Magnetic Fields and the Formation of Aspherical Planetary Nebulae

Abstract

The general evolution of stars with initial mass between 0.8 and 8 solar masses is believed to be well understood until the last stages, when significant mass loss starts. However, an initially spherical star may evolve into an asymmetrical planetary nebula (PN), whereas the underlying mechanism to this process remains as a puzzle. Until about a decade ago, it was believed that stars in the asymptotic giant branch (AGB) phase were still spherically symmetric. Nevertheless, observations performed in the last years show that, for some sources, elongated and asymmetrical envelopes can already be detected during the AGB phase. In the following pre-PN and planetary nebula phases, a variety of morphologies is observed, and the sources are classified into round, elliptical/elongated, bipolar, quadrupolar, multipolar, spiral, collimated lobes and irregular. It is unknown which mechanism or set of mechanisms is responsible for this change of morphology, making this topic to be one of the most discussed by the evolved stars community.<br /> To shed some light on this problem, three AGB stars (IK Tau, R Scl, and V644 Sco) and one red supergiant (VY CMa) were observed at optical wavelengths. We analyzed their dust scattered emission and searched for signs of upcoming asymmetries in their circumstellar envelope. The observations in R band reveal that the dust envelope of the AGB star IK Tau has a global elliptical morphology, and the presence of a central waist is discussed. The observation of VY CMa shows a complex morphology in the very extended nebula that surrounds the source. Furthermore, for the first time the detached shell around the AGB star V644 Sco was imaged, allowing a better investigation of the mass-loss episodes of the source. The detached shell around R Scl was also imaged and analyzed. The results reported in this thesis add together with previous works, confirming that the loss of spherical symmetry in the circumstellar envelope of evolved stars can already start during the AGB phase. <br /> Moreover, we studied one of the mechanisms that can play a role in the shaping process of the circumstellar envelope of these sources: magnetic fields. For this purpose, we investigated 22 GHz H2O maser observations around five sources: four AGB stars (IK Tau, RT Vir, IRC+60370, and AP Lyn) and one pre-PN (OH231.8+4.2). By analyzing the linear and circular polarization in the masers, we detected the presence of magnetic field in four of these five sources. We measured the field strengths to be from a few tens up to a few hundreds of milligauss in the H2O maser region (at a few tens of astronomical units from the star). Comparing our results with magnetic field measurements from the literature, obtained at different distances with respect to the stars, we tried to determine the most plausible geometry of the magnetic fields for the observed sources. However, it is not yet definitive if the observed fields are toroidal, poloidal, or dipole.<br /> The influence of magnetic fields on the shaping process of the circumstellar envelope of evolved stars is still unclear, but their detection around AGB stars, pre-PNe and PNe supports that they might play a role in the process. More measurements of the strength of the fields, also at different distances to the stars, and the investigation of the geometry of the fields are fundamental for providing better constraints to models, and for the better understanding of this subject.