On the APOGEE DR14 sodium spread in the Galactic open cluster NGC188

Abstract

[Na/Fe] is a flagship among chemical abundances as a good indicator of the existence of multiple populations in globular clusters, with spreads of up to 0.6 dex and more; no globular cluster having a null spread (Gratton et al. 2004; Bastian & Lardo 2018). As far as Galactic open clusters are considered, no multiple populations have been found yet (see, Bragaglia et al. 2012; Carrera & Mart´ınez-V´azquez 2013). Enter NGC 188 (age = 6.3±0.2 Gyr, Bonatto 2019). I made use of the publicly available Sloan Digital Sky Survey IV, in particular of APOGEE DR14 (Abolfathi et al. 2018) through the APOGEE Stellar Parameters and Abundances pipeline (ASPCAP, Garc´ıa P´erez et al. 2016), to search for stellar parameters and element abundances of stars in the field of NGC 188. As a result, I gathered 758 stars distributed within a radius of ∼ 1.6◦ from the cluster center and with 2MASS Ks mag down to ∼ 12.5 mag. In order to disentangle bonafide cluster members from field stars I used the diagnostic plane [Fe/H] vs. RV (see Fig. 1), which clearly highlights a group of stars tightly distributed around the known cluster with RV = 42.4±0.1 km/s (Chumak et al. 2010) and metal content 0.12 < [Fe/H] (dex) < 0.20 (Bonatto 2019)). They are red giant stars with 4100 . Tef f (K) . 5000 and 1.5 . log(g) . 3.5, distributed within a circle with a radius of 250 , which is nearly half of the cluster tidal radius (44.780 , Wang et al. 2015).