Candidate Solar Sibling HD 162826: Color-Magnitude Position Compared to Sun and M67
Color-magnitude position (intrinsic B-V color vs. absolute magnitude) for candidate solar sibling HD 162826, the Sun, and M67 open cluster. HD 162826 is mentioned as a candidate solar sibling by Ramirez et al. 2014 (http://arxiv-web3.library.cornell.edu/abs/1405.1723). Color is from Hipparcos catalog, and absolute magnitude is calculated using V magnitude from Hipparcos catalog and parallax from van Leeuwen (2007). The Sun's color is from Ramirez et al. (2012; ApJ, 752, 5; B-V = 0.653+-0.003) and absolute magnitude is taken from the author's notes on the website https://sites.google.com/site/mamajeksstarnotes/basic-astronomical-data-for-the-sun. I plot stars from the open cluster M67 for comparison as it is probably the best-studied open cluster with an age (~4 Gyr) similar to that of the Sun and HD 162826. BV photometry for M67 members is from Sandquist (2004; MNRAS, 347, 101). I deredden the M67 stars assuming E(B-V) = 0.038 (VandenBerg & Steson 2004; PASP 116, 997) and estimating V-band extinction Av = 0.128 mag, and adopt the distance modulus (9.60 mag) from Sandquist (2004). The red lines are 3.8 Gyr and 4.568 Gyr (solar age) isochrones from Bressan+ (2012; MNRAS, 427, 127), assuming a protosolar metal fraction of Z=0.015. Using the Bressan+ isochrones, M67 does indeed look roughly 3.8 Gyr old, similar to previous estimates. Remarkably, a Bressan+ isochrone for the age and adopted chemical composition of the Sun passes right through the color-magnitude diagram position of the candidate solar sibling HD 162826. Hence, not only is the star remarkably similar in chemical composition to the Sun (Ramirez+2014), but its color-magnitude position appears to be remarkably consistent with have an age similar to the Sun. While HD 162826 appears to be the best "candidate" solar sibling found so far, it is worth keeping in mind that there may be field stars with chemical compositions and ages and velocities similar to the Sun that *didn't* form in the same cluster (e.g. during any ~million-year block in the Milky Way's past, thousands of stars are forming in molecular cloud complexes - many associated with spiral arms - and many will be spawned with similar chemical compositions, ages, and even kinematics, but not necessarily in the same birth cluster). It will be interesting to see if more candidates like HD 162826 are found that match the Sun's chemical composition, age, and velocity so well.
