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

An IRAC infrared image of a filament of young star-forming cores, with the intensity of the dense gas emission overlaid in contours. New submillimeter observations have mapped the magnetic field structures in three of the massive cores along the filament, and found that although the fields do not dominate the collapse, at least as compared to gas turbulence, they appear to be influencing the development of disks around the new stars. Credit: NASA/ Spitzer-IRAC, and Wang et al., 2008

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, preferentially 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 their carbon monoxide gas and several other species.

Each of the three cores is slightly different in mass, temperature, gas motions, and substructure, perhaps in part because they are in different stages in their star formation activity. The astronomers find magnetic fields in all three of the clumps, but the strengths also differ slightly from between 1.6 and 0.32 milliGauss (for comparison, the strength of the magnetic field at the Earth’s surface is on average about 500 milliGauss). Their analysis of the energetics shows that turbulence in the gas motions dominates (or compares to) the effects of magnetic fields and that the magnetic force alone cannot prevent gravitational collapse. However the fields may play a key role in another way: There are twelve outflows from the young stars in these cores, and half of them are roughly aligned with the magnetic field directions. Since outflows are related to the disk structures around young stars, it suggests that the fields play a key role in shaping the disks as they develop in the early stages of star formation.

Reference: “Magnetic Fields in the Early Stages of Massive Star Formation as Revealed by ALMA” by Junhao Liu, Qizhou Zhang, Keping Qiu, Hauyu Baobab Liu, Thushara Pillai, Josep Miquel Girart, Zhi-Yun Li and Ke Wang, 5 June 2020, The Astrophysical Journal.
DOI: 10.3847/1538-4357/ab9087