  The nature of non-variable high-excitation emission lines detected in the
optical spectra of normal late-B type and chemically peculiar HgMn and PGa
stars is still poorly understood.
  To better understand the origin of the weak emission lines in B type stars it
is especially important to investigate the spectra of a variety of stars to
search for correlations between the emergence of these lines and fundamental
stellar parameters.
  We have acquired high resolution UVES spectra for the sharp-lined magnetic
helium-variable star a Cen over the rotation period of 8.82 d to search for the
presence of weak emission lines.
  For the first time we present observational evidence for the appearance of
variable high-excitation Si II, Mn II and Fe II emission lines in a magnetic Bp
star. Si II emissions are the strongest at the phase corresponding to the
maximum strength of He I lines. Mn II and Fe II emissions vary in antiphase to
the He I lines. A correlation is found between the probable location of Mn and
Fe surface spots and the strength of the emission lines. On the basis of the
currently available data it seems possible that the same kind of selective
excitation process is working in the atmospheres of objects within a broad
parameter space which could be defined by age, effective temperature, chemical
composition, rotational velocity, and magnetic field. Neutral iron lines
previously reported to appear broad and shallow at certain phases are not
detected in our spectra, although two of them are identified as He I forbidden
lines, showing maximum strength at the phase of the passage of the He rich
region across the visible disk.
