On the nature of the HI infrared emission lines of τ Scorpii
P. A. Zaal, A. de Koter, L. B. F. M. Waters, J. M. Marlborough, T. R. Geballe, J. M. Oliveira, B. H. Foing
We present Hα , He 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 km 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 Teff = 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) Msun 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 Teff ≥ 16 kK and will dominate the profiles of these lines for Teff ≥ 31 and 26 kK, respectively. He i λ 2.058 will be in emission for 20 la Teff la 33 kK and He ii line profiles will contain emission at Teff ≥ 33 kK. The effect of surface gravity on these values is small. based on data obtained with the ESA Infrared Space Observatory (ISO), with the UK infrared telescope (UKIRT, Hawaii) and with the Isaac Newton Telescope (INT, La Palma)
Astronomy and Astrophysics