EDP Sciences
Free access
Volume 366, Number 1, January IV 2001
Page(s) 241 - 253
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20000212

A&A 366, 241-253 (2001)
DOI: 10.1051/0004-6361:20000212

Infrared spectral classification of OB stars with ISO-SWS

P. A. Zaal1, A. de Koter1 and L. B. F. M. Waters1, 2

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

(Received 4 November 1999 / Accepted 6 November 2000)

We present observations of the Br$\alpha$, Br$\beta$and Pf$\alpha$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 $\sim$20 to > 150 and a resolving power varying from $\sim$1400 to 2100. We compare the equivalent widths of these lines with predictions using non-LTE model atmospheres to investigate to what extent these infrared lines can be used to derive effective temperatures. We find that Pf$\alpha$is a sensitive $T_{\rm eff}$ diagnostic for the range of spectral types investigated, and Br$\alpha$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-1for the atmospheric region in which Br$\alpha$is formed to $\sim$15 km s-1for the regime where Pf$\alpha$originates. When this gradient in turbulent velocity is taken into account, the accuracy of the spectral type calibration is improved to $\sim$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 $\lambda$ 2.058is relatively unsuited for spectral classification of O9-B3 stars. Hu$\alpha$, however, is expected to be an even better diagnostic as are Pf$\alpha$and Br$\alpha$. This line may be observed with the VLT Imager and Spectrometer for mid InfraRed when it is installed on the Very Large Telescope.

Key words: line: formation -- radiative transfer -- stars: atmospheres -- stars: early-type -- stars: fundamental parameters -- infrared: stars

Offprint request: A. de Koter, dekoter@astro.uva.nl

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© ESO 2001