Intense
molecular hydrogen formation shown in near infrared image of the
reflection nebula IC 63 in the constellation Cassiopeia. The white bars
represent polarization seen toward stars in the background of the
nebula. The largest polarization shows the most intense emission,
demonstrating that hydrogen formation influences alignment of the dust
grain with a magnetic field. Image: B-G Andersson, USRA
For astrophysicists, the interplay of hydrogen — the most common
molecule in the universe — and the vast clouds of dust that fill the
voids of interstellar space has been an intractable puzzle of stellar
evolution.
The dust, astronomers believe, is a key phase in the life cycle of
stars, which are formed in dusty nurseries throughout the cosmos. But
how the dust interacts with hydrogen and is oriented by the magnetic
fields in deep space has proved a six-decade-long theoretical challenge.
Now, an international team of astronomers reports key observations
that confirm a theory devised by University of Wisconsin-Madison
astrophysicist Alexandre Lazarian and Wisconsin graduate student Thiem
Hoang. The theory describes how dust grains in interstellar space, like
soldiers in lock-drill formation, spin and organize themselves in the
presence of magnetic fields to precisely align in key astrophysical
environments.
Alexandre Lazarian
The effort promises to untangle a theoretical logjam about key
elements of the interstellar medium and underpin novel observational
tactics to probe magnetic fields in space.
The new observations, conducted by a team led by B-G Andersson of the
Universities Space Research Association (USRA), and their theoretical
implications are to be reported in the Oct. 1, 2013 edition of the
Astrophysical Journal. The observations were conducted using a variety
of techniques — optical and near infrared polarimetry, high-accuracy
optical spectroscopy and photometry, and sensitive imaging in the near
infrared — at observatories in Spain, Hawaii, Arizona and New Mexico.
"We need to understand grain alignment if we want to make use of
polarimetry as a means of investigating interstellar magnetic fields,"
says Lazarian, who was encouraged to attack the problem by the renowned
astrophysicist Lyman Spitzer. "Spitzer himself worked on the problem
extensively."
Scientists have long known that starlight becomes polarized as it
shines through clouds of neatly aligned, rapidly spinning grains of
interstellar dust. And the parsing of polarized light is a key
observational technique. But how the grains of dust interact with
hydrogen, become aligned so that starlight shining through becomes
polarized, and are set spinning has been a mystery.
"While interstellar polarization has been known since 1949, the
physical mechanisms behind grain alignment have been poorly understood
until recently," explains Andersson. "These observations form part of a
coordinated effort to — after more than 60 years — place interstellar
grain alignment on a solid theoretical and observational footing."
The observations made by Andersson and his colleagues support an
analytical theory posed by Lazarian and Hoang known as Radiative
Alignment Torque, which describes how irregular grains can be aligned by
their interaction with magnetic fields and stellar radiation. Under the
theory, grains are spun, propeller-like, by photons. Their alignment is
modified by magnetic fields, which orients them with respect to the
field, telling an observer its direction. Impurities and defects on the
dust grains produce catalytic sites for the formation of hydrogen
molecules, which are subsequently ejected, creating miniature "rocket
engines," also called "Purcell thrusters" after Nobel laureate Edwin
Purcell, who studied grain alignment.
The theory devised by Lazarian and Hoang predicts how the molecular
hydrogen thrust changes grain alignment, and was put to the test by
Andersson's team of observers.
Confirming the theory, Lazarian notes, not only helps explain how
interstellar dust grains align, but promises a new ability for
astronomers to use polarized visible and near infrared light to reliably
probe the strength and structure of magnetic fields in interstellar
space, a notoriously difficult phenomenon to measure quantitatively.
Interstellar magnetic fields are ubiquitous in spiral galaxies like
our Milky Way and are believed to be essential regulators of star
formation and the evolution of proto-planetary disks. They also control
the regulation and propagation of cosmic rays.
The murky piece of the astrophysical puzzle, says Lazarian, was how
the irregular grains of interstellar dust were set in spinning motion.
The observations conducted by Andersson demonstrate that intense
molecular hydrogen formation on the surface of the interstellar dust
grains is an important contributor to the dust grains spinning.
Hydrogen does not exist in the element's gas phase in space since the
two atoms of the molecule cannot rid themselves of the formation
reaction energy without a third body. The two hydrogen atoms therefore
use the surfaces of dust grains as a substrate, and the force of the
reaction energy is enough to set the dust grains in motion.
The new work, which was supported by the National Science Foundation,
is especially timely, Lazarian says, as two new observatories — the
ground-based ALMA, the Atacama Large Millimeter Array, and the
space-based Planck Telescope — are poised to build on the new results.
