Magnetic fields around massive protostars as traced by masers and dust emission
Doctoral thesis, 2020

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.

magnetic field – stars: formation – stars: massive – masers – dust – polarization – stars: individual: IRAS 18089-1732 – G9.62+0.19

Stora mötesrummet, Horsalsvägen 11
Opponent: Prof. Anuj Sarma, Department of Physics and Astrophysics, DePaul University, Chicago, USA


Daria Dall` Olio

Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics, Galactic Astrophysics

Methanol masers reveal the magnetic field of the high-mass protostar IRAS 18089-1732

Astronomy and Astrophysics,; Vol. 607(2017)

Journal article

ALMA reveals the magnetic field evolution in the high-mass star forming complex G9.62+0.19

Astronomy and Astrophysics,; Vol. 626(2019)

Journal article

D. Dall’Olio, W. H. T. Vlemmings, B. Lankhaar and G. Surcis "Polarization properties of methanol masers"

In this thesis work I investigate the role played by magnetic fields during the first stages of stellar birth. A star is born from the collapse of a gigantic cloud made of gas and dust: these are the components of the interstellar medium, the material that occupies the space between stars. In the most dense regions of the cloud, the gas and the dust can condense and finally collapse under the action of gravity, generating a primordial star. It is not fully clear how the magnetic field acts at these stages and thus more observations are needed to study its shape and strength. Being aware of the role of the magnetic field is key to build a clear picture not only of the formation but of the entire life of a star.

I use polarized radiation, coming from molecules and dust located in the region where the star is forming, to work out magnetic field properties. Polarized radiation carries information about the direction and the intensity of the magnetic field, and it can be used as a compass to orient oneself in the magnetic field around the young star. In particular, maser emission is a very powerful tool to trace the magnetic field (masers can be considered the astronomical counterpart of lasers), since it is characterized by a typical fingerprint easily recognizable in the signal observed by the telescope.

An instrument with high resolving power is needed to obtain the most detailed view of the magnetic field around the primordial star. Interferometer telescopes such as the Atacama Large Millimeter Array (ALMA) and the Multi-Element Radio Linked Interferometer Network (MERLIN) can fulfil this requirement because they are made of several radio antennas working together like a single, sharp huge eye. They can observe magnetic fields at the small scale of a few astronomical units (an a.u. is the average distance between the Sun and the Earth) in several regions of our galaxy.

In this thesis I present my results about the magnetic field observed around the young star IRAS 18089-1732 and the G9.62+0.19 complex (a region where several stars are forming). I also report on numerical simulations indicating that methanol, a molecule often observed in star forming regions, is a good tool that can be used to study magnetic field properties.

Subject Categories

Physical Sciences


Basic sciences


C3SE (Chalmers Centre for Computational Science and Engineering)

Onsala Space Observatory



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4832


Chalmers University of Technology

Stora mötesrummet, Horsalsvägen 11


Opponent: Prof. Anuj Sarma, Department of Physics and Astrophysics, DePaul University, Chicago, USA

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