Detection of fine-structure line emission from carbon-rich and oxygen-rich AGB stars by the ISO SWS

We detected five forbidden lines of iron, silicon and sulfur in red giants by the ISO SWS. [Fe I] 24 μm emission was detected in two carbon stars, TX Psc and WZ Cas, and [S I] 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 [Fe II] 26μm, [Si II] 34.8μm and [Fe II] 35.3 μm emission lines were detected in the spectrum of 30g Her, while only [Fe II] 26 μm 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 × 10^-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.