| 16 | | 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. |
| | 16 | There are elongated dense structures in Taurus that seem to run perpendicular to the field lines ( Onishi, Goldsmith). These are thought to have formed from along-field contraction (Heyer, Tamura). Star formation has been occuring there, so across field contraction has to have begun happening as well. Palla and Stahler have found Taurus has an accelerating star formation rate. 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 - stars form locally on short timescales, but the gas overall is magnetically supported.They have demonstrated this is possible in 2D sheet-geometry sims. |
