Nicmos Spectrophotometry and Models for A–stars

Absolute flux distributions for eight stars are well measured from 0.8–2.5 μm with NICMOS grism spectrophotometry at a resolution of R ∼ 100 and an accuracy of 1–2%. These SEDs are fit 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 Teff , 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 IR flux distributions from 0.4–40 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 JWST continuous viewing zone and will be important absolute flux standards for the 0.8–30 μm JWST wavelength range. Subject headings: stars: atmospheres — stars: fundamental parameters — stars: individual (HD165459, 1732526, 1739431, 1740346, 1743045, 1802271, 1805292, 1812095, 1812524) — techniques: spectroscopic