Spectroscopic Identification of Young and Active K Dwarfs Within 25 Parsecs

We present the results of an ongoing spectroscopic study of the ages and activity levels of over 1,200 K dwarf stars within 40 parsecs of the Sun and found in the sky between declinations +30^◦ and -30^◦. In an initial study, the CHIRON echelle spectrometer on the SMARTS 1.5m telescope has been used to acquire high resolution (R=80000) spectra for 35 benchmark stars with known ages and an additional 7 field K dwarfs with variable radial velocities (RVV) for which initial CHIRON data did not show obvious companions.The benchmark calibration set of 35 K dwarfs was assembled from nearby moving associations, open clusters, and field stars, with reliable age estimates — it includes 9 stars from the β Pictoris moving group (20 Myr), 6 stars from the Tucana-Horologium association (40 Myr), 7 stars from the AB Doradus moving group (120 Myr), 8 stars from the Hyades cluster (700 Myr), and 5 nearby field K dwarfs (0.3–5 Gyr). Four spectral indicators were tested to determine which might be correlated with age. Three were found to show trends that aligned with the estimated ages of the five sets of stars in the benchmark sample — the Na I Doublet (5895.92 Å & 5889.95 Å), the Hα absorption line (6562.8 Å), and the Li I resonance line (6707.8 Å). The fourth was the Ca II line at 8542 Å, which often showed core emission for the most active stars in our sample, but did not provide a clear trend with age. Of the seven RVV stars, two showed Li I signatures consistent with K dwarfs younger than 700 Myr, while one exhibits stellar activity unusual for older K dwarf field stars and is possibly young. The four remaining stars turn out to be spectroscopic binaries. A larger sample of 287 K dwarfs within 25 parsecs has also been observed with CHIRON and analyzed using results from the benchmark calibration set. Surprisingly, it appears that as many as ~5% of these K dwarfs have spectroscopic features indicating that they are young and/or active. In addition to the age/activity investigation, Empirical SpecMatch has been used to measure various stellar characteristics for all 329 K dwarfs: temperatures range from 3900–5300 K, metallicities range from -0.4 <[Fe/H]< +0.2, 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 have been found to be less reliable than the other quantities.

Upon completion, this study will serve as an excellent resource to evaluate host stars for exoplanet habitability. This effort has been supported by the NSF through grants AST-1517413 and AST-1910130 via observations made possible by the SMARTS Consortium.