High resolution linear spectropolarimetry: a study of the potentiality in probing stellar atmospheres and circumstellar environments

Abstract

Polarimetry is considered as an important searching tool in Astronomy and it deserves to be a standard observational technique for a variety of astrophysical environments. Many phenomena, in fact, can contribute to the polarisation of radiation and so, its observation can potentially provide information about their basic causes. In particular, in a stellar context high resolution spectropolarimetry can potentially: (i) investigate the physics of the shaping mechanisms of the protoplanetary nebulae (PNe) and the possible interaction between the central star and the very close surrounding, (ii) investigate stellar photospheric inhomogeneities (e.g. pulsations, granulations, hot spots and magnetic activity), (iii) play a key role in the exoplanet research. Despite this, however, the difficulties involved in the measurements and in the theoretical interpretation made this technique not as widespread as it deserves.

The main goal of this work is to demonstrate the diagnostic potential of high resolution linear spectropolarimetry. At this purpose, 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} (CAOS) and \emph{HArps-North POlarimeter} (HANPO), in addition to archive data taken with the \emph{Echelle SpectroPolarimtric Device for the Observation of Stars} (ESPaDOnS). We found the existence of linear polarisation in the metal lines in absorption and in some of the metal lines in emission: this is the first \emph{Second Solar Spectrum} ever observed in a star different than the Sun. We have then analyzed the observed polarisation properties. Firstly, regarding the polarisation measured across the absorption lines, we found that the complex Stokes $Q$ and $U$ morphologies vary with the orbital period of the system. We then rule out magnetic field, continuum depolarisation due to pulsations and hot spots as a possible origin. We found that in the framework of optical pumping due to the secondary star, the observed periodic properties of the spectral line polarisation can be justified by two jets, with a flow velocity of a few tens of $km$ $s^{-1}$, at the basis of that hourglass structure characterizing 89 Herculis. Regarding the emission lines, numerical simulations show that these polarised profiles could be formed in an undistrupted circumbinary disk rotating at $\leq 10$ $km$ $s^{-1}$ and with an orientation in the sky in agreement with optical and radio interferometric results. We have concluded that the study of aspherical envelopes, the origin of which is not yet completely understood, of PNe and already present in post-AGBs can benefit from high resolution spectropolarimetry and that this technique can shape envelopes still too far away for interferometry.

In addition, we have conducted a large spectropolarimetric survey on a sample of F-G-K bright stars to understand if the presence of intrinsic linear polarisation is a rare case or it is widespread in these type of stars. Surprisingly, we have found linear polarisation across metal lines in absorption for about $71$ $\%$ of stars of our sample. Some of them clearly show temporal variability, to be characterized with further data. Despite this study is neither exhaustive nor conclusive we speculated that the presence of linear polarisation in the stars is not rare: a broad and in-depth spectropolarimetric study is then necessary.