TheGaia-ESO Survey: impact of extra mixing on C and N abundances of giant stars

Abstract

Context. The Gaia-ESO Public Spectroscopic Survey using FLAMES at the VLT has obtained high-resolution UVES spectra for a large number of giant stars, allowing a determination of the abundances of the key chemical elements carbon and nitrogen at their surface. The surface abundances of these chemical species are known to change in stars during their evolution on the red giant branch (RGB) after the first dredge-up episode, as a result of the extra mixing phenomena. Aims. We investigate the effects of thermohaline mixing on C and N abundances using the first comparison between the Gaia-ESO survey [C/N] determinations with simulations of the observed fields using a model of stellar population synthesis. Methods. We explore the effects of thermohaline mixing on the chemical properties of giants through stellar evolutionary models computed with the stellar evolution code STAREVOL. We include these stellar evolution models in the Besançon Galaxy model to simulate the [C/N] distributions determined from the UVES spectra of the Gaia-ESO survey and to compare them with the observations.Results. Theoretical predictions including the effect of thermohaline mixing are in good agreement with the observations. However, the field stars in the Gaia-ESO survey with C and N abundance measurements have a metallicity close to solar, where the efficiency of thermohaline mixing is not very large. The C and N abundances derived by the Gaia-ESO survey in open and globular clusters clearly show the impact of thermohaline mixing at low metallicity, which explains the [C/N] value observed in lower mass and older giant stars. Using independent observations of carbon isotopic ratio in clump field stars and open clusters, we also confirm that thermohaline mixing should be taken into account to explain the behaviour of 12 C/ 13 C as a function of stellar age. Conclusions. Overall, the current model including thermohaline mixing is able to reproduce very well the C and N abundances over the whole metallicity range investigated by the Gaia-ESO survey data.

Lithium in red giant stars: Constraining non-standard mixing with large surveys in the Gaia era

“…Denissenkov 2010b; Traxler et al 2011;Sengupta & Garaud 2018). The same coefficient also accounts for the observed behaviour of the carbon isotopic ratio and of the abundances of C and N in the upper part of the red giant branch in Population I and II stars (Charbonnel & Zahn 2007a;Charbonnel & Lagarde 2010;Lagarde et al 2012), and for the C and N abundances of giant stars investigated in the Gaia-ESO survey (Lagarde et al 2019). It also leads to a significant reduction of the production of 3 He in low-mass stars, in better agreement with the behaviour of the evolution of this light element in the Galaxy (Charbonnel & Zahn 2007a;Lagarde et al 2011).…”

Photometric characterization of multiple populations in star clusters: the impact of the first dredge-up

“…In a chromosome map the total width of the cluster RGB in one of these pseudocolors is normalized to the value taken two magnitudes above the main sequence turnoff in the F814W filter (Milone et al 2017). This is a level where typically the FDU has either already started or is essentially completed, but still below the RGB bump, beyond which extra mixing processes that further affect the surface C and N abundances appear to be efficient, at least in low mass stars (see, e.g., Lagarde et al 2019, and references therein).…”

Haydn

“…Moreover, the use of stellar photospheric abundances in studies of galactic chemical evolution relies on our understanding of internal mixing, as surface abundances may have changed compared to the initial value due to nuclear processes, and brought to the surface by internal mixing. Our limited understanding of such processes also affects the robustness of spectroscopic proxies of stellar ages like [C/N] [42,128,135,138], which is based on how the efficiency of transport processes in convective and radiative regions depend on the mass and metallicity (hence age) of red giant stars [122,191].…”