Discovery of the first L dwarf plus giant binary system and eight other ultracool dwarfs in wide binaries

Abstract

We identify 806 ultracool dwarfs (of which 34 are newly discovered L dwarfs) from their Sloan Digital Sky Survey (SDSS) riz photometry and obtain proper motions through cross-matching with the United Kingdom Infrared Telescope Infrared Deep Sky Survey (UKIDSS) and Two-Micron All-Sky Survey (2MASS). Proper-motion and distance constraints show that nine of our ultracool dwarfs are members of widely separated binary systems: SDSS 0101 (K5V+M9.5V), SDSS 0207 (M1.5V+L3V), SDSS 0832 (K3III+L3.5V), SDSS 0858 (M4V+L0V), SDSS 0953 (M4V+M9.5V), SDSS 0956 (M2V+M9V), SDSS 1304 (M4.5V+L0V), SDSS 1631 (M5.5V+M8V) and SDSS 1638 (M4V+L0V). One of these (SDSS 0832) is shown to be a companion to the bright K3 giant η Cancri. Such primaries can provide age and metallicity constraints for any companion objects, yielding excellent benchmark objects. η Cancri AB is the first wide ultracool dwarf + giant binary system identified. We present new observations and analysis that constrain the metallicity of η Cancri A to be near-solar, and use recent evolutionary models to constrain the age of the giant to be 2.2–6.1 Gyr. If η Cancri B is a single object, we estimate its physical attributes to be mass = 63–82 MJup, Teff = 1800 ± 150 K, log g= 5.3–5.5, [M/H]= 0.0 ± 0.1 . Its colours are non-typical when compared with other ultracool dwarfs, and we also assess the possibility that η Cancri B is itself an unresolved binary, showing that the combined light of an L4 + T4 system could provide a reasonable explanation for its colours.