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ALMA Reveals Magnetic Fields in the Early Stages of Massive Star Formation

Star formation in molecular clouds usually occurs in a two-step process. Supersonic flows first compress the clouds into dense filaments light-years long, after which gravity collapses the densest material in the filaments into cores. Massive cores, each more than about twenty solar–masses, preferen

tially form at intersections where filaments cross, producing sites of clustered star formation. The process is expected to be efficient yet the observed rate of star formation in dense gas is only a few percent of the rate expected if the material really were freely collapsing. To solve the problem, astronomers have proposed that turbulence and/or magnetic fields support the cores against gravitational collapse.

Magnetic fields are difficult to measure. One common approach is to measure the polarized light, because magnetic fields can align elongated dust grains in the interstellar medium which then scatter light with a preferred polarization direction enabling the field strengths to be estimated. CfA astronomers Junhao Liu and Qizhou Zhang led a team that used the ALMA submillimeter facility to study the polarized emission in three massive cores in a dark cloud with a spatial resolution of about 0.7 light-years, small enough to probe the spatial structures of the cores. The region is in our galaxy, about fifteen thousand light-years away, and is known to have more than ten potentially star forming cores with masses between one hundred and one thousand solar-masses. Three of them show signs that star formation is underway, and the scientists observed these three in their submillimeter continuum emission and the molecular emission from…

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