Magnetic fields in star forming regions

Magnetic fields are dynamically important in star forming regions. The physical characteristics of magnetic fields in molecular clouds have been derived mainly by two methods: (a) measurements of the Zeeman splitting of hyperfine transitions of atoms and molecules, and (b) measurements of polarization of the thermal radiation emitted at millimeter/submillimeter wavelengths by magnetically aligned aspherical dust grains. The measured levels of magnetization imply that molecular clouds are close to the critical value of the mass-to-flux ratio to provide support against gravitational collapse.

Polarization maps of molecular clouds and filaments associated with regions of star formation have revealed rather uniform patterns of magnetic field lines, indicating the presence of a dominating regular component of the field. High-resolution measurements of polarization recently obtained with sub-millimeter arrays (e.g., ALMA) indicate that the magnetic field on scales of ∼1000 AU around young stellar objects has an evident “hourglass” morphology in agreement with the expectations of theoretical models of cloud collapse.

Magnetic fields in filaments, cores, and envelopes

Maps of polarization of the dust emission can be used to infer the magnetic field morphology in clouds, filaments, cores and infalling envelopes. In addition, the Davis-Chandrasekhar-Fermi method can be used to calculate the magnetic field strength if the angular dispersion of the local magnetic field orientations, the gas density, and the one-dimensional velocity dispersion of the gas are known. Our group has been involved since many years in the analysis of polarization maps to provide detailed comparisons with theoretical models and derive fundamental quantities like the magnetic field strength and the mass-to-flux ratio in star forming clouds, see for example Gonçalves, Galli & Walmsley (2005) , Gonçalves, Galli & Girart (2008), Girart, Beltrán, et al. (2009), Frau, Galli & Girart (2011). The figure below, from Beltrán et al. (2019), illustrates an example of modelling dust polarization maps to infer the morphology of the magnetic field in a region of high-mass star formation.

Fig. 1: Panel (a) shows polarization vectors observed with ALMA (in red) and modelled (in blue) for the hourglass-shaped magnetic field in the hot molecular core G31.41+0.31. Panel (b) shows the magnetic field lines of the best-fitting hourglass model inside a radius of 3700 au. Panel (c) shows the same model from a point of view close to pole-on to emphasize the twisting of field lines close to the symmetry axis. From Beltrán et al. (2019).

Magnetic fields in disks

Although magnetic braking has long been believed to be responsible for the loss of angular momentum in cloud cores, an unexpected result has been the theoretical finding that in an ideal MHD flow the magnetic braking becomes so efficient as to prevent the formation of centrifugally supported disks (Lizano & Galli 2015). Therefore, magnetic field diffusion and/or dissipation in the high density regime of gravitational collapse is needed to avoid catastrophic braking. Observations and measurements of magnetic fields in disks around young stars are therefore crucial to constraint theoretical models of disk formation.

The high sensitivity and spatial resolution of the ALMA interferometer have made possible to reveal for the first time the polarization pattern originated by a magnetic field in a young circumbinary disk (Alves et al. 2018). A comparison with models of magnetic field configuration performed with the DustPol module of the ARTIST radiative tool (Padovani et al. 2012) has shown that the field contains a dominant poloidal component resulting from the compression of the field of the parent cloud, and a weaker toroidal component generated by the rotation of the gas and dust.

Polarization from dust self-scattering

Polarization can also arise from mechanisms not related to the magnetic field, as self-scattering of the dust thermal radiation or radiative alignment of non-spherical grains. Recent researches with ALMA and other interferometers have shown that self-scattering is indeed the dominant mechanism for polarization at the scales of protoplanetary disks around young stars. Even if no information on the magnetic field can be derived in this case, the polarization data are a powerful tool for the diagnostics of the properties and the evolution of dust in these disks. In particular, one can constrain with polarization the grain size population and the geometrical distribution of the dust in the disk.

The Arcetri group follows this line of research with studies conducted with ALMA of polarization in disks from young T Tauri stars with jets (Bacciotti et al., 2018). Our results are consistent with dust self-scattering being at the origin of the polarization pattern in the examined targets (see, e.g. Fig 2, for the disk around DG Tau), implying a maximum size of dust grains in the range 50 - 150 μm. The polarization maps give constraints on the scale height of such grains in the disk, and highlight the presence of possible substructures not recognized before. These results confirm that polarization can be a fundamental complement to observations in total emission.

Fig. 2 Polarization in the disk around the young star DG Tau from Bacciotti et al. (2018). Color map: Intensity of the linearly polarized component of the radiation at 0.87 mm. Vector bars: local direction of the polarization angle. Contours: total intensity of the radiation. The arrows indicate the jet orientation, with the disk near-side on the same side of the red arrow. The asymmetry of the polarized intensity indicates a flared geometry for the disk.