Left figure: Colour magnitude diagram showing the absolute magnitude M_V vs (B-V). The contours show the distribution in this diagram of the stars in the Hipparcos Catalogue with σϖ/ϖ≤0.1 and σ(B-V)≤0.05. The contours show the numbers of stars in bins of 0.05×0.2 mag2, where the contour levels are at 5, 20, 50 and 500 stars/bin. The triangles are the candidate siblings selected to be nearby and have small proper motions, and the large dots are the siblings selected from the Geneva-Copenhagen Survey catalogue with ages consistent with that of the Sun (4.6 Gyr). The solid line shows the isochrone at the age and metallicity of the Sun according to the Padova models, the dashed line shows the same isochrone for the Yonsei-Yale models, and the dot-dashed line for the BaSTI models.

Following the predictions by Portegies Zwart on the possible whereabouts of the lost siblings of the Sun (i.e. stars born in the same cluster as the Sun, see PoW published on 21/04/2009 ), a search for potential siblings was conducted in the Hipparcos Catalogue (using the new reduction, see PoW published on 24/02/2009).

This search is based on the predicted phase space distribution of the Sun's siblings from simple simulations of the orbits of the cluster stars in a smooth Galactic potential. For stars within 100 pc the simulations show that it is interesting to examine those that have small space motions relative to the Sun. From amongst the candidate siblings thus selected there are six stars with ages consistent with that of the Sun.

Considering their radial velocities and abundances only one potential candidate (HIP 21158) remains, but essentially the result of the search is negative. This is consistent with predictions on the number of siblings near the Sun.

The research was carried out by Anthony Brown, Simon Portegies Zwart (both Leiden Observatory) and Jennifer Bean (Missouri State University) and is published in MNRAS in September 2010 . Brown and Portegies Zwart will continue the theoretical work of understanding how star clusters dissolve in a realistic Galactic potential in order to better prepare future searches fo r the Sun's siblings in the Gaia catalogue.

[Published: 27/08/2010]