Detection of Unresolved Circumstellar Lines in Stellar Infrared Spectra and Discovery of Quasi-Static Molecular Envelope Around Red Giant Stars

Abstract

It is found that the low excitation lines of the first overtone band of CO in the infrared spectra of normal red giant and supergiant stars include excess absorption that cannot be explained by the photospheric absorption alone. the excess absorption is shown to be due to unresolved circumstellar absorption originating from hitherto unrecognized quasi-static molecular envelope around normal red giant and supergiant stars. As contrasted to the previously known expanding circumstellar envelope recognized by Doppler-shifted absorption lines, the newly found static envelope has the following characteristics: (1) excitation temperature determined from CO lines is between 500 and 1000K, indicating that the envelope may be located at a few stellar radii above the photosphere, (2) turbulent velocity may be as high as 5km/s, (3) CO column density increases from 10 +19 /cm 2 at early M giant to 10 +20 /cm 2 at the latest non-Mira M-giant, and (4) molecular envelope and stellar photosphere show slight relative motion in general. Some implications of such a quasi-static molecular envelope on circumstellar chemistry as well as on stellar mass-loss are discussed.