A chromospheric scenario for the activity ofβ Pictoris, as revealed by FUSE

Abstract

We investigate a chromospheric scenario to explain the characteristics of the far-UV emission lines of Pictoris, revealed by FUSE spectra of this famous circumstellar disk system. The model assumes a thin region heated up to a few 10 5 K located close to the stellar photosphere. The resonance lines of C III at 977 A and O VI at 1032-1037 A, seen in emission, are produced in this chromosphere-transition region complex. Using complementary data in the mid and near UV (specifically, a resonance doublet of C IV at 1548-1550 A and Mg II h & k), we show that the whole dataset is remarquably well reproduced by the model. In addition, we investigate the properties of the C III* multiplet at 1176 A and conclude that this line likely forms in a weak warm wind, originating from the prominent circumstellar disk of Pictoris. Finally, radiative losses have been calculated and have provided estimates of the amount of non radiative energy dissipated in the external atmosphere of Pictoris, which is a measure of the star's activity level. Such behavior for an A5 V star presents a challenge for both evolution and activity models since the former predict that main-sequence A stars should not be active, while the latter are unable to quantitatively account for the characteristics of the chromospheric heating of Pictoris.