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Astron. Astrophys. 333, L19-L22 (1998)


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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

1 Institute of Astronomy, The University of Tokyo, Osawa, Mitaka, Tokyo, 181-8588, Japan
2 Department of Astronomy, School of Science, The University of Tokyo, Bunkyo-ku, Tokyo, 113-0033, Japan

Received 28 November 1997 / Accepted 19 February 1998

Abstract

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 [FORMULA] 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
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