Magnetic fields around massive protostars as traced by masers and dust emission
Sammanfattning: It is not fully clear how the magnetic field acts during the first stages of star formation. A possible way to clarify its role is to observe the polarized light coming from masers and thermal dust emission. By measuring linear polarization angles and Zeeman splitting of different maser species it is possible to study the magnetic field morphology and strength in different parts of the protostar. Polarized emission of thermal dust has also been used extensively to probe the magnetic field at the onset of star formation. In this thesis we study the magnetic field properties of two well-known sources: the massive protostar IRAS18089-1732, showing a hot core chem- istry and a disc-outflow system, and the high-mass star forming complex G9.62+0.19, presenting several cores at different evolutionary stages. We also investigate the polarization properties of selected methanol masers, con- sidering newly-calculated methanol g-factors and hyperfine components. We compare our results with previous maser observations and we evaluate the contribution of preferred hyperfine pumping and non-Zeeman effects. We make use of MERLIN and ALMA observations and we analyse the polarized emission by 6.7 GHz methanol masers and thermal dust. Simulations were run using the radiative transfer code CHAMP for different magnetic field values, hyperfine components and pumping efficiencies. We observe that the large scale field probed by dust continuum emission is consistent with the small scale magnetic field probed by masers. Moreover, in the G9.62+0.19 complex we resolved several cores showing polarized emission. We propose an evolutionary sequence of magnetic field in this complex, where the less evolved stellar embryo exhibits a magnetic field stronger than the more evolved one. From our simulations, we find that preferred hyperfine pumping can explain some high levels of linear and circular polarization. We also notice that non-Zeeman effects need to be considered in magnetic field studies. In conclusion, our work indicates that there is a link between the magnetic field at different scales. More masers observations will help in evaluating the relevance of non-Zeeman effects and obtain good estimates of magnetic fields close to the protostar. Future multi-wavelength and multi-scale observations, aimed at detecting polarized light from masers, thermal dust and thermal molecular lines, will help to constrain magnetic field properties around massive protostars.
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