The Gaia-ESO Survey and the stellar chemical evolution. Published on MNRAS the study: “The Gaia-ESO Survey: evidence of atomic diffusion in M67?” of C. Bertelli Motta (ZAH)

It has been suggested that stars formed from the same molecular cloud should share the same primordial chemical abundances. This has led several authors to consider the possibility that stars formed in a star forming region now disappeared could be identified by studying their chemical composition (chemical tagging theory, Freeman & Bland-Hawthorn 2002). However, the evolution of chemical abundances of stars makes this approach difficult.

 

During their main sequence, in fact, in low-mass stars chemical elements sink toward stellar interior under the effect of gravity, with the radiative acceleration working against this process, called atomic diffusion (Michaud, Alecian & Richer 2015). The efficiency of atomic diffusion increases with stellar mass, and it decreases surface abundances during stellar evolution. When stars leave the main sequence becoming giants (turn-off, which is the age at which stars with a given mass leave the main sequence), the depth of the convective zone increases, making the chemical mixing more efficient. As a consequence, during the giant phase the surface chemical abundances increase again becoming similar to primordial values.

 

In the study: “The Gaia-ESO Survey: evidence of atomic diffusion in M67?” of C. Bertelli Motta (Zentrum für Astronomie der Universität Heidelberg), recently published on the Monthly Notices of the Royal Astronomical Society with the collaboration of astronomers of the INAF – Astronomical Observatory of Palermo, the surface chemical abundances of the stars belonging to the stellar cluster M67 are calculated using the high-resolution spectra obtained in the frame of the Gaia-ESO Survey. With an age of 3.75-4 billion years, M67 is a perfect target to study the evolution of stellar chemical abundances. In fact, this cluster host both old main sequence solar types stars and few giant stars. The authors have calculated the stellar abundances of several elements (C, O, Mg, Al, Na, Si, Ca, Ti, Cr, Mn, Fe, and Ni), showing that they decrease with stellar mass and with the proximity with the turn-off. Besides, they have shown that the members of M67 in the giant phase have similar abundances of stars in the young cluster NGC 6633, whose abundances are at primordial values. Taken together, these results indicate that atomic diffusion is at work in M67.

 

The figure (link) shows an optical image of M67