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

Abstract

Abstract. We investigate a chromospheric scenario to explain the characteristics of the far-UV emission lines of β 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 Å and O VI at 1032-1037 Å, 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 Å 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 Å and conclude that this line likely forms in a weak warm wind, originating from the prominent circumstellar disk of β Pictoris. Finally, radiative losses have been calculated and have provided estimates of the amount of non radiative energy dissipated in the external atmosphere of β 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 β Pictoris.

Accurate magnetic field measurements of Vega-like stars and Herbig Ae/Be stars

“…The most accurate determination of a magnetic field, at 2.6 σ level, has been performed for the Vega-like star ι Cen, for which we measured B z = −77 ± 30 G. In spite of expecting to find a magnetic field in β Pictoris, the prototype of Vega-like stars, which shows conspicuous signs of chromospheric activity (Bouret et al 2002), a longitudinal magnetic field is measured only at ∼1.5 σ level. A longitudinal magnetic field at a level higher than 3 σ was diagnosed for the Herbig Ae stars HD 31648, HD 139614, and HD 144432.…”

FUSE observations of H2 around the Herbig AeBe stars HD 100546 and HD 163296

“…As already observed in AB Aur and β Pic (Roberge et al 2001;Deleuil et al 2001), bright emission from C  and O  are also detected in the two spectra. All these features indicate the presence of circumstellar material, chromospheric activity and/or accretion (Bouret et al 2002). A detailed analysis of the whole FUSE spectrum will be made in a forthcoming paper (Deleuil et al 2003).…”

Burn Out or Fade Away? On the X-Ray and Magnetic Death of Intermediate Mass Stars

“…Tantalizing evidence that young mid-A type stars can have residual X-ray activity came from the FUSE detection of transition region lines in the spectrum of the 12 Myr old A6 V star β Pic (Bouret et al 2002). A tentative detection of the O vii emission was subsequently made from XMM-Newton observations, indicating a plasma temperature of 6 × 10 5 K (Hempel et al 2005).…”