An IRAC infrared image of a filament of young star-forming nuclei with the intensity of the dense gas emission superimposed in contours. New sub-millimeter observations have mapped the magnetic field structures in three of the massive cores along the filament and found that the fields, while not dominating the collapse, at least when compared to gas turbulence, appear to affect the development of disks around the filament in new stars. Photo credit: NASA / Spitzer-IRAC and Wang et al., 2008
Star formation in molecular clouds usually takes place in a two-step process. Supersonic currents initially compress the clouds into dense filaments for light years, after which gravity breaks the densest material in the filaments down into cores. Massive nuclei, each with more than twenty solar masses, are preferably formed at points of intersection where filaments cross and form places where stars are formed. The process is expected to be efficient, but the observed rate of star formation in dense gas is only a few percent of the rate expected when the material actually freely collapses. To solve the problem, astronomers have suggested that turbulence and / or magnetic fields assist the nuclei against gravitational collapse.
Magnetic fields are difficult to measure. A common approach is to measure the polarized light, since magnetic fields can align elongated grains of dust in the interstellar medium, which then scatter light with a preferred direction of polarization, whereby the field strengths can be estimated. CfA Astronomers Junhao Liu and Qizhou Zhang led a team that created the ALMA Submillimeter facility to study polarized emission in three massive nuclei in a dark cloud with a spatial resolution of around 0.7 light years, which is small enough to study the spatial structures of the nuclei. The region is located in our galaxy about fifteen thousand light years away and is known to have more than ten potentially star-forming nuclei with masses between one hundred thousand solar masses. Three of them show evidence that star formation is underway, and the scientists observed these three in their sub-millimeter continuum emission and the molecular emission of their carbon monoxide gas and several other species.
Each of the three nuclei differ slightly in mass, temperature, gas movement, and substructure, possibly in part because they are in different stages of their star-forming activity. Astronomers find magnetic fields in all three clusters, but the strengths also differ slightly between 1.6 and 0.32 milliGauss (for comparison, the strength of the magnetic field at the earth’s surface is around 500 milliGauss on average). Your analysis of energetics shows that turbulence in gas movements dominates (or compares with) the effects of magnetic fields and that magnetic force alone cannot prevent gravitational collapse. However, the fields can play a key role in another way: in these nuclei there are twelve outflows from the young stars, half of which are roughly aligned with the magnetic field directions. Since runoff is related to the disk structures around young stars, this 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 according to ALMA” by Junhao Liu, Qizhou Zhang, Keping Qiu, Hauyu Baobab Liu, Thushara Pillai, Josep Miquel Girart, Zhi-Yun Li and Ke Wang, June 5, 2020, The astrophysical journal.
DOI: 10.3847 / 1538-4357 / ab9087
