Water in Emission in the ISO Spectrum of the Early M Supergiant Star mu Cephei

摘要

We report a detection of water in emission in the spectrum of the M2supergiant atar mu Cep (M2Ia) observed by the Short Wavelength Spectrometer(SWS) aboard Infrared Space Observatory (ISO) and now released as the ISOArchives. The emission first appears in the 6 micron region (nu2 fundamental)and then in the 40 micron region (pure rotation lines) despite the ratherstrong dust emission. The intensity ratios of the emission features are farfrom those of the optically thin gaseous emission. Instead, we could reproducethe major observed emission features by an optically thick water sphere of theinner radius about two stellar radii (1300Rsun), Tex = 1500K, and Ncol (H2O) =3.0E+20/cm2. This model also accounts for the H2O absorption bands in the nearinfrared (1.4, 1.9, and 2.7 micron) as well. The detection of water in emissionprovides strong constraints on the nature of water in the early M supergiantstars, and especially its origin in the outer atmosphere is confirmed againstother models such as the large convective cell model. We finally confirm thatthe early M supergiant star is surrounded by a huge optically thick sphere ofthe warm water vapor, which may be referred to as MOLsphere for simplicity.Thus, the outer atmosphere of M supergiant stars should have a complicatedhierarchical and/or hybrid structure with at least three major constituentsincluding the warm MOLsphere (T about 1.0E+3K) together with the previouslyknown hot chromosphere (T about 1.0E+4K) and cool expanding gas-dust envelope(T about 1.0E+2K).