Spectral variations and evidence for edge and/or line locking mechanisms in the low-excitation P.N. HD 138403.

Spectral variations detected within the last three decades for He II 4686 A, forbidden lines O III and other high excitation lines in the low-excitation planetary nebula HD 138403, are shown to suggest appreciable activity in the deep atmospheric layers of the nucleus, and an increase of the excitation of the nebula. The spectrum of the nebula is found to be unusual due to the presence of red and blue emission satellites around the central Balmer (H-beta - H9) lines and forbidden lines O II 3726, 3729 A, with a velocity separation equal to plus 122 and minus 126 km/s, respectively. These features are interpreted as evidence for mass outflow from the central object via the selective radiation process of edge and/or line locking mechanisms.