We present revised stellar properties for 172 K2 target stars that were identified as possible hosts of transiting planets during Campaigns 1-17. Using medium-resolution near-infrared spectra acquired with the NASA Infrared Telescope Facility/SpeX and Palomar/TripleSpec, we found that 86 of our targets were bona fide cool dwarfs, 74 were hotter dwarfs, and 12 were giants. Combining our spectroscopic metallicities with Gaia parallaxes and archival photometry, we derived photometric stellar parameters and compared them to our spectroscopic estimates. Although our spectroscopic and photometric radius and temperature estimates are consistent, our photometric mass estimates are systematically {Delta}M_=0.11 M{sun} (34%) higher than our spectroscopic mass estimates for the least massive stars (M,phot_<0.4 M_{sun}). Adopting the photometric parameters and comparing our results to parameters reported in the Ecliptic Plane Input Catalog, our revised stellar radii are {Delta}R*=0.15 R{sun} (40%) larger, and our revised stellar effective temperatures are roughly {Delta}T_eff=65 K cooler. Correctly determining the properties of K2 target stars is essential for characterizing any associated planet candidates, estimating the planet search sensitivity, and calculating planet occurrence rates. Even though Gaia parallaxes have increased the power of photometric surveys, spectroscopic characterization remains essential for determining stellar metallicities and investigating correlations between stellar metallicity and planetary properties.

Cone search capability for table J/AJ/158/87/table2 (Stellar classifications)

Cone search capability for table J/AJ/158/87/table4 (Candidate stellar companions identified in Gaia DR2 (Cat. I/345))