Southern Spectrophotometric Standards

One of the goals in astrophysics is to characterize the properties of the stars which, by comparison with models, allows us to test the physics of their interiors and atmospheres, and learn about their nature. Atmosphere parameters like effective temperature and surface gravity provide important constraints to the models. These parameters can be determined from spectroscopic observations, as long as the observed fluxes can be calibrated. In principle this calibration can be performed with spectrophotometric standards. However, if the grid of fluxes of such standards is coarse, significant interpolation errors can be introduced in the calibration. Furthermore, uncertain or inaccurate fluxes of the standards can hamper the determination of the physical parameters of interest.

Since this was the actual situation in 1990, I initiated a long-term program, in collaboration with CTIO astronomers N. Suntzeff, M. M. Phillips, S. Heathcote, and A. Walker, to obtain high-quality CCD observations for the secondary (Hayes) and tertiary (Baldwin & Stone) spectrophotometric standards, in order to improve their flux calibration. This work provided monochromatic fluxes at steps of 50 A for 20 tertiary standards, and a finer grid (16 A) for 10 secondary standards in the range 3300-10500 A, relative to the primary standard (Vega). These data has proved to be a fundamental reference for spectroscopic studies and the calibration of photometric systems from spectrophotometry.