Characterizing high‐latitude K dwarfs and giants with Sloan SDSS colors and Kepler 2 light curves

This work describes how Sloan SDSS ugr colors and emphKepler 2 (K2) light curves can go a long way towards identifying and characterizing two unbiased tracers of the halo population, K dwarfs ∼1 kpc distant and red K giants of the remote halo, without additional spectroscopy. We first compare observed stellar effective temperatures T eff and dereddened ( g – r ) 0 colors for K stars to Castelli & Kurucz (2003) models. This shows that in lightly‐reddened halo fields, T eff is determined to 150K for K dwarfs (and to 100 K for K giants) from ( g – r ) 0 alone. We then consider the u – g vs. g – r diagram for K stars with 15 < r < 18.5 in the K2 field C8 at l ∼ 129°.4, b ∼ –57°.4. The high‐quality Bhatti et al. (2010) photometry in the Stripe 82 region of C8 suggests 99% are likely to be K dwarfs, located at a height z ∼ 750 pc above the plane of the Galaxy. The metallicity sensitivity of the u – g vs. g – r diagram reveals that most of these K dwarfs have [Fe/H] > –1.0 and [α/Fe] ∼ 0, which is typical for their Galactic distance and position (Hayden et al. 2015, Fig. 4). Finally, we suggest Kepler 2 as an efficient way to identify the trace population of remote K giants, using asteroseismology rather than spectroscopy. In giants but not in dwarfs, p‐mode oscillations like those of the Sun occur at frequencies detectable at long cadence. To reduce dwarf contamination, we have proposed K giant targets for K2 that are largely restricted to the u – g vs. g – r region where most cool red giants fall but few dwarfs do. The metallicity bias so introduced is small compared to that of previous wide‐field surveys such as SEGUE (Xue et al. 2014). Although halo giants are rare, K2 imaging of the halo fields C8 and C10 with SDSS photometry could locate and estimate [Fe/H] for 50 to 100 red giants > 50 kpc distant.