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.