Infrared spectral classification of OB stars with ISO-SWS

¹ Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
² Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200 B, 3001 Heverlee, Belgium

Corresponding author: A. de Koter, dekoter@astro.uva.nl

Received: 4 November 1999
Accepted: 6 November 2000

Abstract

We present observations of the Brα, Brβand Pfαlines 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 Pfαis a sensitive T_eff diagnostic for the range of spectral types investigated, and Brαfor 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 5 km s^-1for the atmospheric region in which Brαis formed to ∼15 km s^-1for the regime where Pfαoriginates. 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 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 λ 2.058is relatively unsuited for spectral classification of O9-B3 stars. Huα, however, is expected to be an even better diagnostic as are Pfαand Brα. This line may be observed with the VLT Imager and Spectrometer for mid InfraRed when it is installed on the Very Large Telescope.