HST Stellar Standards with 1% Accuracy in Absolute Flux.pdf

a r X i v : a s t r o - p h / 0 6 0 8 7 1 5 v 1 3 1 A u g 2 0 0 6 THE FUTURE OF PHOTOMETRIC, SPECTROPHOTOMETRIC AND POLARIMETRIC STANDARDIZATION ASP Conference Series, Vol. 999, 2007 C. Sterken HST Stellar Standards with 1% Accuracy in Absolute Flux Ralph C. Bohlin Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 email: bohlin@ Abstract. Free of any atmospheric contamination, the Hubble Space Tele- scope provides the best available spectrophotometry from the far-UV to the near-IR for stars as faint as V ～ 16. The HST CALSPEC standard star net- work is based on three standard candles: the hot, pure hydrogen white dwarf (WD) stars G191B2B, GD153, and GD71, which have Hubeny NLTE model flux calculations that require the atomic physics for only one atom. These model flux distributions are normalized to the absolute flux for Vega of 3.46×10?9 erg cm?2 s?1 A??1 at 5556 A? using precise Landolt V band photometry and the V band- pass function corrected for atmospheric transmission by M. Cohen. The three primary WD standards provide absolute flux calibrations for FOS, STIS, and NICMOS spectrophotometry from these instruments on the HST. About 32 stel- lar spectral energy distributions (SEDs) have been constructed with a primary pedigree from the STIS data, which extends from 1150 A? for the hot stars to a long wavelength limit of 1 μm. NICMOS grism spectrophotometry provides an extension to 1.9 μm in the IR for 17 of the HST standards and longward to 2.5 μm for a few of the brighter stars. Included among these HST standards are Vega, the Sloan standard BD+17?4708, three bright solar analog candidates, three cool stars of type M or later, and five hot WDs. In addition, four K giants and four main sequence A-stars have NICMOS spectrophotometry from 0.8– 2.5 μm. The WD fluxes are compared to their modeled SEDs and demonstrate an internal precision of 1–2%, while the A-stars agree with the Cohen IR fluxes to ～2%. Three solar analog candidate sta