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The role of magnetic fields in star formation

The star forming molecular clump W43-MM1 is very massive and dense, containing about 2100 solar masses of material in a region only one-third of a light year across (for comparison, the nearest star to the Sun is a bit over four light years away).

The role of magnetic fields in star formation
A false-color far-infrared image of the star forming region W43; the contours are for molecular gas density. The subregion 
MM1 located just left of center in not conspicuous in the image but is the site of massive star formation and fragmentation. 
A new study has mapped the magnetic fields in this region, and found they are not strong enough to prevent further 
gravitational collapse [Credit: ESA/Herschel and L.Q. Nguyen et al.]
Previous observations of this clump found evidence for infalling motions (signaling that material is still accumulating onto a new star) and weak magnetic fields. These fields are detected by looking for polarized light, which is produced when radiation scatters off of elongated dust grains aligned by magnetic fields.

The Submillimeter Array recently probed this source with high spatial resolutions and found evidence for even stronger magnetic fields in places. One of the outstanding issues in star formation is the extent to which magnetic fields inhibit the collapse of material onto stars, and this source seems to offer a particularly useful example.

The role of magnetic fields in star formation
A schematic illustration of the magnetic field and motions found in a massive star forming cluster. 
The core (gray-filled ellipse) is a flattened, rotating cloud of gas and dust (blue and red arrows indicate
 the sense of the rotation). It is fragmenting into new stars (evidenced by the three condensations/gray dots), 
and is threaded by an hourglass magnetic field ("B field" green lines) largely aligned with 
a bipolar outflow indicated with the arrows [Credit: Qiu et al.]
CfA astronomers Josep Girart and TK Sridharan and their colleagues have used the ALMA submillimeter facility to obtain images with spatial scales as small as 0.03 light years. Their detailed polarization maps show that the magnetic field is well ordered all across the clump, which itself is actually fifteen smaller fragments, one of which (at 312 solar masses) appears to be the most massive fragment known.

The scientists analyze the magnetic field strengths and show that, even in the least massive fragment the field is not strong enough to inhibit gravitational collapse. In fact, they find indications that gravity, as it pulls material inward, drags the magnetic field lines along. They are, however, unable to rule out possible further fragmentation. The research is the most precise study of magnetic fields in star forming massive clumps yet undertaken, and provides a new reference point for theoretical models.

The findings are published in The Astrophysical Journal.

Source: Harvard-Smithsonian Center for Astrophysics [July 29, 2016]

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