Finding the elusive RR Lyrae companions via speckle imaging

Aims . Despite their key role in astrophysics, the binary properties of RR Lyrae stars (RRLs) are almost completely unknown since only a single RRL is confirmed as belonging to a binary system. Finding companions to RRLs is difficult since most of them will be at wider orbits, given that close orbits will likely cause mass transfer, thus disrupting the conditions for developing stellar pulsations. These wide orbits open the possibility that RRL companions may be more easily found by high-resolution imaging. Methods . We observed 81 RRLs with the speckle interferometers Zorro and ’Alopeke at the Gemini telescopes, reaching the diffraction limit of ∼20 mas of these 8m-class telescopes, and therefore exploring a new parameter space around RRLs. Results . We have detected ten newly identified companions around these 81 RRLs, with projected separations between 20 AU to 220 AU. An analysis of the field contamination shows that all of these detected companions are most likely gravitationally bound binaries. From these observations we can estimate an RRL binary fraction higher than 12%, ruling out a binary fraction higher than 25% at the 99% confidence level. These numbers are significantly more elevated than previous estimations which were close to a binary fraction of only 1%, although derived with methods exploring a different parameter space. For RRLs with thin disk kinematics, we find that the binary fraction is significantly lower, around 6%, with a single thin disk RRLs having a companion out of the 16 observed. The nature of the companions, found to be stars in the lower red giant branch and upper main sequence, is also studied via the measurement of the minimum light colors of the RRL, which is a useful method for the search and analysis of RRLs in binary systems.