Water in Arcturus' Atmosphere
Abstract
We report on a detection of water vapor in the K1.5III red-giant star Arcturus (α Bootis) using high-resolution, infrared spectra covering the regions 806-822 cm-1 (12.2-12.4 μ m) and 884-925 cm-1 (10.8-11.3 μ m). 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 τ 500=-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
- Publication:
-
American Astronomical Society Meeting Abstracts #200
- Pub Date:
- May 2002
- Bibcode:
- 2002AAS...200.7807R