Computed Hβ indices from ATLAS9 model atmospheres

Aims. Grids of H-beta indices based on updated (new-ODF) ATLAS9 model atmospheres were computed for solar and scaled solar metallicities [+ 0.5], [+ 0.2], [0.0], [- 0.5], [- 1.0], [- 1.5], [- 2.0], [- 2.5] and for alpha enhanced compositions [+ 0.5a], [0.0a], [- 0.5a], [- 1.0a], [- 1.5a], [- 2.0a], [- 2.5a], and [- 4.0a]. Methods. Indices for T-eff > 5000 K were computed with the same methods as described by Lester et al. (1986, LGK86) except for a different normalization of the computed natural system to the standard system. LGK86 used special ODFs to compute the fluxes. For T-eff <= 5000 K we computed the fluxes using the synthetic spectrum method. In order to assess the accuracy of the computed indices comparisons were made with the indices computed by Smalley & Dworetsky ( 1995, A&A, 293, 446, MD95) and with the empirical relations T-eff-H-beta given by Alonso et al. (1996, A&A, 313, 873) for several metallicities. Furthermore, for cool stars, temperatures inferred from the computed indices were compared with those of the fundamental stars listed by MD95. The same kind of comparison was made between gravities for B-type stars. Results. The temperatures from the computed indices are in good agreement, within the error limits, with the literature values for 4750 K <= T-eff <= 8000 K, while the gravities agree for T-eff > 9000 K. The computed H-beta indices for the Sun and for Procyon are very close to the observed values. The comparison between the observed and computed H-beta indices as function of the observed H-beta has shown a very small trend which almost completely disappears when only stars hotter than 10 000 K are considered. The trend due to the cool stars is probably related with the low accuracy of the fundamental T-eff which are a. ected by large errors for most of the stars.