Theoretical Isochrones with Extinction in the K Band

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 μm using the Padova stellar evolutionary models by Girardi et al. Even when displayed in the same Vega magnitude system, the near‐infrared colors of the same isochrone can differ by up to 0.18 mag at its bright end, depending on the filter. We present magnitude transformations between K‐band filters as a function of color from H and K band filters. Isochrones with extinction at K of up to 6 mag are also presented. We find that care is needed when comparing extinction 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 reduce this error, we introduce an “effective extinction slope” for each filter set and isochrone model, which describes the extinction behavior of isochrones in the color‐magnitude diagram more correctly than the actual extinction law. Our calculation also suggests that the extinction law implied by the observations of Rieke et al. for wavelengths between the H and K bands is better described by a power‐law function with an exponent of 1.61 instead of 1.55, which is commonly used with an assumption that the transmission functions of H and K filters are Dirac delta functions.