MagneticallyRegulatedStarFormationinThreeDimensions:TheCaseoftheTaurusMolecularCloudComplex

Intro

There is a long standing debate on the relative importance of turbulence vs. magnetic support in SF. Early studies in the 80's and 90's focused on magnetic support — studying how stars can form out of quiescent, magnetically supported clouds (Nakano, Shu, Mouschovias). This ties into the idea of long lived clouds. More recently, 2000's, people have been looking at how turbulence manifests in star forming environments, and how the SF is effected by it. The ultimate test will be from observations - they need to tell us something about the magnetic field strengths inside of clouds — which is a hard thing to nail down.

There isn’t a lot of evidence of magnetization in clouds. Nearby Taurus and Riegel-Crutcher HI cloud appear to be magnetized at least in their diffuse regions (Heyer '08, McClure-Griffiths '06). Field strength is estimated on the 10's of micro Gauss scale in these regions. These regions may or may not be representative.

While ultimately, the importance of fields (relative to other factors such as turbulence) in star formation will come down to observations, currently, this is lacking. So in the meantime, they want to study how *diffuse*, magnetized structures, similar to the Taurus cloud, evolve to form stars.

Dynamics of diffuse clouds - how do they collapse??
If they’re only marginally magnetically critical, they will have a tendency to collapse along field lines rather than across them. This will result in an increase of the density in the collapsing cloud, while not effecting the field strength. (i.e., were collapse perpendicular to the field lines, drag between the field and collapsing gas would cause the field to get pulled in with the gas/distorted — increasing its strength). Zeeman measurements support this concept — showing that up until ~ 10^{3 cm}(-3), field strength is measured as constant, but above that density, the field grows with density — indicating collapse perpendicular to the field. So first the cloud collapses along field lines (making sheets), and then once the self-gravity wins out, it will begin collapsing radially within the sheets (forming filaments).

Properties of Taurus specifically??
Aside from morphological and field topology discussed earlier, Taurus shows an accelerating star formation rate (Stahler and Palla 2002). That is, stars started forming at a low rate 10 Myr ago - in a spatially dispersed fashion, but that the majority of them have begun forming in the last 3 Myr. This means that as early as 10 Myr, cross-field collapse began, but the field has largely remained dominant, keeping the bulk of the gas magnetically sub-critical or critical. (Note another explanation could be turbulence, but the authors aren’t in that camp). The support for this is that even in dense gas (where you would expect everything to be collapsing to form stars), we measure a moderately low SFR (Goldsmith 2008), M-dot~5x10e-5 sol. mass per yr, which is 2 orders of magnitude lower than the freefall rate of the dense gas (n~10^{4 /cc). This is possible if the cloud is only marginally magnetically critical, so that pockets of it might collapse, but overall it is stable. They have demonstrated this is possible in 2D sheet-geometry sims.}