Observational properties of visual binaries as modelled using a synthetic catalogue, I: For visual apparent magnitudes $m_v < 15$

Abstract

A simulated binary star catalogue is created that represents the full sky distribution of stars, excluding those with galactic latitude $|b|<10\degr$, down to $m_v < 15$. The catalogue is compatible with observed magnitude and number distributions of the HST Guide Star Catalog. The simulated catalogue is analyzed for observational properties of visual binary stars, both in a general context without taking any observational constraints into account, and using different observability criteria that model the observational capability of the astrometric DIVA satellite. It is shown that several observational bias effects should be taken into account when final output catalogues are analyzed. The results indicate that $Δm_v$ distribution peaks at $Δm_v \approx 7$ and to maximize the number of binary detections with large mass ratios, the magnitude threshold should be set as high as possible. This is especially important if low mass $M<0.2 M_{\sun}$ secondaries are studied. The predicted number of optical pairs is calculated and for large magnitude differences close to the galactic plane and toward the galactic center it may be difficult to discriminate between a red companion and a low-mass foreground/background star for distant double stars for which no parallax measurements are available. Using optimal detection procedures for optical double stars it is estimated that, depending on the assumed double star distributions, 60%-70% of all the visual binary stars in the catalogue with separations $0.0625\arcsec<θ<3.5\arcsec$, $m_{v}(\mathrm{secondary})<22$, and $Δm<10$ could be observed during the DIVA or similar missions.