Márcia Leão

Publications (4) View all

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  Available from: Márcia Leão

  Article: Magnetic Flux Transport by turbulent reconnection in astrophysical flows

  Elisabete M. de Gouveia Dal Pino, Márcia R. M. Leão, Reinaldo Santos-Lima, Gustavo Guerrero, Grzegorz Kowal, Alex Lazarian
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  ABSTRACT: The role of MHD turbulence in astrophysical environments is still highly debated. An important question that permeates this debate is the transport of magnetic flux. This is particularly important, for instance, in the context of star formation. When clouds collapse gravitationally to form stars, there must be some magnetic flux transport. otherwise the new born stars would have magnetic fields several orders of magnitude larger than the observed ones. Also, the magnetic flux that is dragged in the late stages of the formation of a star can remove all the rotational support from the accretion disk that grows around the protostar. The efficiency of the mechanism which is often invoked to allow the transport of magnetic fields in the different stages of star formation, namely, the ambipolar diffusion, has been lately put in check. We here discuss an alternative mechanism for magnetic flux transport which is based on turbulent fast magnetic reconnection. We review recent results obtained from 3D MHD numerical simulations that indicate that this mechanism is very efficient for decoupling and transport magnetic flux from the inner denser regions to the outskirts of collapsing clouds in the different stages of star formation. We also discuss this mechanism in the context of dynamo processes and speculate that it can play a role both in the solar dynamo and in accretion disk dynamo processes.
  12/2011;
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  Conference Proceeding: Star formation triggered by SNR impact into magnetized neutral clouds

  M. R. M. Leao (IAG-USP, E. M. de Gouveia Dal Pino (IAG-USP, D. Falceta-Goncalves (UNICSUL, C. Melioli (University of Bologna, F. G. Geraissate (IAG-USP
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  ABSTRACT: Considering the physical conditions that are relevant for triggering star formation in interactions involving SN shocks and neutral clouds, we have built diagrams of the SNR radius versus the cloud density in which these conditions constrain a shaded zone where star formation is allowed. The diagrams are also tested with fully 3-D MHD radiative cooling simulations involving a SNR and a self-gravitating cloud and we find that the numerical analysis is consistent with the results predicted by the diagrams. While the inclusion of a homogeneous magnetic field approximately perpendicular to the impact velocity of the SNR with an intensity ~1 $mu$G results only a small shrinking of the star formation zone in the diagrams, a larger magnetic field (~10 $\mu$G) causes a significant shrinking, as expected. Applications of our results to real star formation regions in our own galaxy have revealed that their formation could have been triggered by a SN shock wave. Finally, we have evaluated the effective global star formation efficiency of this sort of interactions and found that it is 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 suggests that the mechanism presently investigated, though very powerful to drive structure formation, supersonic turbulence and eventually, local star formation, does not seem to be sufficient to drive global star formation in normal star forming galaxies.
  Magnetic Fields In The Universe II: from Laboratory and Stars to the Primordial Structures; 01/2009
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  Article: Local Star formation triggered by SN shocks in magnetized diffuse neutral clouds

  M. R. M. Leao, E. M. de Gouveia Dal Pino, D. Falceta-Goncalves, C. Melioli, F. G. Geraissate
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  ABSTRACT: In this work, considering the impact of a SNR with a neutral magnetized cloud we derived analytically a set of conditions which are favorable for driving gravitational instability in the cloud and thus star formation. We have built diagrams of the SNR radius, versus the cloud density, that constrain a domain in the parameter space where star formation is allowed. The diagrams are also tested with fully 3-D MHD simulations involving a SNR and a self-gravitating cloud and we find that the numerical analysis is consistent with the results predicted by the diagrams. While the inclusion of a homogeneous magnetic field approximately perpendicular to the impact velocity of the SNR with an intensity ~1 $ mu$G results only a small shrinking of the star formation triggering zone in the diagrams, a larger magnetic field (~10 $ mu$G) causes a significant shrinking, as expected. Applications of the diagrams to a few regions of our own galaxy have revealed that star formation in those sites could have been triggered by shock waves from SNRs. Finally, we have evaluated the effective star formation efficiency for this sort of interaction and found that it is smaller than the observed values in our own Galaxy (sfe ~0.01-0.3). This result is consistent with previous work in the literature and also suggests that the mechanism presently investigated, though very powerful to drive structure formation, supersonic turbulence and eventually, local star formation, does not seem to be sufficient to drive global star formation in normal star forming galaxies, not even when the magnetic field in the neutral clouds is neglected. (abridged) Comment: 19 pages, 13 figures, accepted for pubblication in MNRAS
  Monthly Notices of the Royal Astronomical Society 12/2008; · 4.90 Impact Factor
• Article: Removal of magnetic flux from self-gravitating clouds due turbulent reconnection

  Márcia R. M. Leão, Elisabete M. de Gouveia Dal Pino, Alexandre Lazarian, Reinaldo Santos-Lima, Grzegorz Kowal
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  ABSTRACT: Understanding the star formation is a central problem of modern astrophysics. The interstellar medium, where stars are formed, is known to be magnetized and turbulent. The concentration of magnetic flux inside clouds of gas in the interstellar medium is an obstacle for the gravitational collapse of such clouds, that ultimately will give rise to stars. Ambipolar diffusion is considered, by some authors, the most relevant mechanism to transport magnetic field out of these clouds, allowing the collapse to proceed. We are going to present a numerical study about self-gravitating clouds in the presence of magnetic field and turbulence, aiming to test the efficiency of removal of magnetic flux from the interior of these clouds due the turbulent reconnection process, instead of ambipolar diffusion. It allows one to compare the relative importance between the ambipolar diffusion and the turbulent transport of magnetic field in some scenarios of star formation.
  04/2010; 12:8924.