P.A. Zaal

Infrared spectral classification of OB stars with ISO-SWS

We present observations of the Br ,B r and Pf 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 of20 to > 150 and a resolving power varying from1400 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 eective temperatures. We nd that Pf is a sensitive Te diagnostic for the range of spectral types investigated, and Br for types O9{B2, yielding agreement with optical results to within 1 4 kK or one-three spectral sub-types. We nd evidence for a gradient in the turbulent velocity, increasing from < 5k m s 1 for the atmospheric region in which Br is formed to 15 km s 1 for the regime where Pf originates. When this gradient in turbulent velocity is taken into account, the accuracy of the spectral type calibration is improved to1 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 eects in the line core and line wing. Our line predictions show that He i 2.058 is relatively unsuited for spectral classication of O9{B3 stars. Hu, however, is expected to be an even better diagnostic as are Pf and Br .T his line may be observed with the VLT Imager and Spectrometer for mid InfraRed when it is installed on the Very Large Telescope.

Astrophysics and Space Science

Astrophysics and Space Science publishes original contributions and invited reviews covering the entire range of astronomy, astrophysics, astrophysical cosmology, planetary and space science and the astrophysical aspects of astrobiology. This includes both observational and theoretical research, the techniques of astronomical instrumentation and data analysis and astronomical space instrumentation. We particularly welcome papers in the general fields of high-energy astrophysics, astrophysical and astrochemical studies of the interstellar medium including star formation, planetary astrophysics, the formation and evolution of galaxies and the evolution of large scale structure in the Universe. Papers in mathematical physics or in general relativity which do not establish clear astrophysical applications will no longer be considered. The journal also publishes topically selected special issues in research fields of particular scientific interest. These consist of both invited reviews and original research papers. Conference proceedings will not be considered. All papers published in the journal are subject to thorough and strict peer-reviewing. Astrophysics and Space Science features short publication times after acceptance and colour printing free of charge. Commonly used title abbreviations: Astrophys Space Sci, Ap&SS

The ISO-SWS Flux Standard Stars: Synthetic Spectra and Observations

We present flux-calibrated, synthetic spectra for the calibration stars of the Short Wavelength Spectrometer of the Infrared Space Observatory ISOSWS (cf. Kessler et al. 1996). ISO-SWS covers a wavelength range of 2.4 to 45µm (cf. de Graauw et al. 1996), and although in the NIR the flux calibration of Vega can be used, at longer wavelengths the flux calibration has to be extrapolated by other means, because (i) Vega has at λ ≥ 25μm an IR excess due to circumstellar dust and (ii) there is an, as yet, unresolved controversy about Vega’s flux at 10µm (cf. Rieke et al. 1985). The majority of the standard stars of SWS are of MK class G and K III, as cool giants are amongst the brightest objects in the IR, and model atmospheres for cool giants are available and well-studied (cf. Jorgensen and Gustafsson 1994).