TheGaia-ESO Survey: Sodium and aluminium abundances in giants and dwarfs

Abstract

Context. Stellar evolution models predict that internal mixing should cause some sodium overabundance at the surface of red giants more massive than ∼1.5-2.0 M . The surface aluminium abundance should not be affected. Nevertheless, observational results disagree about the presence and/or the degree of Na and Al overabundances. In addition, Galactic chemical evolution models adopting different stellar yields lead to very different predictions for the behavior of [Na/Fe] and [Al/Fe] versus [Fe/H]. Overall, the observed trends of these abundances with metallicity are not well reproduced. Aims. We readdress both issues, using new Na and Al abundances determined within the Gaia-ESO Survey. Our aim is to obtain better observational constraints on the behavior of these elements using two samples: i) more than 600 dwarfs of the solar neighborhood and of open clusters and ii) low-and intermediate-mass clump giants in six open clusters. Methods. Abundances were determined using high-resolution UVES spectra. The individual Na abundances were corrected for nonlocal thermodynamic equilibrium effects. For the Al abundances, the order of magnitude of the corrections was estimated for a few representative cases. For giants, the abundance trends with stellar mass are compared to stellar evolution models. For dwarfs, the abundance trends with metallicity and age are compared to detailed chemical evolution models. Results. Abundances of Na in stars with mass below ∼2.0 M , and of Al in stars below ∼3.0 M , seem to be unaffected by internal mixing processes. For more massive stars, the Na overabundance increases with stellar mass. This trend agrees well with predictions of stellar evolutionary models. For Al, our only cluster with giants more massive than 3.0 M , NGC 6705, is Al enriched. However, this might be related to the environment where the cluster was formed. Chemical evolution models that well fit the observed [Na/Fe] vs. [Fe/H] trend in solar neighborhood dwarfs cannot simultaneously explain the run of [Al/Fe] with [Fe/H], and vice versa. The comparison with stellar ages is hampered by severe uncertainties. Indeed, reliable age estimates are available for only a half of the stars of the sample. We conclude that Al is underproduced by the models, except for stellar ages younger than about 7 Gyr. In addition, some significant source of late Na production seems to be missing in the models. Either current Na and Al yields are affected by large uncertainties, and/or some important Galactic source(s) of these elements has as yet not been taken into account.

Multiple Stellar Populations and Their Evolution in Globular Clusters: A Nucleosynthesis Perspective

“…Denissenkov 2010; Denissenkov and Merryfield 2011;Traxler et al 2011;Brown et al 2013;Garaud and Brummell 2015), it should not reach deep and hot enough layers to induce O and Na variations in RGB stars. This is in agreement with the fact that no sign of sodium and oxygen abundance variations could be found at that phase in field and open cluster stars (Gratton et al, 2000;Smiljanic et al, 2016) nor in GCs 4 . Additionally, low-mass red giant branch stars never reach internal temperatures high enough to activate the Mg-Al chain (see § 5 and Fig.…”

The Evolution of the Inner Regions of Protoplanetary Disks

“…Out of 557 stars with optical photometry (V, I c ) and IRAC photometry in all bands (Hernández et al 2008), we compile 125 stars with effective temperatures lower than 5000 K (e.g. spectral type K0 or later; Pecaut & Mamajek 2013) reported by Spina et al (2014), Frasca et al (2015) or Smiljanic et al (2016). From the sample of 125 stars, 11 do not have reported spectral types or effective temperature, thus we estimate their stellar mass using the [V − I c ] vs stellar mass relation from Prisinzano et al (2016), then we estimate their effective temperature from the relation T eff = 1005.78 M * + 3042.1 (Frasca et al 2015).…”

Chemical Abundances of Main-sequence, Turnoff, Subgiant, and Red Giant Stars from APOGEE Spectra. I. Signatures of Diffusion in the Open Cluster M67

“…This is a possible effect for the abundances in this study, as the targets cover a large range in T eff -log g parameter space, although M67 has solar metallicity and departures from LTE are expected to be more significant for red giants in the metal-poor regime (e.g., Asplund 2005; Asplund et al 2009). Several studies in the literature have investigated non-LTE effects for lines in the optical (e.g., Korn et al 2007;Andrievsky et al 2008;Lind et al 2011;Bergemann et al 2012;Osorio & Barklem 2016;Smiljanic et al 2016); however, to date, few non-LTE studies have investigated the behavior of transitions in the H band and, in particular, in the APOGEE region. Cunha et al (2015) presented non-LTE abundance corrections for Na I lines in the APOGEE region for stars in the RC and on the RGB in the very metal-rich ([Fe/H]=+0.35) open cluster NGC 6791; the departures from LTE were found to be minimal.…”