Detection of variable Si~II, Mn~II and Fe~II emission lines in the magnetic Bp star a~Centauri

S. Hubrig$^1$, J.F. Gonzalez$^2$

1 - European Southern Observatory, Casilla 19001, Santiago 19, Chile;
2 - Complejo Astronomico El Leoncito, Casilla 467, 5400 San Juan, Argentina

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.

Reference: Accepted for publication in A&A
Status: Manuscript has been accepted