Turbulence in Zeeman Measurements from Molecular Clouds

Magnetic fields (B fields) play an important role in molecular cloud fragmentation and star formation but are very difficult to detect. The temporal correlation between the field strength (B) and gas density (n) of an isolated cloud has been suggested as an indication of the dynamical importance of B fields relative to self-gravity. This temporal B–n relation is, however, unobservable. What can be observed using Zeeman measurements are the "spatial B–n relations" from the current plane of the sky. Nevertheless, the temporal B–n relation argument has still been widely used to interpret observations. Here we present the first numerical test of the legitimacy of this interpretation. From a simulation that can reproduce the observed Zeeman spatial B ∝ n2/3 relation, we found that temporal B–n relations of individual cores bear no resemblance to the spatial B–n relations. This result inspired us to discover that the true mechanism behind the 2/3 index is random turbulence compression instead of symmetrical gravitational contraction.