The K giant star Arcturus: the hybrid nature of its infrared spectrum ^*

Institute of Astronomy, School of Sciences, The University of Tokyo, Mitaka, Tokyo, 181-0015, Japan e-mail: ttsuji@ioa.s.u-tokyo.ac.jp

Received: 14 April 2009
Accepted: 28 May 2009

Abstract

Aims. We investigate the infrared spectrum of Arcturus to clarify the nature of the cool component of its atmosphere, referred to as the CO-mosphere, and its relationship to the warm molecular envelope or the MOLsphere in cooler M (super)giant stars.

Methods. We apply the standard methods of spectral analysis to the CO lines measured from the “Infrared Atlas of the Arcturus Spectrum” by Hinkle, Wallace, and Livingston.

Results. We found that the intermediate-strength lines (with : W is the equivalent width and ν the wavenumber) of CO fundamentals as well as overtones cannot be interpreted with the line-by-line analysis based on the classical line formation theory, while the weaker lines can and provide some information on the photosphere (e.g. log A_C = 7.97/log , = 1.87 km s^-1, and  km s^-1). The nature of CO lines shows an abrupt change at and the lines stronger than this limit indicate large excess absorption that cannot be photospheric in origin. This difficulty also appears as an unpredictable upturn (at ) in the flat part of the curves-of-growth of CO lines. We confirm the same phenomenon in dozens of M giant stars, whose infrared spectra are known to have hybrid origins in the photosphere and extra-molecular constituent referred to as the MOLsphere. Thus the curve-of-growth analysis provides a simple means by which to recognize the hybrid nature of the infrared spectra. We note that the curves-of-growth of red supergiants and Mira variables found in the literature show similar peculiar patterns. The intermediate-strength lines of CO in Arcturus show only minor expansion (0.6 km s^-1) relative to the weak lines and only small line-asymmetry, but the strong lines of the CO fundamentals exhibit higher turbulent velocity than the other CO lines.

Conclusions. The large excess absorption of the CO fundamental lines in Arcturus appears to be the same phenomenon as that found in the CO overtone lines of cooler M giant stars. Thus, molecular condensation referred to as the MOLsphere in cool luminous stars may also exist in Arcturus. The MOLsphere, however, is not necessarily a separate “sphere” but may be an aggregation of molecular clouds formed within the outer atmosphere. The formation of molecular clouds (referred to as MOLsphere in our modeling) in the outer atmosphere appears to be a basic feature of all the red giant stars from early K to late M types (and red supergiants).