From the Princeton Weekly Bulletin, February 10, 1997
Astrophysicists solve puzzle of interstellar dust
By Jacquelyn Savani
Bruce Draine, professor of astrophysical sciences, and graduate student Joseph Weingartner have solved a 50-year-old puzzle: the mysterious alignment of interstellar dust particles. They presented their conclusions at the American Astronomical Society meeting in Toronto on January 13.
"Dust particles, present in the space between the stars, dim and redden the light reaching us from distant stars," explains Draine. "These interstellar dust grains--approximately the size of particles of smoke--have irregular shapes. Fifty years ago astronomers discovered, to their surprise, that these tiny particles are somehow aligned with one another: the separate particles are oriented with their long dimensions tending, on average, to be parallel. Most stars emit unpolarized light, containing equal amounts of the two polarizations. However, the light reaching us from stars behind a dust cloud becomes linearly polarized after passing through such a cloud. The cloud of aligned dust grains acts like a giant polaroid filter, preferentially dimming one polarization more than the other.
"For nearly 50 years, astrophysicists have struggled to understand what force could be responsible for the observed alignment of these tiny particles. Because the particles appeared to be aligned with their long axes perpendicular to the interstellar magnetic field, magnetic effects were often considered, but careful studies always found the magnetic forces to be too weak to accomplish the observed alignment."
The dust particles in interstellar space are illuminated by light from the many stars in the galaxy. Draine and Weingartner used a specialized computer program, DDSCAT.5a, developed by Draine and atmospheric scientist Piotr Flatau of the University of California, San Diego, to study the absorption and scattering of light by small, irregular particles.
Draine and Weingartner's theoretical computations demonstrated that moderately anisotropic starlight can have important consequences. "Starlight is anisotropic," Draine says, "because the stars are not uniformly distributed over the sky--there tend to be more stars in certain directions and fewer in others--so that more light comes from some directions than from others."
When starlight is anisotropic
A careful study of how starlight is absorbed and scattered by small irregular particles led Draine and Weingartner to conclude that "two important things happen when the starlight is anisotropic."
One is that such starlight will cause a dust grain to spin extremely rapidly --typically millions of revolutions per second. The anisotropic starlight illuminating the grain acts like a breeze blowing on a pinwheel.
The second is that the anisotropic starlight exerts a torque on the spinning grain that will cause the spin axis to line up gradually with the galactic magnetic field. "The alignment process is slow," Draine points out. "It may take a million years for a spinning grain to come into alignment--but a million years is a short time compared to the hundreds of millions of years the dust grains spend in space in our galaxy. The alignment process does not depend on the strength of the magnetic field, provided only that it exceeds a very small minimum value. The galactic field is known to satisfy this condition.
"For decades researchers have been trying to understand what causes grains to be aligned," he added. "It was quite a surprise to find that starlight itself can do it."
The necessary calculations required six months of computing on fast scientific workstations. The research was carried out at Princeton with support from the National Science Foundation.