The Tale of the Lobster: Over-luminous Stars in Wide Binaries and a Search for Higher Order Multiples

Abstract - We present a search for higher-order multiples (triples, quadruples, etc.) among K+K and K+M wide binaries identified in the SUPERWIDE all-sky catalog of wide binaries. The SUPERWIDE catalog was assembled from a Bayesian analysis of the high proper motion (> 40 mas/yr) stars in Gaia Data Release 2 (DR2) using their positions, proper motions, and parallaxes. Examining the color-magnitude diagram of the primary and secondary components clearly shows a doubling of the main sequence in the K and early-M dwarf regime, consistent with a normal single star main sequence plus an over-luminous sequence due to some of the components being unresolved binaries. To better identify the over-luminous stars in our wide binaries, we define an over-luminosity factor, which measures the difference between the absolute magnitude of a star and a reference line which runs parallel to the main sequence in the K dwarf region. A “lobster” diagram is then created, which plots the over-luminosity factor of the primary as a function of the over-luminosity factor of the secondary. An examination of this plot reveals that for K+K wide binaries, the higher order multiplicity of the selected sample is at least ~40%. We expand this technique to the lower mass regime (mid-M to late-M) by matching subsets of K+M pairs to the apogee catalog, and by assuming the metallicity of the low-mass secondaries is the same as that of the higher mass primaries. Using the resulting metallicity “tracks”, we determine the shape of the main sequence in the M dwarf regime, which allows us to define a proper reference and identify over-luminous components in the M dwarf range as well. To test if our over-luminous components are caused by binarity rather than other potential sources, we crossmatch our sample with TESS, K2 and Kepler to identify eclipsing systems among the over-luminous components and take speckle imaging results from previous observations and the POKEMON survey.