High resolution spectroscopy of CO in the infrared spectra of cool stars. II - Quasi-static molecular dissociation zone in the outer atmosphere of red giant stars

Details High resolution spectroscopy of CO in the infrared spectra of cool stars. II - Quasi-static molecular dissociation zone in the outer atmosphere of red giant stars High resolution spectroscopy of CO in the infrared spectra of cool stars. II - Quasi-static molecular dissociation zone in the outer atmosphere of red giant stars

May 1988

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988a%26a...197..185t&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 197, no. 1-2, May 1988, p. 185-199.

Astronomy and Astrophysics

Astrophysics

51

Astronomical Spectroscopy, Carbon Monoxide, Cool Stars, Infrared Spectra, Red Giant Stars, Stellar Atmospheres, Stellar Envelopes, Abundance, Chromosphere, Cosmic Dust, Dissociation, High Resolution, Inhomogeneity, Molecular Clouds, Photosphere, Stellar Mass

Scientific paper

Low excitation lines of the CO first overtone bands in high resolution infrared spectra of red giant stars show intensity anomalies that cannot be explained by photospheric absorption alone. Also, these CO lines show shifts and asymmetries that indicate excess absorption in blue wing in some stars and in red wing in other stars. Thus, a part of low excitation CO lines should be originating in an extra molecular component, not yet expanding at large velocity, but well distinct from the photo- sphere. The contribution of such an extra molecular component has been separated from the observed profile by subtracting the photospheric contribution, which can be predicted by a model atmosphere. A curve-of-growth analysis of equivalent widths of the separated excess CO absorption revealed that the extra molecular component is characterized by an excitation temperature between 1000 and 2000K, a turbulent velocity as large as 10 km s-1 and a CO column density up to 1020 cm-2. Thus, a quasi-static, turbulent molecular formation zone should exist as an extra component in the outer atmosphere. This new component may represent a transition zone between the chromosphere and the cool wind in red giant stars, and may be a cool component of the chromospheric inhomogeneity and/or a clumpy molecular condensation in an inhomogeneous outer atmosphere. Observational manifestation of such a molecular formation zone in the outer atmosphere of red giant stars may lend a support to a recent theory of an autocatalytic molecular formation by the thermal instability due to molecular cooling. Also, such a turbulent layer can provide a necessary environment for the formation of dust grains, which could initiate mass-loss outflow by means of radiation pressure. Also, the quasi-static molecular formation zone can be regarded as an inhomogeneity in cool corona expanded by the turbulent pressure, which could directly accelerate the cool wind. Thus, turbulence plays probably an essential role in determining the physical structure of the outer atmosphere and massloss in red giant stars.

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