KI emission from envelopes around N-type stars : Spectroscopic observations and interpretations

• 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 choronagraphic 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 estimates from the K I line observations agree rather well with those obtained from CO (rangning from 1/4 to 1/1 of the CO mass loss), which suggest 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.

Nyckelord

stars: carbon -circumstellar matter - stars: individual: R Scl

X TrA

V Aql - stars: mass-loss - stars: AGB

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