Atacama Large Millimeter / Submillimeter Array - Taiwan
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MAGNETIC FIELD STRUCTURES IN STAR FORMING REGIONS
Polarization studies in the submillimeter
wavelengths have shown that the dust emission is polarized.In
contrast to optical and infrared studies where polarization is from
reflection and scattering, submm studies directly see the emission
from the dust grains.Hence polarization implies that the dust grains
are elongated and aligned.The alignment mechanism is presumably the
magnetic field.Figure 15 shows the polarized dust emission towards
NGC1333-IRS4, a young star forming region.The dust emission is seen
to be polarized over an elongated part of the structure.The
polarization of the dust grains are clearly aligned.Since the dust
grains are aligned perpendicular to the magnetic field, we can
deduce the field directions across the line of sight.The field lines
appear to be pinched towards the center of the cloud
core.Furthermore, the percentage of polarization appears to decrease
from the outside to the inside of the cloud core.This may suggest
that the magnetic field lines are twisted in the inner region, or
perhaps the magnetic field is getting weaker as the density
increases.The elongated appearance of the dust polarization may also
be due to the orientation of the magnetic field relative to the line
of sight for different parts of the cloud.Heretofore, there has been
insufficient angular resolution to study the dust polarization in
the inner parts of a dense cloud core.ALMA will provide greater
sensitivity and resolution.This will greatly improve our ability to
study the magnetic field structures for a variety of sources and on
much finer spatial scales.
Figure 15. SMA images of the dust continuum emission from NGC1333-IRS4 (Girart et al. 2006). Left: Polarization vectors are plotted on the color image of the polarized dust continuum emission, and the white contours of the total dust continuum emission at 1mm. The polarization vectors are aligned. The length of the polarization vectors correspond to the strength of the polarized flux. Right: The magnetic field vectors in orange are plotted on the blue contours of the total dust continuum emission. The length of the vectors correspond to the percentage of polarization.
