K-Band Calibration of the Red Clump Luminosity

The average near-infrared (K-band) luminosity of 238 Hipparcos red clump giants is derived and then used to measure the distance to the Galactic center. These Hipparcos red clump giants have been previously employed as I-band standard candles. The advantage of the K-band is a decreased sensitivity to reddening and perhaps a reduced systematic dependence on metallicity. In order to investigate the latter, and also to refer our calibration to a known metallicity zero point, we restrict our sample of red clump calibrators to those with abundances derived from high-resolution spectroscopic data. The mean metallicity of the sample is [Fe/H]=-0.18 dex (σ=0.17 dex). The data are consistent with no correlation between M_K and [Fe/H] and only weakly constrain the slope of this relation. The luminosity function of the sample peaks at M_K=-1.61+/-0.03 mag. Next, we assemble published optical and near-infrared photometry for ~20 red clump giants in a Baade's window field with a mean metallicity of [Fe/H]=-0.17+/-0.09 dex, which is nearly identical to that of the Hipparcos red clump. Assuming that the average (V-I)₀ and (V-K)₀ colors of these two red clumps are the same, the extinctions in the Baade's window field are found to be A_V=1.56, A_I=0.87, and A_K=0.15, in agreement with previous estimates. We derive the distance to the Galactic center: (m-M)₀=14.58+/-0.11 mag, or R=8.24+/-0.42 kpc. The uncertainty in this distance measurement is dominated by the small number of Baade's window red clump giants examined here.