Solar twins in M 67

Abstract

Context. The discovery of true solar analogs is fundamental to a better understanding of the Sun and of the solar system. Despite a number of efforts, this search has brought only limited results for field stars. The open cluster M 67 offers a unique opportunity to search for solar analogs, because its chemical composition and age are very similar to those of the Sun. Aims. We analyse FLAMES spectra of a large number of M 67 main sequence stars to identify solar analogs in this cluster. Methods. We first determined cluster members that are not likely binaries, by combining proper motions and radial velocity measurements. We concentrate our analysis on determining stellar effective temperature, using analyses of line-depth ratios and Hα wing and making a direct comparison to the solar spectrum obtained with the same instrument. We also computed the lithium abundance for all the stars. Results. Ten stars have the temperature derived both by line-depth ratios and by the Hα wings within 100 K from the Sun. From these stars we derive, assuming a cluster reddening E(B − V) = 0.041, the solar color (B − V) = 0.649 ± 0.016 and a cluster distance modulus of 9.63. Five stars are most similar (within 60 K) to the Sun and candidates to be true solar twins. These stars also have a low Li content, comparable to the photospheric abundance of the Sun, likely indicating a similar mixing evolution. Conclusions. We find several candidates for the best solar analogs ever. These stars are amenable to further spectroscopic investigations and planet searches. The solar colors are determined with fairly high accuracy with an independent method, as is the cluster distance modulus.

Climbing the cosmic ladder with stellar twins

“…The member stars with twins of Hipparcos are plotted with red colour in the colour-magnitude diagrams of Fig. 7 as taken from WEBDA 3 combined with Pasquini et al (2008) for the twins found in M67.…”

“…8 represents the median of the distances obtained for each star individually, the dashed lines represent the values reported in the literature (Twarog, Ashman & Anthony-Twarog 1997;Kharchenko & et al 2013;Güneş, Karataş & Bonatto 2012;Pasquini et al 2008). Not all literature studies listed here report distances for both clusters.…”

“…Not all literature studies listed here report distances for both clusters. Pasquini et al (2008) determined the distance only for M67 while Güneş, Karataş & Bonatto (2012) only for NGC2360. In Fig.…”

“…8 we show in the legend the agreement with Twarog, Ashman & Anthony- Twarog (1997), who analysed both clusters and agrees with us better than 3%. Note that all literature values except that one of Pasquini et al (2008) determined the distance using evolutionary models by fitting the main-sequence to the colour-magnitude diagram of the cluster. Our method is more similar to Pasquini et al (2008), which considered the averaged magnitude of ten solar analogs in M67 and the absolute magnitude of the Sun to compute the distance modulus.…”

“…Note that all literature values except that one of Pasquini et al (2008) determined the distance using evolutionary models by fitting the main-sequence to the colour-magnitude diagram of the cluster. Our method is more similar to Pasquini et al (2008), which considered the averaged magnitude of ten solar analogs in M67 and the absolute magnitude of the Sun to compute the distance modulus. The agreement with Pasquini et al (2008) is remarkable.…”