| 4 | There isn’t a lot of evidence of magnetization on large, GMC scale clouds, but at least there is fairly good evidence that nearby Taurus, and the Riegel-Crutcher cloud, are magnetically dominated. 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. Again, it is unclear how representative these clouds are, but for them, they’re trying to pin down these kinds of examples.. |
| 5 | |
| 6 | '''Dynamics of diffuse clouds - how do they collapse?? [[br]] |
| 7 | ''' |
| 8 | 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). |
| 9 | |
| 10 | '''Properties of Taurus specifically?? |
| 11 | '''[[br]] |
| 12 | 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. |
| 13 | |