NICMOS Spectrophotometry and Models for A-Stars

Abstract

Absolute flux distributions for eight stars are well measured from 0.8-2.5mu m 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 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 IR flux distributions from 0.4-40mu m are always less than the quoted CWW uncertainty of 5% because of compensating changes in the measured extinction. At wavelengths longward of the 2.5mu m 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 0.8-30mu m JWST wavelength range.

20 pages, 6 figures, 2 tables; to be published in AJ, 2008 September

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• New bright optical spectrophotometric standards: A-type stars from the STIS Next Generation Spectral Library
• A reevaluation of the 2MASS zero points using CALSPEC spectrophotometry complemented with Gaia Data Release 2 parallaxes
• Bridging Model and Observed Stellar Spectra
• The James Webb Space Telescope Absolute Flux Calibration. I. Program Design and Calibrator Stars