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N. Ryde, D. L. Lambert, M. J. Richter, J. H. Lacy (The University of Texas at Austin)
We report on a detection of water vapor in the K1.5III red-giant star Arcturus (\alpha Bootis) using high-resolution, infrared spectra covering the regions 806-822 cm-1 (12.2-12.4 \mum) and 884-925 cm-1 (10.8-11.3 \mum). Arcturus is the hottest star yet to have shown water-vapor features in its disk-averaged spectrum. We argue that the water vapour lines originate from the photosphere of the star. In calculating the synthetic spectra, we find that no existing water-vapor line list has wavelength assignments sufficiently accurate for our high-resolution spectra. Therefore, we assembled a new water line-list based on laboratory measurement found in the literature. The photosphere of Arcturus is modeled with the MARCS model-atmosphere code with atmospheric parameters from a detailed study of Arcturus based on the entire mid-infrared, low-resolution Infrared Space Observatory spectrum (Decin et al. 1997). Using a photospheric model based on these parameters, we are able to model several OH lines of different excitation and the water-vapor lines satisfactorily first after lowering the temperature structure of the very outer parts of the photosphere at \tau500=-3.8 and beyond compared to a flux-constant, hydrostatic, standard MARCS model photosphere. Our new model is consistently calculated from the given temperature structure. Possible reasons for a temperature decrease in the outer-most parts of the photosphere and the assumed break-down of the assumptions made in classical model-atmosphere codes will be discussed.
This work was supported by the Swedish Foundation for International Cooperation in Research and Higher Education, Stiftelsen Blanceflor Boncompagni-Ludovisi, née Bildt, and the Robert A. Welch Foundation of Houston, Texas
The author(s) of this abstract have provided an email address for comments about the abstract: ryde@astro.as.utexas.edu
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.