1998A&A...333L..19A

We detected five forbidden lines of iron, silicon and sulfur in red giants by the ISO SWS. [FeI] 24µm emission was detected in two carbon stars, TX Psc and WZ Cas, and [SI] 25.2µm emission was also detected in TX Psc, while none of these emission lines were detected in an oxygen-rich giant, 30g Her. On the other hand, the [FeII] 26µm, [SiII] 34.8µm and [Fe II] 35.3µm emission lines were detected in the spectrum of 30g Her, while only [Fe II] 26µm was detected in TX Psc. Namely the emission lines of neutral metals were detected in carbon stars while the lines of ions were detected in an oxygen-rich giant. This result suggests that the ionization states of metals in the outer atmosphere as well as the ultraviolet radiation field of carbon stars are different from those of M giants. We estimate that the mass of the line forming region is several x10^–6M_☉ and the temperature is several hundred Kelvin or higher. This implies that there is a rather dense and warm region close to the star in addition to the well known expanding envelope.