Nicmos Spectrophotometry and Models for a Stars

Absolute flux distributions for eight stars are well measured from 0.8 to 2.5 μm with NICMOS grism spectrophotometry at a resolution of R ∼ 100 and an accuracy of 1–2%. These spectral energy distributions (SEDs) are fitted with Castelli & Kurucz model atmospheres, and the results are compared with the Cohen–Walker–Witteborn (CWW) template models for the same stars. In some cases, the T eff , log g , and log z parameters of the best-fitting model differ by up to 1000 K from the earlier CWW model. However, differences in the continua of the modeled infrared flux distributions from 0.4 to 40 μm are always less than the quoted CWW uncertainty of 5% because of compensating changes in the measured extinction. At wavelengths longward of the 2.5 μm NICMOS limit, uncertainties still approach 5%, because A star models are not yet perfect. All of these A stars lie in the James Webb Space Telescope ( JWST ) continuous viewing zone and will be important absolute flux standards for the 0.8–30 μm JWST wavelength range.