No abstract

Context. Given the closeness of the two open clusters Collinder 135 and UBC 7 on the sky, we investigate the possibility that the two clusters are physically related. Aims. We aim to recover the present-day stellar membership in the open clusters Cr 135 and UBC 7 (300 pc from the Sun) in order to constrain their kinematic parameters, ages, and masses and to restore their primordial phase space configuration. Methods. The most reliable cluster members are selected with our traditional method modified for the use of Gaia DR2 data. Numerical simulations use the integration of cluster trajectories backwards in time with our original high-order Hermite4 code φ−GRAPE. Results. We constrain the age, spatial coordinates, velocities, radii, and masses of the clusters. We estimate the actual separation of the cluster centres equal to 24 pc. The orbital integration shows that the clusters were much closer in the past if their current line-of-sight velocities are very similar and the total mass is more than seven times larger than the mass of the most reliable members. Conclusions. We conclude that the two clusters Cr 135 and UBC 7 might very well have formed a physical pair based on the observational evidence as well as numerical simulations. The probability of a chance coincidence is only about 2%.

The Binary star database contains data on about 100 000 stellar systems of multiplicity 2 to 22, taken from a large variety of published catalogues for all types of binary stars: visual, orbital, astrometric, interferometric, spectroscopic, photometric, eclipsing, etc. Positional, kinematic, photometric, spectroscopic, orbital and astrophysical parameters are provided when available. The database can be queried by identifier, coordinates, catalogue and stellar/orbital parameters (including binary type) of objects. Lists of objects can be submitted as well. Also, the database provides links to some other online services, both of general purpose and on binary stars. A pilot version of BDB can be accessed at http://bdb.inasan.ru

Stellar masses and ages are not directly observable parameters, and the methods used to determine them are based on the calibrating relations. In particular, the mass-luminosity relation, based on the masses of less than 200 well-studied binaries, is virtually the only way to estimate the mass of single stars. Thus, the development of methods for estimating stellar masses with accuracy comparable to direct methods is a problem of vital importance.Here, we describe a method for estimating stellar masses and ages, which is based on the geometric similarity of evolutionary tracks for the stars at the same evolutionary stage in the Hertzsprung-Russell (HR) diagram. To examine the proposed approach, it has been applied to various test data sets. Application of the method, using synthetic stellar spectra Basel Stellar Library (of theoretical spectra; BaSeL), demonstrates that it allows determination of masses and ages of stars with a predictable distribution of uncertainties.This statistical approach allows us to demonstrate the viability of the method using it on the set of double-lined eclipsing binaries with intermediate-mass and low-mass components which allows us to compare calculated characteristics with observational ones. As a result, the uncertainties of the stellar masses estimated with the proposed method are comparable with the accuracy of ones obtained from direct observations. This allows us to recommend the method for mass estimates of masses of single stars by the localization in the HR diagram.As for the ages, the estimates for intermediate-mass stars are more reliable, while those obtained for low-mass stars are very uncertain, due both to slower movement of these stars in the HR diagram with age at stages close to the main sequence and to certain disagreements between theoretical models for this mass range.Well-known correlation between mass of the star and its observational characteristics (radius, luminosity, temperature) provides observational relations like 'mass-luminosity', 'mass-radius', etc.