Infrared spectral classification of OB stars with ISO-SWS

Abstract

We present observations of the Bralpha , Brbeta and Pfalpha 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 Pfalpha is a sensitive T_eff diagnostic for the range of spectral types investigated, and Bralpha for types O9-B2, yielding agreement with optical results to within 1-4 kK or one-three spectral sub-types. We find evidence for a gradient in the turbulent velocity, increasing from la 5 km s^-1 for the atmospheric region in which Bralpha is formed to ~ 15 km s^-1 for the regime where Pfalpha originates. When this gradient in turbulent velocity is taken into account, the accuracy of the spectral type calibration is improved to ~ 1 kK 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 Hei lambda 2.058 is relatively unsuited for spectral classification of O9-B3 stars. Hualpha , however, is expected to be an even better diagnostic as are Pfalpha and Bralpha . This line may be observed with the VLT Imager and Spectrometer for mid InfraRed when it is installed on the Very Large Telescope.