Polarimetry and Direct Imaging of Detached Shells Around Carbon Stars

Abstract

This poster presents polarimetry and direct images in the atomic Na D (λ₁= 5890 Å, λ₂= 5896 Å) and K I (λ= 7699 Å) resonance lines, showing shells around the nearby bright carbon stars R Scl and U Ant. They confirm results obtained in CO radio line observations. A simple analytical model shows that the observed intensities are consistent with resonance line scattering of stellar light provided that about 1% of the K and Na has remained in neutral form.

In the case of R Scl, the direct images, both in K and Na, show an almost perfectly circular disk with essentially uniform intensity out to a sharp outer radius of ≈ 21”. Crude Wollaston polarimetry also shows that this shell is polarised up to ≈ 30%. The K image of U Ant shows a geometrically thin shell of radius ≈ 43”. However, the Na image suggests the presence of at least two other shells inside the ≈ 43” shell. Polarimetry imaging of U Ant still remains to be done.

These narrow shells might be one of the few observable direct consequences of strong and rapid variations in the mass loss rate, induced by thermal pulses, that must have been involved in their origin.