Polarization properties of methanol masers
  (Abridged) Astronomical masers have been effective tools to study magnetic
fields for many years. In particular, methanol can be used to probe different
parts of protostars such as accretion discs and outflows, since it produces one
of the strongest and the most commonly observed masers in massive star-forming
regions. We investigate the polarization properties of selected methanol maser
transitions in light of newly calculated methanol Land\'e g-factors and
considering hyperfine components. We compare our results with previous
observations and we evaluate the effect of preferred hyperfine pumping and
non-Zeeman effects. We run simulations using the radiative transfer code CHAMP.
We find a dependence of linear and circular polarization fractions on the
hyperfine transitions. Preferred hyperfine pumping can explain some high levels
of linear and circular polarization and some of the peculiar features seen in
the S-shape of observed V-profiles. Methanol masers are not significantly
affected by non-Zeeman effects. Our models show that for methanol maser
emission, both the linear and circular polarization percentages depend on which
hyperfine transition is masing and the degree to which it is being pumped.
Since non-Zeeman effects become more relevant at high values of brightness
temperatures, it is important to obtain good estimates of these quantities and
on maser beaming angles. Better constraints on the brightness temperature will
help in understand about the extent to which non-Zeeman effects contribute to
the observed polarization percentages. In order to detect separate hyperfine
components, an intrinsic thermal line width significantly smaller than the
hyperfine separation is required.
