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Astron. Astrophys. 318, 535-542 (1997)

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K I emission from envelopes around N-type stars - Spectroscopic observations and interpretations

Bengt Gustafsson 1, Kjell Eriksson 1, Dan Kiselman 2, Nils Olander 3 and Hans Olofsson 4

1 Uppsala Astronomical Observatory, Box 515, S-751 20 Uppsala, Sweden
2 The Royal Swedish Academy of Sciences, Stockholm Observatory, S-133 36 Saltsjöbaden, Sweden
3 Dept. of Physics and Mathematics, Mid Sweden University, S-851 70 Sundsvall, Sweden
4 Stockholm Observatory, S-133 36 Saltsjöbaden, Sweden

Received 15 April 1996 / Accepted 10 July 1996

Abstract

Circumstellar envelopes around three bright N-type stars, R Scl, X TrA, and V Aql have been detected in emission in resonance lines from K I. This radiation, which is most probably scattered photospheric radiation, was first found spectroscopically, but has later been imaged with coronographic and polarimetric techniques. In the present paper, which is the first in a series, the spectroscopic K I observations are discussed.

From the observations of the K I 769.9 nm emission we find systemic and expansion velocities in fair agreement with those obtained from the CO millimetre lines. We find a decline of the emission with distance from the star, in rough agreement with the assumption of a constant expansion velocity, mass-loss rate and K I abundance. Our mass loss rate estimates from the K I line observations agree rather well with those obtained from CO (ranging from 1/4 to 1/1 of the CO mass loss), which suggests that a considerable fraction of the potassium stays neutral through the envelope. This puts strong upper limits on the photoionizing chromospheric UV emission from the stars. Some indirect indications that the envelopes have inhomogeneous structures, clumps, are discussed.

Key words: stars: carbon – circumstellar matter – stars: individual: R Scl, X TrA, V Aql – stars: mass-loss – stars: AGB

Send offprint requests to: B. Gustafsson

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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