Resolving stellar atmospheres

I. The H line and comparisons to microlensing observations

¹ Research School of Astronomy and Astrophysics, Australian National University, Mt. Stromlo Observatory, Cotter Road, Weston ACT 2611, Australia e-mail: cthurl@mso.anu.edu.au
² Sternwarte Hamburg, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany

Received: 23 December 2005
Accepted: 2 March 2006

Abstract

Context.We present work on spectral line characteristics in PHOENIX stellar model atmospheres and their comparison to microlensing observations.

Aims.We examine in detail the equivalent width (EW) and the line shape characteristics for effective temperatures of  K where is a strong spectral feature.

Methods.We find that EW in models calculated under the assumption of local thermodynamic equilibrium (LTE) is up to 15% smaller than in models without this assumption, non-LTE models (NLTE) and that line shapes vary significantly for the two model types. A comparison with available high quality microlensing data, capable of tracing absorption across the face of one G5III giant, shows that the LTE model that fits the EW best is about 100 K hotter than and the best-fitting NLTE model has a similar as predicted by the spectral type analysis of the observed star but agree within the uncertainties of the observationally derived temperature.

Results.Neither LTE nor NLTE models fit the line shape well. We suspect unmodelled chromospheric emission. Line shape diagnostics suggest lower gravities than derived for the star and are unacceptable low in the case of the LTE models. We show that EW alone is insufficient for comparison to stellar model atmospheres, but combined with a new shape parameter we define is promising. In stellar parameter ranges where the line is strong, a NLTE approach of modeling stellar atmospheres is not only beneficial but mandatory.