Master´s Thesis Defense by Jonathan Oftedahl Vivanco – Niels Bohr Institute - University of Copenhagen

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Master´s Thesis Defense by Jonathan Oftedahl Vivanco

In this M.Sc. thesis I present elaborate radiation transfer simulations of the polarized thermal emission from dust grains. The purpose is to investigate to what degree one can use such observations to uncover the magnetic field structure in star forming regions.

Magnetic fields are ubiquitous in the Galaxy and have a large impact on star formation in molecular clouds. Along with gravity and turbulence they play a significant part in controlling their dynamics.During the earliest formation phase the magnetic fields provides support which delays the accretion of matter by the protostar. At later times they also play an important role in the formation and fragmentation of disks where excess angular momentum is transmitted to outflows and jets.These physical markers are seen in simulations and to constrain the physics behind them, an observational approach must be taken.

Observations of magnetic fields around low-mass protostars can be performed by observing the cold thermal continuum emission from dust grains. Dust grains are predominantly elongated and as a result they emit more thermal radiation along their longest axis. At the same time they rotate with their rotation axis aligned towards the local magnetic field. As a consequence the observed thermal dust emission is linearly polarized and this can be used to map the plane-of-sky component of the magnetic field. Since the last decade this has been a useful tool for observing the magnetic field structure around protostars. Such observations support the standard formation theory in which the field has an hourglass morphology due to the ongoing process of ambipolar diffusion. By modelling the polarization signature one can obtain further insight on the role of magnetic fields during star formation. Ultimately this can serve as a link between theory and observations.

I have simulated the polarization signature for analytical models and a full 3D magnetohydrodynamical simulation by using the radiation transfer code LIME/DustPol. In addition I have simulated SMA and ALMA observations of the polarization signature with the instrument simulator of CASA.The results shows that it is possible to recover details of the magnetic field by modelling the polarized thermal dust emission and by simulating observations. But the interpretation of the field morphology is sensitive to the modelled noise levels for a given instrument. Regions with low or absent polarization can occur when the magnetic field change direction along the line of sight. Moreover, ALMA will have much better capabilities to observe magnetic fields because of its ability to recover more details than any previous instruments.