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 radiat ion, was first found spectroscopically, but has later been imaged with coronographic and polarimetric techniques. In the present paper, whic h is the first in a series, the spectroscopic K I observations are dis cussed. From the observations of the K I 769.9 nm emission we find sys temic and expansion velocities in fair agreement with those obtained f rom the CO millimetre lines. We find a decline of the emission with di stance from the star, in rough agreement with the assumption of a cons tant expansion velocity, mass-loss rate and KI abundance. Our mass los s rate estimates from the KI 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 neu tral through the envelope. This puts strong upper limits on the photoi onizing chromospheric UV emission from the stars. Some indirect indica tions that the envelopes have inhomogeneous structures, clumps, are di scussed.