4. Discussion and conclusions
The variations in UV emission lines in the spectrum of PC 11 and the F-type stellar continuum in the wavelength range 2650Å to 3200Å suggest that the central star of PC 11 is a close binary system with an early-F dwarf companion. The jet-like structures observed in the light of [O III] could be triggered by episodic physical interactions between the two components of the binary system.
On the other hand, the changes observed in the Mg II 2800Å profile might be originated in the chromosphere of the cooler star, heated by the hot companion. Other explanations, however, cannot be ruled out, like effects produced by eclipse or partial eclipse phenomena or rotating star spots in the cool star.
The F-type spectral type assigned to the companion star indicates that the distance to PC 11 must be of a few hundred parsecs. The age of the nebula turns out to be of only a few hundred years.
The absence of a strong UV continuum in the wavelength region 1150Å to 1900Å indicates that the hot whitedwarf or hot sub-dwarf like central star may be obscured by a dusty disk. GMM98 also discussed the possibility of a dusty disk obscuring the central star. The variations in the UV emission lines may be due to the formation and evolution of jet-like structures and clumpy knots in the shell surrounding the central star of PC 11, ejected from the binary nucleus and observed in the high resolution images of PC 11 taken with HST WFPC2 in the light of [O III]. These high excitation knots of emission might be similar to those observed in the PN K1-2, which also shows evidence for jet-like structures ejected from its binary nucleus. The formation of these jets might indicate the action of hydromagnetic winds, similarly to what it is observed in bipolar flows associated to protostellar objects. The formation and evolution of jet-like structures and collimated outflows in PNe remains unclear. However it appears that it is directly or indirectly related to the presence of a binary companion (Soker and Livio 1994; Bobrowsky et al. 1998).
© European Southern Observatory (ESO) 2000
Online publication: March 9, 2000