Ultra-wide, Low-mass Binaries: Constraints on Binary Formation Theory and Calibration of Fundamental Stellar Parameters

Abstract

I identified and used a large catalog of ultra-wide binaries to investigate observationally two related questions in low-mass star astrophysics: (1) What is the frequency and distribution of ultra-wide binary systems, and what can they tell us about formation processes? and (2) How can we use wide binaries as coeval laboratories to measure the fundamental parameters—specifically, metallicity and age—of low-mass stars? The Sloan Low-mass Wide Pairs of Kinematically Equivalent Stars (SLoWPoKES) catalog of ultra-wide (10^3-10^5.5) AU), low-mass (K5–M7) common proper motion binaries is comprised of 1342 disk dwarf, subdwarf, and white dwarf–red dwarf systems, making it the largest catalog of wide binaries. A Galactic model, based on empirical stellar number density and space velocity distributions, was constructed to select only bona fide pairs with probability of chance alignment <5%, making SLoWPoKES an efficient sample for followup observations. Our findings suggest that the SLoWPoKES binaries could have formed via a combination of two modes of wide binary formation---dynamical widening and cluster dissipation. Comparison with dynamical dissipation timescales of wide binaries and the high incidence of hierarchical multiples suggest the widest systems could be the result of dynamical widening. On the other hand, the distribution of physical separations of wide binary and the tight binary (when present) is similar to results from numerical simulations of cluster dissipation. From spectroscopic followup of SLoWPoKES pairs, we confirmed that the ζ_(TiO/CaH) index traces iso-metallicity loci for most of our sample of M dwarfs. However, we found a small systematic bias in ζ_(TiO/CaH), especially for early-type M dwarfs. We used our sample to recalibrate the definition of ζ_(TiO/CaH). While representing a small change in the definition, the new ζ_(TiO/CaH) is a significantly better predictor of iso-metallicity for early-type M dwarfs.