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H-rich, DA-type white dwarfs are particularly suited as primary standardstars for flux calibration. State-of-the-art NLTE models consider opacities ofspecies up to trans-iron elements and provide reliable syntheticstellar-atmosphere spectra to compare with observation.  We establish a database of theoretical spectra of stellar flux standards thatare easily accessible via a web interface.  In the framework of the Virtual Observatory, the German Astrophysical VirtualObservatory developed the registered service TheoSSA. It provides easy accessto stellar spectral energy distributions (SEDs) and is intended to ingest SEDscalculated by any model-atmosphere code. In case of the DA white dwarf G191-B2B, we demonstrate that the model reproduces not only its overallcontinuum shape but also the numerous metal lines exhibited in its ultravioletspectrum.  TheoSSA is in operation and contains presently a variety of SEDs for DA whitedwarfs. It will be extended in the near future and can host SEDs of all primaryand secondary flux standards. The spectral analysis of G 191-B2B has shown thatour hydrostatic models reproduce the observations best at an effectivetemperature of 60000 +/- 2000K and a surface gravity of log g = 7.60 +/- 0.05.We newly identified Fe VI, Ni VI, and Zn IV lines. For the first time, wedetermined the photospheric zinc abundance with a logarithmic mass fraction of-4.89 (7.5 times solar). The abundances of He (upper limit), C, N, O, Al, Si,O, P, S, Fe, Ni, Ge, and Sn were precisely determined. Upper abundance limitsof 10% solar were derived for Ti, Cr, Mn, and Co.  The TheoSSA database of theoretical SEDs of stellar flux standards guaranteesthat the flux calibration of all astronomical data and cross-calibrationbetween different instruments can be based on the same models and SEDscalculated with different model-atmosphere codes and are easy to compare.

astronomy and astrophysics

The virtual observatory service TheoSSA: Establishing a database of  synthetic stellar flux standards. I. NLTE spectral analysis of the DA-type  white dwarf G 191-B2B