HD 131861, a double-line spectroscopic triple system

摘要

Our red-wavelength spectroscopic observations of HD 131861, a previously known single-line multiple system, span 20 years. Now lines of two components, the short-period F5 V primary and G8 V secondary, have been detected. The inner orbit is circular with a period of 3.5507439 days, while the outer orbit of the system has a period of 1642 days or 4.496 yr and a relatively low eccentricity of 0.10. Analysis of the Hipparcos data produces a well-determined astrometric orbit for the long-period system that has an inclination of 52 degrees. Our photometric observations show shallow primary and secondary eclipses of the short-period pair, and eclipse solutions result in an inclination of 81 degrees. Thus, the long- and short-period orbits are not coplanar. The mass of the unseen third component is 0.7 M-circle dot, corresponding to a mid-K dwarf. The total mass of the system, 3.08M(circle dot), leads to a semimajor axis of 4 AU for the outer orbit. The F5 V primary is rotating more slowly than it would if it were synchronously rotating, while the G8 V secondary may be synchronously rotating. The lithium abundance of the F5 V primary is similar to the initial lithium abundance found for Population I dwarfs and so indicates no significant dilution.