2016MNRAS.460.2611S

We present the effective temperatures (T_eff), metallicities, and colours in Sloan Digital Sky Survey (SDSS), Two Micron All Sky Survey, and Wide-field Infrared Survey Explorer filters, of a sample of 3834 late-K and early-M dwarfs selected from the SDSS Apache Point Observatory Galactic Evolution Experiment (APOGEE) spectroscopic survey ASPCAP (APOGEE Stellar Parameters and Chemical Abundances Pipeline) catalogue. We confirm that ASPCAP T_eff values between 3550 < T_eff < 4200 K are accurate to ∼100 K compared to interferometric T_eff values. In that same T_eff range, ASPCAP metallicities are accurate to 0.18 dex between -1.0 <[M/H]<0.2. For these cool dwarfs, nearly every colour is sensitive to both T_eff and metallicity. Notably, we find that g - r is not a good indicator of metallicity for near-solar metallicity early-M dwarfs. We confirm that J - K_S colour is strongly dependent on metallicity, and find that W1 - W2 colour is a promising metallicity indicator. Comparison of the late-K and early-M dwarf colours, metallicities, and T_eff to those from three different model grids shows reasonable agreement in r - z and J - K_S colours, but poor agreement in u - g, g - r, and W1 - W2. Comparison of the metallicities of the KM dwarf sample to those from previous colour-metallicity relations reveals a lack of consensus in photometric metallicity indicators for late-K and early-M dwarfs. We also present empirical relations for T_eff as a function of r - z colour combined with either [M/H] or W1 - W2 colour, and for [M/H] as a function of r - z and W1 - W2 colour. These relations yield T_eff to ∼100 K and [M/H] to ∼0.18 dex precision with colours alone, for T_eff in the range of 3550-4200 K and [M/H] in the range of -0.5-0.2.