Searching for Li-rich giants in a sample of 12 open clusters

Abstract

Aims. The aim of this work is to search for Li-rich giants in a sample of clusters where planets have been searched, thus we can study the planet engulfment scenario to explain Li replenishment using a proper comparison sample of stars without detected giant planets. Methods. We derived Li abundances for a sample of 67 red giant stars in 12 different open clusters using standard spectral synthesis techniques and high-resolution spectra (from HARPS and UVES). We also determined masses, ages, and radius from PARSEC stellar isochrones to constrain the evolutionary stage of these stars. Results. We found three stars in different clusters with clearly enhanced Li abundances compared to other stars within the cluster. Interestingly, the only two stars with a detected substellar companion in our sample belong to that group. One of the planet hosts, NGC 2423 No. 3, might lie close to the luminosity bump on the HR diagram, a phase where Li production by the Cameron-Fowler process is supported by extra-mixing to bring fresh Li up to the surface. On the other hand, NGC 4349 No. 127 is a more massive and more evolved giant that does not seem to be in the evolutionary phase where other Li-rich stars are found. We discuss the possibility that the Li enhancement of this star is triggered by the engulfment of a planet, considering that close-in planets hardly survive the RGB tip and the early AGB phases.

Planets around evolved intermediate-mass stars

“…1 we show the HR diagram for this cluster. We use PARSEC isochrones (Bressan et al 2012) to derive masses and ages (see Table B.2 of Delgado Mena et al 2016). The isochrone which better matches our stars has an age of 1.41 Gyr, slightly higher than the 1.14 Gyr provided by the WEBDA database 1 .…”

“…In total, 142 stars were monitored within 17 open clusters using the HARPS spectrograph at the ESO 3.6m telescope (R ∼ 115000). The stellar parameters, namely the effective temperature (T eff ), surface gravity (log g), metallicity ([Fe/H]), and microturbulence (ξ t ) for most of the clusters were presented in Santos et al (2009Santos et al ( , 2012 and improved upon in Delgado Mena et al (2016) (by using higher S/N data and a Fe linelist optimized for cool stars) together with the derivation of stellar ages, masses and radii. In this paper we will focus on the results for three stars in the clusters IC4651, NGC2423 and NGC4349.…”

“…In this paper we will focus on the results for three stars in the clusters IC4651, NGC2423 and NGC4349. The parameters for those targets are summarized in Table 1 Table B.1 of Delgado Mena et al 2016). In Fig.…”

“…9122 is -30.26 km s −1 , thus this star is likely a cluster member. We estimated a v sin i of 3.89 km s −1 in 1 http://www.univie.ac.at/webda/ Delgado Mena et al (2016) for a fixed macroturbulence velocity of 2.36 km s −1 (with the empirical formula by Valenti & Fischer 2005). This leads to a maximum rotational period of ∼115 days.…”

“…We note that we have also added here an error of 15 m s −1 before fitting the data 6 . By using the updated stellar mass (2.26±0.07 M ) from Delgado Mena et al (2016) we derive a planetary mass m 2 sin i = 9.6 M J (see Table 2). The period 6 In Paper I an error of 20 m s −1 was added, which was shown later to be overestimated, thus we decided to use here 15 m s −1 as done for the previous star.…”