Refining the Parallax in Visual Double Stars Using Orbital and Spectral Data: Application to the System of the K0 Giants, A 1808

Abstract

In this paper, we show how Hipparcos parallaxes of distant visual binaries (π_Hip <~ 10 mas) can be refined. This is accomplished by using relevant orbital and spectral data. To this end, we study the pair of K0 giants, A 1808. A revised orbit of this system is presented that was calculated by taking into account a set of interferometric and visual measurements covering almost one orbital revolution. In addition, by analyzing its spectrum obtained with the 2.6 m telescope of the Byurakan Astrophysical Observatory (Armenia), we confirm the MK spectral type, K0III. On the basis of B - V photometric data, we derived the extinction A_V = 0.23 ± 0.08 mag as well as obtained a spectroscopic parallax (π _sp = 3.19 ± 0.21 mas) with a much smaller relative uncertainty of 6.6% compared with that of Hipparcos (42.1%; π_Hip = 2.43 ± 1.00 mas). Accordingly, this system is now placed at 313 ± 21 pc. With the obtained spectroscopic parallax, a dynamical mass of 4.52^{+1.24}_{-0.90} \,M_{\odot } is obtained, which agrees well with that expected from standard calibrations.