Arquivo do trabalho - IAG - USP

tion and found that it is generally smaller than the observed values in our Galaxy (sfe ∼0.01−0.3). This result is consistent with previous work in the literature and also suggeststhat the mechanism presently investigated, though very powerful to drive structure formation,supersonic turbulence and eventually, local star formation, does not seem to besufficient to drive global star formation in normal star forming galaxies, not even whenthe magnetic field is neglected.Besides the study above, we have also explored star formation considering a prioriinjection of turbulence (by an arbitrary physical mechanism) in magnetized clouds. Fora molecular cloud clump to form stars some transport of magnetic flux may be requiredfrom the denser, inner regions to the outer regions of the cloud, otherwise this can preventthe gravitational collapse. We have considered here a new mechanism. Fast magneticreconnection which takes place in the presence of turbulence can induce a process ofreconnectiondiffusionofthemagneticfield. Inthiswork, wehaveinvestigated thisprocessbymeansof3DMHDnumericalsimulationsconsidering itsimplicationsonstarformation.Wehave extended aprevious studywhich considered cloudswithcylindrical geometryandno self-gravity (Santos-Lima et al. 2010). Here, we considered more realistic clouds withspherical gravitational potentials (from embedded stars) and also accounted for the effectsof the gas self-gravity. We demonstrated that reconnection diffusion takes place. We havealso, for the first time, determined the conditions under which reconnection diffusion isefficient enough to make an initially subcritical cloud clump to become supercritical andcollapse.Ourresultsindicatethattheformationofasupercritical coreisregulatedbyacomplexinterplay between gravity, self-gravity, magnetic field strength and nearly transonic andtrans-Alfvénic turbulence. In particular, self-gravity helps reconnection diffusion and, asa result, the magnetic field decoupling from the collapsing gas becomes more efficient thanin the case when only an external gravitational field is present. We have demonstratedthat reconnection diffusion is able to remove magnetic flux from most of the collapsingclumps analysed, but only a few of them develop nearly critical or supercritical cores,which is consistent with the observations. Their formation is restricted to a range ofinitial conditions for the clouds as follows: thermal to magnetic pressure ratios β ∼ 1 to 3,viii