recent studies, based on the second Gaia data release, place it at about 400 pc (Cantat-Gaudin et al 2018;Reddy & Lambert 2019). The average reddening is E(B−V) ≈ 0.35, but it seems to be variable across the cluster field (Krzeminski & Serkowski 1967;Spagna et al 2009;Ye et al 2021).…”

“…On the one hand, the cluster might be coeval or slightly older than the Pleiades (100-275 Ma, e.g. Krzeminski & Serkowski 1967;Robichon et al 1999;Reddy & Lambert 2019;Ye et al 2021) but on the other hand, Sciortino et al (2000), from the analysis of the cluster X-ray luminosity function, found it to have and age similar to the Hyades (τ 625 Ma). Spagna et al (2009), based on the TO region shape and the distribution of the giants on the CMD, reported an age within the 200-500 Ma range.…”