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

Abstract

Context. 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. Aims. 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. Methods. 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. Results. 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.

Doppler imaging of the helium-variable star a Centauri

“…We obtained a tentative hint of the presence immediately obvious that the star showed pronounced variations of metal lines as later reported by Hubrig & Gonzalez (2007). Even though the v sin i of a Cen is at the extreme low end of what is useful for Doppler imaging and our phase resolution is quite coarse, the strong spectrum variability encouraged us to try to map the surface abundance distribution for the elements He, Fe, N, and O to see if there were indications of axial symmetry in these distributions that would augment the impressions of the orientation of the magnetic axis gleaned from the earlier work of Borra et al (1983).…”

“…We have already noted the discovery of weak metallic emission lines in a Cen by Hubrig & Gonzalez (2007); the incidence of this phenomenon appears to be almost universal among the helium-3 stars.…”

Ultraviolet variability of the helium-peculiar staraCentauri

“…Since the analysis by Norris (), many studies of a Cen have been carried out (Underhill & Klinglesmith ; Underhill, Klinglesmith & Frey ; Underhill, Fahey & Klinglesmith ) with the aim of understanding the complex spectral behaviour of this star. Qualitative discussions of the distribution of helium on the surface of a Cen have been given by Norris & Baschek () and Mihalas (), while Hubrig & González () were the first to present observational evidence for the appearance of variable high‐excitation Si ii , Mn ii and Fe ii emission lines. Recently, Bohlender, Rice & Hechler () obtained abundance maps of several elements over the surface of a Cen, using Doppler imaging techniques.…”

Electron-impact excitation of Fe II

“…Of the lines that were identified 346 were found to be emission lines of Fe ii. More recently, Hubrig & Gonzalez (2007) have for the first time presented observational evidence for the appearance of variable high-excitation Fe ii emission lines in the magnetic Bp star aCentauri. D 'Elia et al (2007) presented UVES/VLT high resolution spectroscopy of the optical afterglow of Gamma Ray Burst GRB 050730 and detected strong Fe ii fine-structure absorption lines in the Inter Stellar Medium of the GRB host galaxy.…”