Theoretical Isochrones with Extinction in theKBand

We calculate theoretical isochrones in a consistent way for five filters (K,K', Ks, F205W, and F222M) in the atmospheric window between 1.9 and 2.5 micronusing the Padova stellar evolutionary models by Girardi et al. Even whendisplayed in the same Vega magnitude system, the near-infrared colors of thesame isochrone can differ by up to 0.18 mag at its bright end, depending on thefilter. We present magnitude transformations between K-band filters as afunction of color from H and K band filters. Isochrones with extinction at K ofup to 6 mag are also presented. We find that care is needed when comparingextinction values that are estimated using different filter sets in the K band,in particular when comparing those of atmospheric and space filter sets:extinction values for space filters can be in error by up to 0.3 mag. To reducethis error, we introduce an "effective extinction slope" for each filter setand isochrone model, which describes the extinction behavior of isochrones inthe color-magnitude diagram more correctly than the actual extinction law. Ourcalculation also suggests that the extinction law implied by the observationsof Rieke et al. for wavelengths between the H and K bands is better describedby a power-law function with an exponent of 1.61 instead of 1.55, which iscommonly used with an assumption that the transmission functions of H and Kfilters are Dirac delta functions