Chemical (in)homogeneity and atomic diffusion in the open cluster M 67

Abstract

Context.The benchmark open cluster M67 is known to have solar metallicity and similar age as the Sun. It thus provides us a great opportunity to study the properties of solar twins, as well as the evolution of Sun-like stars. Aims. Previous spectroscopic studies reported to detect possible subtle changes in stellar surface abundances throughout the stellar evolutionary phase, namely the effect of atomic diffusion, in M67. In this study we attempt to confirm and quantify more precisely the effect of atomic diffusion, as well as to explore the level of chemical (in)homogeneity in M67. Methods. We presented a strictly line-by-line differential chemical abundance analysis of two groups of stars in M67: three turn-off stars and three sub-giants. Stellar atmospheric parameters and elemental abundances were obtained with very high precision using the Keck/HIRES spectra. Results. The sub-giants in our sample show negligible abundance variations (≤ 0.02 dex), which implies that M67 was born chemically homogeneous. We note there is a significant abundance difference (∼ 0.1 -0.2 dex) between sub-giants and turn-off stars, which can be interpreted as the signature of atomic diffusion. Qualitatively stellar models with diffusion agree with the observed abundance results. Some turn-off stars do not follow the general pattern, which suggests that in some cases diffusion can be inhibited, or they might suffered some sort of mixing event related to planets. Conclusions. Our results pose additional challenges for chemical tagging when using turn-off stars. In particular, the effects of atomic diffusion, which could be as large as 0.1 -0.2 dex, must be taken into account in order for chemical tagging to be successfully applied.

The chemical composition of HIP 34407/HIP 34426 and other twin-star comoving pairs

“…High-precision chemical analyses of twin-star binaries have revealed statistically-significant differences of 0.05 dex, with a couple of extreme cases where the differences amount to about 0.20 dex, 1 including the system investigated in this work. These values are comparable to the chemical abundance inhomogeneities (star-to-star scatter) found in high-precision studies ( 0.01 dex errors in relative abundances) of stars in open clusters like the Hyades (Liu et al 2016), Pleiades (Spina et al 2018), and M67 (Liu et al 2019). We must note, however, that strictly speaking the stellar pairs utilized in this type of binary studies are by definition comoving objects.…”

The Gaia-ESO survey: 3D NLTE abundances in the open cluster NGC 2420 suggest atomic diffusion and turbulent mixing are at the origin of chemical abundance variations

“…Empirical evidence of the impact of atomic diffusion on the surface chemical composition of stars is continuously emerging from careful observational studies of Galactic clusters with modern space-and ground-based astronomical facilities (e.g. Korn et al 2007;Gruyters et al 2014Gruyters et al , 2016Blanco-Cuaresma et al 2015;Husser et al 2016;Gao et al 2018;Bertelli Motta et al 2018;Souto et al 2018Souto et al , 2019Liu et al 2019). For example, it is known that the abundances of light elements (Li, Be, and B) can be depleted in MS, TO, and SGB stars (Smiljanic et al 2010;Deliyannis et al 2019;Boesgaard et al 2020), and these signatures have been linked to the effects of rotation-induced mixing, internal gravity waves, atomic diffusion, and thermohaline mixing.…”

Super-Earth ingestion can explain the anomalously high metal abundances of M67 Y2235

“…The analysis in the preceding sections shows that the latetime ingestion of one or more super-Earth planets is a plausible mechanism to explain the enhanced elemental abundances displayed by M67 Y2235 compared to the other two turn-off stars studied by Liu et al (2019). We show that two dynamical mechanisms -Lidov-Kozai cycles and planetplanet scattering -can drive planets into collision with their central star.…”

“…Any late-time ingestion of planetary material into the envelopes of solar-like stars will lead to a spread in elemental abundances, limiting the power of the technique. Whilst the sample size in Liu et al (2019), and hence this study, is small, it suggests that this is a process that should not be ignored when evaluating the potential of chemical tagging as a tool to probe the history of the Milky Way. The problems become less pronounced when considering lower-mass stars.…”

“…The analysis that we have carried out here ignores the effects of differential gravitational settling. The results of Liu et al (2019) show that, for most elements, gravitational settling provides a good description of the evolution of surface elemental abundances seen between the main sequence, turn-off and subgiant branch. Our assumption here is that, since 0.128 dex is a small abundance difference, the relative rates of settling between polluted and unpolluted stars will remain roughly the same, and hence the abundance difference will be retained through subsequent settling.…”