NICMOS Spectrophotometry and Models for A-Stars

TL;DR

This study provides precise NICMOS spectrophotometry for eight A-stars, compares them with model atmospheres and existing templates, and discusses their suitability as flux standards for JWST across 0.8-30 micrometers.

Contribution

It offers high-accuracy flux measurements and model comparisons for A-stars, improving their reliability as flux standards for JWST.

Findings

NICMOS flux measurements have 1-2% accuracy.

Model parameters can differ significantly from previous templates.

A-stars are suitable as flux standards for JWST in the 0.8-30μm range.

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.