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

Abstract

We detected five forbidden lines of iron, silicon and sulfur in red giants by the ISO SWS. [FeI] 24mu m emission was detected in two carbon stars, TX Psc and WZ Cas, and [SI] 25.2mu m 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] 26mu m, [SiII] 34.8mu m and [FeII] 35.3mu m emission lines were detected in the spectrum of 30g Her, while only [Fe II] 26mu m 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 x 10(-6}M_{sun) 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. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA.