Finding the elusive RR Lyrae companions via speckle imaging

Abstract

Despite their key role in astrophysics, the binary properties of RR Lyrae stars (RRL) remain almost completely unknown since only a single RRL is confirmed as belonging to a binary system. Finding companions to RRL is difficult since most of them will be at wider orbits, given that close orbits will likely ensue mass transfer disrupting the conditions to develop stellar pulsations. These wide orbits open the possibility that RRL companions may be more easily found by high-resolution imaging. We observed 81 RRL with the speckle interferometers Zorro and 'Alopeke at the Gemini telescopes, reaching the diffraction limit of $\sim$20 mas of these 8m-class telescopes, and therefore exploring a new parameter space around RRL. We have detected 10 newly identified companions around these 81 RRL, 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%, albeit derived with methods exploring a different parameter space. For RRL with thin disc kinematics, we find that the binary fraction is significantly lower, at around 6%, with a single thin disc RRL 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 appears as a useful method for the search and analysis of RRL in binary systems.

Figures (19)

• Figure 1: 5-$\sigma$ contrast curves from speckle observations of RRL BH Aur in both filters EO562 and EO832, together with reconstructed images shown as insets. The shape of the contrast curves is the typical for speckle observations, with a sharp decline from the diffraction limit until 0.1, followed by a more gentle decline until the end of the fov. Contrast curves in the redder bandpasses are always deeper given the relatively lower impact of turbulence. Observations of BH Aur reveal no companions above the contrast curves. Contrast curves for all of the observed targets are given in Appendix \ref{['app:contrast']}.
• Figure 2: Reconstructed images for the 10 RRL with binary companions. In each plot, the position of the RRL is marked with a green cross, while its companion is depicted in red. The right upper corner indicates which filter is displayed. Each reconstructed image show as well a "mirror" image of the companion at 180$\degr$ (more clearly seen in AT And). The real companion is found using bi-spectral analysis weigelt77.
• Figure 3: EO562--EO832 vs EO832 color magnitude diagrams for the 10 RRL (blue stars) with their respective companions (orange stars). In light blue are shown MIST isochrones in the speckle filters with a fixed age of 12 Gyr and the metallicity of each RRL as shown in Table \ref{['tab:sample']}. In the case of non-detection in the blue camera (DN Aqr, IU Car and WZ Hya), a lower limit for the color is given based on the contrast obtained at that separation from the RRL.
• Figure 4: Color-color plane for a sample of RRL with speckle companions (in red stars) and a control sample of RRL without companions (grey dots). BB Vir is indicated in green. The large blue cross shows the mean and 1-sigma dispersion for $(g-r)_0^{\rm min}$ and $(r-i)_0^{\rm min}$ for our control sample, while the orange one shows the same for RRab stars in M 5 from vivas17.
• Figure 5: Distribution of the separations of speckle companions (in red), compared to those found via $O-C$ analysis in hajdu21 (in blue).