Dust in Comets

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Grain Alignment in Starless Cores
Terry Jay Jones University of Minnesota
Megan Krejny Kathleen DeWahl Pierre Bastien B.G.
Andersson
IR Astronomy
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Goal Measure the magnetic field geometry in the
interstellar medium in the Milky Way and other
galaxies. Why? We want to learn what role
magnetic fields play in physical processes in the
ISM. In large volumes we find
Does the magnetic field control phenomena such as
star formation, bulk gas motion, coupling of
turbulence, confinement, formation of giant
molecular clouds etc??
IR Astronomy
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Heiles Compilation
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Synchrotron Polarization from Sukumar and Allen
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Mantra
We can NOT claim to use interstellar polarization
to measure the magnetic field geometry without a
solid understanding of the grain alignment
mechanism. This is critical in dense, star
forming regions, where we do not have the
corroborating evidence we see in the very diffuse
ISM.
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efficient alignment turbulence
efficient alignment turbulence
??
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Whittet et al. 2008
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Whittet et al. 2008
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Ward-Thompson et al. preprint
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What about the Near IR?
J band K band
JHK imaging by Pierre Bastien
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What about polarization in EMISSION?
L183 850um (Ward-Thompson et al.) So called
Starless Core
M17 West 100um (Dotson et al.) Warm dense core
with Star Formation
IR Astronomy
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Grain Size Distribution
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Radiative Alignment Theory
Hoang and Lazarian, 2009
IR Astronomy
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Without a central light source, the polarizing
efficiency of dust grains in molecular clouds
decreases with optical depth with a power law
slope
This power law slope steepens beyond AV 10 to
about -0.7. By AV 40, Sub-MM observations
suggest the slope steepens to -1, which indicates
there is NO grain alignment at these high optical
depths.
IR Astronomy
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In the diffuse ISM and in molecular clouds with a
luminous, embedded YSO i.e., where there is a
strong, asymmetric radiation field, grains are
very efficiently aligned. In molecular cloud
regions without an external or internal radiation
field, grain alignment is inefficient, and
becomes absent at high optical depths. We must
conclude that the presence of a radiation field
is an essential component to the grain alignment
mechanism.
IR Astronomy