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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
Received 15 April 1996 / Accepted 10 July 1996
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
Online publication: July 8, 1998