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MagneticallyRegulatedStarFormationinThreeDimensions:TheCaseoftheTaurusMolecularCloudComplex

Abstract
Model strong fields + diffuse clouds, motivated by observations of Taurus & Pipe Nebula which show large scale ordered fields that run perpendicular to diffuse elongated clouds. Sims include ambipolar diffusion and outflow feedback. Find that stars begin to form once clouds are marginally ​magnetically critical. However, only a small portion of the nearly magnetically critical material is forming stars - about ~1% of the gas ​per local free-fall time. The regulation of the SF in these regions seems to be due to magnetic fields rather than turbulence, as these regions are only marginally supersonic. The quiescent condensations are surrounded by turbulent diffuse halos, reminiscent of Taurus. Notable figures ​here.

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??
For marginally magnetically critical clouds, collapse first occurs along field lines rather than across them, as there is no magnetic support in that direction. This results in an increase in density without a concomitant change in field strength (were collapse occurring perpendicular to the field lines, drag between charged particles and field would cause field distortion, increasing its strength). Zeeman measurements support this concept — showing that up until ~ 10³ cm^-3, field strength is constant. Above that density, the field grows with density — indicating collapse perpendicular to the field. This type of collapse can lead to sheets, filaments, and knots, depending on the degree of initial anisotropy in the mass distribution. As an example, you can imagine a cloud collapsing along field lines will make sheets, and then once the self-gravity wins out, it will begin collapsing radially within the sheets, forming filaments.

Properties of Taurus specifically??
There are elongated dense structures in Taurus that seem to run perpendicular to the field lines (Onishi '96, '02, Goldsmith '08). These are thought to have formed from along-field contraction (Heyer, Tamura '87). Star formation has been occuring there, so across field contraction has to have begun happening as well. Palla and Stahler ('02) have found Taurus has an accelerating star formation rate - SF has begun ~10 Myr ago, but most has been happening in the last ~3 Myr. This is conducive with a 2 stage collapse — along field lines and then across them once enough material has collected to make you critical/supercritical. The star formation rate in the last 3 Myr in the dense gas is ~ M-dot = 5x10^-5 solar masses/year — ~2 orders below free fall rate in the dense gas (Goldsmith '2008). Krumholz and Tan '07 have found this holds for a wide variety of objects. This can be explained by turbulent support, people have found marginallly supersonic motions in Taurus (~M=2). However, marginal magnetic criticality also explains it quite naturally (Basu & Ciolek '04) - stars form locally on short timescales, but the gas overall is magnetically supported. The authors have demonstrated this is possible in 2D sheet-geometry sims. The pipe nebula might be an earlier phase of evolution for a magnetically dominated cloud (Lada '08, Alves '08).