Improved observations and models

In the field of hot stars, high-quality IR-spectra have been obtained during the last decade (Morris et al. 1996; Hanson et al. 1996; Figer et al. 1997; Blum et al. 1997). Most of these spectra were obtained with low-mid (R ~ 5002,000) resolution, which is enough to classify the stars but insufficient in most cases to perform accurate quantitative spectroscopic studies. An example of

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: Fe ll Fe ll

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Figure 13.1. The impact of high signal-to-noise ratio and mid-high spectral resolution and new blanketed models for early-type stars in the infrared, showing UKIRT-CGS4 (R ~ 5,000) H-, K-, and L-band and Bra observations of the "Pistol Star" in the Quintuplet cluster and new model fits.

mid-high-resolution observations of early-type stars with strong winds is shown in Figure 13.1, which displays the spectra for several IR bands of the "Pistol Star," an LBV in the Quintuplet cluster, which were obtained with UKIRT-CGS4 with a resolution of R ~ 5,000. The number of new observational constraints provided by the new spectroscopic data is striking in comparison with the limited quality of previous low-mid-resolution observations, from which we could gather information only from some H and He lines. On inspecting Figure 13.1 we immediately note that the key diagnostic lines would be fully blurred at R ~ 1,000. The availability of high-quality IR spectroscopic data has been improved substantially with a new generation of IR-spectrographs on 8-m class telescopes (ISAAC, NIRSPEC, SINFONI, etc.).

The updated model (Hillier & Miller 1998a) is a line-blanketing method based on the standard iterative, non-LTE method to solve the radiative-transfer equation for the expanding atmospheres of early-type stars. New species, O, Mg, Ca, Si, Na, Al, Fe, etc., are included and the blanketing ensures that the effect of continua on lines and of lines on the continua as well as overlapping lines are handled automatically; see Hillier & Miller (1998a, 1999) for a detailed discussion of the method. The new model is then prescribed by the stellar radius, R*, the stellar luminosity, L*, the mass-loss rate M, the velocity field, v(r), the volume-filling factor f, and the abundances of the element considered.

In this review, we assume the Solar abundances derived by Anders & Grevesse (1989) because they are the ones entering evolutionary-model calculations.

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