On the analysis of band 3 of the ISO–SWS calibration sources

Abstract

Abstract. We analyse ISO-SWS 01 (R ∼ 1500) 12−27.5 µm (band 3) spectra of the 10 standard calibration stars with the highest flux using synthetic spectra generated from () atmosphere models. The comparison between the observed and synthetic spectra reveals the quality of (1) the atmospheric model construction and subsequent synthetic spectra computation and of (2) the (OLP 10.1) calibration and data reduction of the spectrometer at these wavelengths. The models represent the general features of the observations, but the synthetic spectrum computation is hampered by the lack of comprehensive molecular and atomic line lists. We also suspect some problems with the temperature distribution in the outer layers of the model and inaccuracies in the extrapolation of the collision-induced absorption coefficients of H 2 pairs. We detect baseline ripples and fringes in the observed spectra, that survive the calibration and detailed reduction process. Photometric calibration uncertainties are estimated by means of the scaling factors between the synthetic and observed spectra.

Amorphous alumina in the extended atmosphere of α Orionis

“…about 0.25 mag in V (Gray 2000) vs. 3 mag for e.g. T Cep (Van Malderen 2003), the low surface gravity and high luminosity may help the levitation of the upper layers. As for Mira stars, the temperatures in this levitated matter can be low enough for molecules to form.…”

“…For a detailed description of these procedures, we refer to and Van Malderen et al (2004). Furthermore, we check the correction for memory effects (Kester et al 2001) by comparing both up and down scans with the final spectrum.…”

“…For a detailed description of the need for this procedure (problematic dark current subtraction, pointing errors, etc. ), we refer to Van Malderen et al (2004).…”

“…They range from plane-parallel slabs placed in front of the star (e.g. Yamamura et al 1999;Matsuura et al 2002;Van Malderen 2003) over infinitesimally thin spherical shells (Perrin et al 2004b) to extended spherical layers with temperature and density distributions Ohnaka 2004a,b). Slab models are not suitable for our purposes: the molecular layers are expected to be fairly optically thin, in which case the slab-approximation is far too rough (in a first order because these models do not take into account that the visible column density is twice as high next to the stellar disk as it is in front of it, and to a second order because they miss sphericity effects which are important for large scale heights).…”

“…However, for computational reasons, we included only lines with log g f − χθ ≥ −9, with g the ground level statistical weight, f the oscillator strength, θ = 5040/3500 and χ the excitation energy in eV. The consequences of neglecting the weaker lines on the final opacity are below 1 percent for a spectral resolution of 300 (Van Malderen et al 2004). Differences between the available line lists for H 2 O (e.g.…”