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
H-rich, DA-type white dwarfs are particularly suited as primary standard
stars for flux calibration. State-of-the-art NLTE models consider opacities of
species up to trans-iron elements and provide reliable synthetic
stellar-atmosphere spectra to compare with observation.
We establish a database of theoretical spectra of stellar flux standards that
are easily accessible via a web interface.
In the framework of the Virtual Observatory, the German Astrophysical Virtual
Observatory developed the registered service TheoSSA. It provides easy access
to stellar spectral energy distributions (SEDs) and is intended to ingest SEDs
calculated by any model-atmosphere code. In case of the DA white dwarf G
191-B2B, we demonstrate that the model reproduces not only its overall
continuum shape but also the numerous metal lines exhibited in its ultraviolet
spectrum.
TheoSSA is in operation and contains presently a variety of SEDs for DA white
dwarfs. It will be extended in the near future and can host SEDs of all primary
and secondary flux standards. The spectral analysis of G 191-B2B has shown that
our hydrostatic models reproduce the observations best at an effective
temperature 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, we
determined 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 limits
of 10% solar were derived for Ti, Cr, Mn, and Co.
The TheoSSA database of theoretical SEDs of stellar flux standards guarantees
that the flux calibration of all astronomical data and cross-calibration
between different instruments can be based on the same models and SEDs
calculated with different model-atmosphere codes and are easy to compare.Comment: 42 pages, 27 figure