Stellar Companions to Stars with Planets

A combination of high-resolution and wide-field imaging reveals two binary stars and one triple star system among the sample of the first 11 stars with planets detected by radial velocity variations. High-resolution speckle or adaptive optics (AO) data probe subarcsecond scales down to the diffraction limit of the Keck 10 m or the Lick 3 m, and direct images or AO images are sensitive to a wider field, extending to 10'' or 38'', depending on the camera. One of the binary system—HD 114762—was not previously known to be a spatially resolved multiple system; additional data taken with the combination of Keck adaptive optics and NIRSPEC are used to characterize the new companion. The second binary system—τ Boo—was a known multiple with two conflicting orbital solutions; the current data will help constrain the discrepant estimates of periastron time and separation. Another target—16 Cyg B—was a known common proper motion binary, but the current data resolve a new third component, close to the wide companion 16 Cyg A. Both the HD 114762 and 16 Cyg B systems harbor planets in eccentric orbits, while the τ Boo binary contains an extremely close planet in a tidally circularized orbit. Although the sample is currently small, the proportion of binary systems is comparable to that measured in the field over a similar separation range. Incorporating the null result from another companion search project lowers the overall fraction of planets in binary systems, but the detections in our survey reveal that planets can form in binaries separated by less than 50 AU.