On the nature of the H i infrared emission lines of τ Scorpii

We present Ha, Hα I λ 2.058 μm and 6 hydrogen Brackett and Pfund lines of τ Sco (B0.2V) obtained using the ground-based INT and UKIRT instruments as well as satellite data from ISO. The infrared lines all show core emission. We have investigated the formation of these lines using sophisticated non-LTE models. The observed emission in the most pronounced hydrogen lines, such as Brα and Pfα, is stronger than predicted by our models. The velocities of peak emission are blue-shifted by 5-10 kms^-1 with respect to the stellar velocity. This together with the surprisingly strong width of Brα and the peculiar profile of He I λ 2.058 suggests that shock-induced turbulent velocity fields may be present in or somewhat above the stellar photosphere, as has already been suggested from analysis of optical and ultraviolet data. We derive T_eff = 32±2 kK from the infrared data alone, a value consistent with previous optical analysis. The good agreement indicates that quantitative analysis of infrared lines alone (e.g. for hot stars in regions of high extinction) can be used to characterize photospheres accurately. We also investigate the mass loss of τ Sco and find an upper-limit of 6 10^-9 M⊙yr^-1. A parameter study of the infrared hydrogen and helium lines indicates that emission may be expected in Brα and Pfα for stars with T_eff ≳16 kK and will dominate the profiles of these lines for T_eff ≳31 and 26 kK, respectively. He I λ 2.058 will be in emission for 20 ≲ T_eff ≲ 33 kK and He II line profiles will contain emission at T_eff ≳ 33 kK. The effect of surface gravity on these values is small.