2001A&A...366..241Z

We present observations of the Brα, Brβ and Pfα lines of 16 dwarf and (sub)giant stars in the spectral range O9-B3. The observations were done using the Short Wavelength Spectrometer on board the Infrared Space Observatory, and have a signal-to-noise of ∼20 to >150 and a resolving power varying from ∼1400 to 2100. We compare the equivalent widths of these lines with predictions using non-LTE model atmospheres to investigate to what extent these infrared lines can be used to derive effective temperatures. We find that Pfα is a sensitive T_eff diagnostic for the range of spectral types investigated, and Brα for types O9-B2, yielding agreement with optical results to within 1-4kK or one-three spectral sub-types. We find evidence for a gradient in the turbulent velocity, increasing from ≲5km/s for the atmospheric region in which Brα is formed to ∼15km/s for the regime where Pfα originates. When this gradient in turbulent velocity is taken into account, the accuracy of the spectral type calibration is improved to ∼1kK or one spectral sub-type. The gravity dependence of the strengths of the investigated infrared lines is relatively weak, and could not be used to constrain luminosity class. This failure is in part a result of the modest S/N and resolution and in part a result of a cancelation of gravity effects in the line core and line wing. Our line predictions show that He I λ 2.058 is relatively unsuited for spectral classification of O9-B3 stars. Huα, however, is expected to be an even better diagnostic as are Pfα and Brα. This line may be observed with the VLT Imager and Spectrometer for mid InfraRed when it is installed on the Very Large Telescope.