WIYN Open Cluster Study. LXXVI. Li Evolution Among Stars of Low/Intermediate Mass: The Metal-deficient Open Cluster NGC 2506

Abstract

HYDRA spectra of 287 stars in the field of NGC 2506 from the turnoff through the giant branch are analyzed. With previous data, 22 are identified as probable binaries; 90 more are classified as potential non-members. Spectroscopic analyses of ∼60 red giants and slowly rotating turnoff stars using line equivalent widths and a neural network approach lead to [Fe/H] = -0.27 ± 0.07 (s.d.) and [Fe/H] = -0.27 ± 0.06 (s.d.), respectively. Li abundances are derived for 145 probable single-star members, 44 being upper limits. Among turnoff stars outside the Li-dip, A(Li) = 3.04 ± 0.16 (s.d.), with no trend with color, luminosity, or rotation speed. Evolving from the turnoff across the subgiant branch, there is a well-delineated decline to A(Li) ∼1.25 at the giant branch base, coupled with the rotational spindown from between ∼20 and 70 km s −1 to less than 20 km s −1 for stars entering the subgiant branch and beyond. A(Li) remains effectively constant from the giant branch base to the red giant clump level. A new member above the clump redefines the path of the first-ascent red giant branch; its Li is 0.6 dex below the first-ascent red giants. With one exception, all post-He-flash stars have upper limits to A(Li), at or below the level of the brightest first-ascent red giant. The patterns are in excellent qualitative agreement with the model predictions for low/intermediate-mass stars which undergo rotation-induced mixing at the turnoff and subgiant branch, first dredge-up, and thermohaline mixing beyond the red giant bump.

Li in Open Clusters: Cool Dwarfs in the Young, Subsolar-metallicity Cluster M35 (NGC 2168)*

“…It is predicted that because of the higher [Fe/H], the Hyades/Praesepe stars likely formed with a higher primordial Li abundance relative to M35. As discussed in Anthony- Twarog et al (2018), the cluster data of Cummings (2011) produce a relation for the primordial cluster A(Li) between [Fe/H] = -0.2 and +0.1 with a slope of 0.96. If M35 is more metal-poor by 0.30 (0.15) dex, the dark green symbols should be shifted down by 0.29 (0.14) dex.…”

Light Elements in the Universe

“…This program has started about 20 years ago and the results for a number of open clusters have been published during the years, following the evolution of Li from the MS to the tip of the red giant branch. The observed clusters cover a large range in metallicity but are mostly older Twarog et al, 2018b). Specifically, the existence of the Li dip has been confirmed in many clusters; additionally, the new data suggested that the blue-, warm-, or high-mass side of the Li dip is very sharp (they defined it as the "cliff") and its position (magnitude or temperature) does not appear to change with the cluster age; the mass at which the cliff is seen is determined by the cluster metallicity, with higher masses for more metal-rich clusters.…”

Do Hertzsprung‐gap stars show any chemical anomaly?

“…Even so, we are not sure whether such a more sophisticated modeling with noncanonical mixing is really superior to the simple classical theory. For example, Anthony‐Twarog et al () reported based on their Li abundance study of 287 low‐to‐intermediate mass stars in the open cluster NGC 2506 that the observed abundance trend agrees with the prediction from theoretical models including rotational + thermohaline mixing. On the other hand, Smiljanic et al () argued in their spectroscopic investigation on the C, N, O, Na, and Al abundances of 20 intermediate‐mass red giants in 10 open clusters that models with rotational mixing tend to overestimate the mixing effects and thus not preferable.…”