Norbert Przybilla and Keith Butler
MODELLING OF STELLAR HYDROGEN AND HELIUM LINE SPECTRA IN THE IR (Poster)

MODELLING OF STELLAR HYDROGEN AND HELIUM LINE SPECTRA IN THE IR

Norbert Przybilla and Keith Butler
Dr. Remeis-Observatory, Bamberg, Germany
University-Observatory, Munich, Germany

The quantitative interpretation of the hydrogen and helium line spectra is one of the foundations of modern astrophysics. The modelling of the spectral features in the visual has been brought close to perfection in the last few decades, allowing astrophysical plasma parameters to be determined with high accuracy. Despite fewer applications so far, quantitative IR-spectroscopy offers great potential in view of upcoming instrumentation and diffraction-limited observations with large telescopes using adaptive optics. However, we have to ask ourselves whether the status of the modelling in the IR is as good as in the visual.
We report on results obtained in an ongoing project to develop reference model atoms for NLTE computations in early-type stars that are able to reproduce observed spectra in the optical and in the $I$ to $L$-bands simultaneously. Our approach takes advantage of the amplification of NLTE effects on the line source function in the Rayleigh-Jeans limit. This makes IR lines in hot stars very susceptible to small changes in the atomic data, allowing us to discriminate between different model atoms empirically.
In the case of hydrogen $[1]$ it is found that even for a standard star like Vega neither the classical LTE approach nor traditional NLTE recipes allow for highly accurate modelling of the IR line spectrum. The discrepancies become more pronounced for OB-stars and for supergiants. Equivalent widths $W_{\lambda}$ of the IR lines computed under the assumption of LTE or with standard NLTE model atoms may differ from observation by factors up to $>$3. Our improved reference model atom on the other hand succeeds in reproducing the optical and IR line spectra over the whole parameter space. For helium, our efforts have concentrated on the He I $\lambda$10 830 Å transition so far. Our modelling reproduces the observed trend of $W_{\lambda}$ with $T_{\rm eff}$ for early-A to late-O main sequence stars - including the transition from absorption to emission characteristics - for the first time $[2]$. Modelling deficiencies in extreme helium stars could also be resolved $[3]$. Again, changes in the computed $W_{\lambda}$ for the IR line by factors up to $>$3 are found relative to LTE or NLTE computations with older model atoms, while classical results in the optical are retained.
Besides the immediate relevance for quantitative analyses of early-type stars there is a further impact. The reference model atoms are also of interest beyond stellar astrophysics because of the universality of atomic data. Finally, telluric line removal from IR spectra involving A-type dwarfs as telluric standards may benefit from the improved spectrum synthesis.

$[1]$ Przybilla, N., & Butler, K. 2004, ApJ, 609, 1181
$[2]$ Przybilla, N. 2005, A&A, 443, 293
$[3]$ Przybilla, N., Butler, K., Heber, U., & Jeffery, C.S. 2005, A&A, 443, L25