17th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun

Type: Oral

Topic: Fundamental Parameters of Cool Stars and Brown Dwarfs

Abstract
We present results from the Sloan Low-mass Wide Pairs of Kinematically Equivalent Stars (SLoWPoKES) catalogs of ultra-wide (10$^{3--5.5}$~AU), low-mass (K5--M7) common proper motion binaries. Our initial catalog contains 1342 disk dwarf, subdwarf, and white dwarf-red dwarf systems and is the largest collection of low-mass, wide binaries ever assembled. We constructed a Galactic model, based on empirical stellar number density and 3D velocity distributions, to select bona fide pairs with probability of chance alignment $<$5\%, making SLoWPoKES an efficient sample for followup observations. The diversity---in mass, metallicity, age, and evolutionary states---of SLoWPoKES pairs makes it a valuable resource of {\it coeval laboratories} to examine and constrain the physical properties of low-mass stars. We use followup spectroscopic observations to recalibrate the metallicity-sensitive $\zeta_{TiO/CaH}$ index by assuming that both members of the binary system were formed from the same material. Our new formulation is a significantly better predictor of metallicity, particularly for the early-type M dwarfs. SLoWPoKES appears to contain two populations of wide binaries, with a break at projected physical separation of $\sim$0.1~pc: 1) tightly bound "wide" systems that are expected to last 10 Gyr or longer; and 2) wide, weakly bound "ultra-wide" systems that are expected to dissipate within a few Gyr (based on binary disruption timescales from dynamical calculations). Followup high-resolution imaging has revealed that the multiplicity in "individual" stars in the ultra-wide binaries (higher-order multiplicity) is significantly higher than in the wide binaries or in low-mass field stars. This is consistent with the premise that ultra-wide systems are the result of dynamical widening via transfer of angular momentum from the outer orbit to the inner orbit, followed by dissipation via interactions with Galactic tide and giant molecular clouds. The observed bimodality, however, is also consistent with recent theoretical predictions, which show that the ultra-wide binaries are not formed primordially but during dissipation of star clusters. Our data do not rule out either scenario but indicate neither mechanism can form all of the observed wide binaries. We conclude that multiple processes, not all of which are primordial, are likely responsible for the observed distribution of stellar binaries.