Lithium in M 67: From the main sequence to the red giant branch

Abstract

Context.Lithium abundances in open clusters are a very effective probe of mixing processes, and their study can help us to understand the large depletion of lithium that occurs in the Sun. Owing to its age and metallicity, the open cluster M 67 is especially interesting on this respect. Many studies of lithium abundances in M 67 have been performed, but a homogeneous global analysis of lithium in stars from subsolar masses and extending to the most massive members, has yet to be accomplished for a large sample based on high-quality spectra. Aims. We test our non-standard models, which were calibrated using the Sun with observational data. Methods.We collect literature data to analyze, for the first time in a homogeneous way, the non-local thermal equilibrium lithium abundances of all observed single stars in M 67 more massive than ∼0.9 M . Our grid of evolutionary models is computed assuming a non-standard mixing at metallicity [Fe/H] = 0.01, using the Toulouse-Geneva evolution code. Our analysis starts from the entrance into the zero-age main-sequence. Results. Lithium in M 67 is a tight function of mass for stars more massive than the Sun, apart from a few outliers. A plateau in lithium abundances is observed for turn-off stars. Both less massive (M ≤ 1.10 M ) and more massive (M ≥ 1.28 M ) stars are more depleted than those in the plateau. There is a significant scatter in lithium abundances for any given mass M ≤ 1.1 M . Conclusions. Our models qualitatively reproduce most of the features described above, although the predicted depletion of lithium is 0.45 dex smaller than observed for masses in the plateau region, i.e. between 1.1 and 1.28 solar masses. More work is clearly needed to accurately reproduce the observations. Despite hints that chromospheric activity and rotation play a role in lithium depletion, no firm conclusion can be drawn with the presently available data.

No X-rays from WASP-18

“…Compared to the values reported by Takeda et al (2013) and Pace et al (2012), WASP-18 has a slightly higher Li abundance. Lithium depletion is due to the convective mixing during the main sequence life of a star, so Li abundance is a rough indicator of youth in solar type stars with deep convective zone.…”

“…However, the scenario becomes more puzzling when considering that WASP-18 shows Li absorption at 6708 Å, with a value typical of, or even stronger than values found in F-late stars of Hyades (Takeda et al 2013) and M 67 (Pace et al 2012). Figure 3 shows a portion of the spectrum of WASP-18 around the Li doublet lines at 6708 Å, and Fig.…”

“…Figure 3 shows a portion of the spectrum of WASP-18 around the Li doublet lines at 6708 Å, and Fig. 4 shows the Li equivalent widths (EWs) vs. T eff given by Pace et al (2012) and Takeda et al (2013) for Hyades and the older M 67 cluster. For comparison we show the FEROS spectrum of τ Boo, which has no Li absorption.…”

Li depletion in solar analogues with exoplanets

“…This spread has been noticed in previous works, such as Pace et al (2012), where they show how Li abundances do not depend on mass for solar type stars (one solar mass or lower) in M67, and they suggest that an extra variable, in addition to mass, age, and metallicity, is responsible for the scatter in Li abundances, since those stars are very similar. A similar spread in Li is observed for field stars around one solar mass and −0.2 < [Fe/H] < 0 in Lambert & Reddy (2004).…”

“…Chaboyer et al 1995;Charbonnel & Talon 2005), that account for the destruction of Li observed in solar-type stars in open clusters and in the field (e.g. Pace et al 2012). As seen in Fig.…”