HD 131861, a Double-Line Spectroscopic Triple System

Abstract

Our red-wavelength spectroscopic observations of HD 131861, a previously known single-line multiple system, span20years.Nowlinesoftwocomponents,theshort-periodF5VprimaryandG8Vsecondary,havebeendetected. Theinnerorbitiscircularwithaperiodof3.5507439days,whiletheouterorbitofthesystemhasaperiodof 1642days or 4.496 yr and a relatively low eccentricity of 0.10. Analysis of theHipparcosdata produces a well-determined astrometric orbit for the long-period system that has an inclination of 52 � . Our photometric observations show shallow primary and secondary eclipses of the short-period pair, and eclipse solutions result in an inclination of 81 � . Thus, the long- and short-period orbits are not coplanar. The mass of the unseen third component is 0.7 M� ,c orrespondingtoamid-Kdwarf.Thetotalmassofthesystem,3.08M� ,leadstoasemimajoraxis of4AUfortheouter 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.