We present the results of a spectroscopic study to determine the stellar properties, activity levels, space motions, and ages of the nearest K dwarfs. The targeted stars are members of RKSTAR (RECONS K Star) Survey that includes ∼5000 K dwarf primaries within 50 pc. An initial study established a benchmark calibration set of 35 K dwarfs with known ages of 20 Myr to 5 Gyr and high resolution (R=80000) spectra acquired on the CHIRON echelle spectrometer on the SMARTS 1.5m telescope at CTIO. Two known spectral indicators of activity and youth — the Hα absorption line (6562.8 Å) and the Li I resonance line (6707.8 Å) — showed strong trends in the training set. A sample of 615 K dwarfs within 33 parsecs and found between declinations +30 and −30 degrees has now been observed with CHIRON and analyzed relative to the stars in the benchmark calibration set. Surprisingly, it appears that as many as ∼8% of these K dwarfs have spectroscopic features indicating that they are young and/or active. As expected, Galactic U V W space motions indicate that most of the stars fall into the thin (80%) and thick (20%) disk populations, with a single outlier, HD 134439, which is a known halo star. Overall, we find a set of metal poor K dwarfs with [Fe/H]values of −0.5 dex or less that account for 4% of the population in the solar neighborhood.

Empirical SpecMatch has been used to measure stellar properties for this sample of K dwarfs: temperatures range from 3600–5500 K, metallicities range from −0.6 < [Fe/H]< +0.55, and rotational velocities (v sin i) range from less than 10 km/s to more than 50 km/s. Surface gravity values (log g values) have also been determined, although they are less reliable than the other quantities. Cross-matching with NASA’s Exoplanet Archive reveals that only 42 exoplanet host stars are among our sample, highlighting the limited focus on mid and late-type K dwarfs (Teff < 4800 K) in large exoplanet surveys. As a result of this work, we have identified 500 K dwarfs that are calm host stars worthy of detailed efforts to detect terrestrial planets. This work will serve as a key resource for assessing host star suitability for exoplanet habitability.