The discovery of a solar twin in the open star cluster M67 (Önehag et al. 2011) implies a near-solar chemical composition for the cluster. This study uses high-resolution spectroscopic data of five main-sequence stars in M67 to analyze their abundance of a few key elements and compare results to the solar-twin composition and the composition of field twins (Melendez et al. 2009). The derived composition was also compared to predictions of stellar-structure models including the effects of  element diffusion. (Richard, private communication).  It is found that all analyzed elements are, to varying degree, less abundant in the five main sequence stars than in the solar twin. With the possible exception of iron, all derived abundances also fall clearly below the diffusion predictions.

Some stars with similar properties to our sun (solar twins) have differ- ent chemical composition than the rest of the solar twins. One explanation might be planet engulfment. Therefore we did a large number of simu- lations where a disturbing star passed a sun and a planet at a distance closer than 100 AU to see how often the planet was engulfed. The result was that the planet in most cases was thrown out of the system, but it was engulfed in about 10 − 30% of the simulations when the planet was close to its star. The conclusion was that planet engulfment indeed can be a good explanation for the different chemical compositions of solar twins, at least in dense stellar clusters where such close passages should be quite common. / Vissa solliknande stja ̈rnor (s ̊a kallade soltvillingar) har en annorlunda kemisk sammansa ̈ttning ja ̈mfo ̈rt med resten av soltvillingarna. En mo ̈jlig fo ̈rklaring kan vara att dessa stja ̈rnor har slukat planeter. Da ̈rf ̈or gjorde vi ett stort antal simuleringar d ̈ar en sto ̈rande stja ̈rna passerade ett sys- tem best ̊aende av solen och en planet. Stj ̈arnan passerade p ̊a ett avst ̊and under 100 AU och vi observerade hur ofta planeten slukades. Resultatet var att planeten oftast slungades ut i rymden, men den slukades i 10-30% av simuleringarna da ̈r planeten kretsade na ̈ra sin stja ̈rna. Slutsatsen vara att planetslukning kan vara en rimlig f ̈orklaring till de annorlunda kemiska sammansa ̈ttningarna hos soltvillingar, ̊atminstone i ta ̈ta stja ̈rnhopar da ̈r na ̈ra stja ̈rnpassager bo ̈r vara ganska vanliga.

The open cluster Messier 67 is known to have chemical composition, metallicity and age (~ 4 Gyr) close to the Sun. Therefore, it is advantageous for stellar physical studies and of stellar evolution, in particular for solar like stars within the cluster. This work considers three such stars, the formerly studied solar twin M67-1194 and two more recently suggested solar twins M67-1787 & 2018. Most solar twins show a ratio of volatile to refractory elements that systematically depart from the Sun’s. Our targets do not follow this trend as closely. Their composition is closer to the Sun and they are, therefore, exquisite targets for studies of stellar evolution within the cluster. However, their solar likeness also provides studies regarding the origin and evolution of the Solar system. The stellar ages of the solar twins are established through a chemical clock [Y/Mg] and via stellar isochrones from BaSTI. The latter age assessment of the solar twins is supplemented with the analysis of two subgiant stars M67-1442 & 1844. We approach the isochrone-based method using spectroscopically, astrometrically and photometrically derived parameters. The different ages of the stars and methods thus estimate the age of the cluster itself. The chemical ages of the stars suggest a cluster age of 4.56  ±0.44 Gyr and the isochrone-based estimates suggests a cluster age within the range 3.30-5.51 Gyr. Our results thus affirm and imply a near solar age of the cluster.

A solar twin is a star with characteristics very similar to those of the Sun. This is, a Sun-like star. Earlier studies show that M67-1194 is one of the best solar twins found to date. But this star is not exactly like the Sun. There exist differences between them, in terms of chemical composition, that were defined as well. The goal of this study is to apply a new method to study the similarities of the mentioned star with the Sun. Hopefully a method that provides more precise results. The method uses difference spectra, result of subtracting the spectrum of the star from the solar spectrum, instead of the regular spectra. Then the question is: can this method cast more precise results than those obtained by Önehag et al. in 2011? The answer is that it does. The earlier study gave an uncertainty of ±0.02 dex and ours gave uncertainties between ±0.01 dex and ±0.02 dex. Nevertheless, this conclusion must be taken with caution as the study is not as conclusive as the one out carried by Önehag et al.. / En soltvilling är en stjärna som liknar solen väldigt mycket. Tidigare studier visar att M67-1194 är en av de bästa upptäckta soltvillingarna hittills. Men stjärnan är inte exakt som solen. Det finns skillnader mellan dem angående deras kemiska sammansättningar vilket har också visats i tidigare studier. Den här studiens syfte är att tillämpa en ny metod för att granska likheterna mellan den nämnda stjärnan och solen. En metod som förhoppningsvis ger noggrannare resultat. Metoden använder differensspektra som erhålls genom att subtrahera stjärnans spektrum från solens spektrum, i stället för att använda de vanliga spektra. Frågan är om den här nya metoden kan ge noggrannare resultat än dem som Önehag et al. fick 2011. Svaret är ja. Den tidigare studien visade en osäkerhet på ±0,02 dex medan vår gav osäkerheter mellan ±0,01 dex och ±0,02 dex. Icke desto mindre måste våra slutsatser tas försiktigt ty vår studie är inte lika säker som Önehags et al..