The solar-like "Second Spectrum" and the polarised metal lines in emission of the post-AGB binary 89 Herculis

TL;DR

This study investigates the polarised spectra of the post-AGB binary 89 Herculis, revealing optical pumping polarisation, complex line morphologies, and circumbinary disk features, advancing understanding of aspherical envelopes in evolved stars.

Contribution

It provides the first detailed spectropolarimetric analysis of 89 Herculis, identifying optical pumping as the polarisation mechanism and linking spectral features to circumbinary disk and jet structures.

Findings

Detection of linear polarisation in metal lines and CaII 8662Å line.

Evidence of optical pumping polarisation, ruling out magnetic fields.

Circumbinary disk rotation at ≤10 km/s consistent with interferometric data.

Abstract

We studied the polarised spectrum of the post-AGB binary system 89\,Herculis on the basis of data collected with the high resolution \emph{Catania Astrophysical Observatory Spectropolarimeter}, \emph{HArps-North POlarimeter} and \emph{Echelle SpectroPolarimetric Device for the Observation of Stars}. We find the existence of linear polarisation in the strongest metal lines in absorption and with low excitation potentials. Signals are characterized by complex Q and U morphologies varying with the orbital period. As possible origin of this "Second Solar Spectrum"-like behaviour, we rule out magnetic fields, continuum depolarisation due pulsations and hot spots. { The linear polarisation we detected also in the Ca{\sc ii}\,8662\AA\, line is a clear evidence of optical pumping polarisation and it rules out the scattering polarisation from free electrons of the circumbinary environment.} In the framework of optical pumping due to the secondary star, the observed periodic properties of the spectral line polarisation can be justied by two jets, flow velocity of few tens of km\,s$^{-1}$, at the basis of that hour-glass structure characterising 89\,Herculis. We also discovered linear polarisation across the emission profile of metal lines. Numerical simulations show that these polarised profiles could be formed in an undisrupted circumbinary disk rotating at $\le$10 km\,s$^{-1}$ and whose orientation in the sky is in agreement with optical and radio interferometric results. We conclude that the study of those aspherical enevlopes, whose origin is not yet completely understood, of PNe and already present in the post-AGB's, can benefit of high resolution spectropolarimetry and that this technique can shape envelopes still too far for interferometry.