Context :

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 -4.75 < { log } { W / \nu } \la - 4.4 : W is the equivalent width and \nu the wavenumber ) of CO fundamentals as well as overtones can not 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 A _ { H } = 12.00 , \xi _ { micro } = 1.87 km s ^ { -1 } , and \xi _ { macro } = 3.47 km s ^ { -1 } ) .
The nature of CO lines shows an abrupt change at { log } { W / \nu } \approx - 4.75 and the lines stronger than this limit indicate large excess absorption that can not be photospheric in origin .
This difficulty also appears as an unpredictable upturn ( at { log } { W / \nu } \approx - 4.75 ) 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 ( \la 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 ) .