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Letter to the Editor
Detection of fine-structure line emission from carbon-rich and oxygen-rich AGB stars by the ISO SWS *
W. Aoki 1, 2,
T. Tsuji 1 and
K. Ohnaka 1
Received 28 November 1997 / Accepted 19 February 1998
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] 25.2 µm 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] 26 µm, [Si II] 34.8 µm 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 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.
Key words: stars: AGB and post-AGB stars: chromospheres stars: circumstellar matter stars: late-type stars: mass-loss infrared: stars
* 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.
Present address: Institut für Astronomie und Astrophysik, Technische Universität Berlin, Hardenbergstr. 36, D-10623 Berlin, Germany
© European Southern Observatory (ESO) 1998
Online publication: April 20, 1998