ALMA captures polarization image of young star in unprecedented detail

Using the Atacama Large Millimeter/submillimeter Array (ALMA), the world’s most powerful millimeter/ submillimeter telescope, scientists have captured a polarization image of a young star in unprecedented detail. The star, HL Tauri, is located just 480 light-years away and is surrounded by a protoplanetary disk with visible gaps which could be where young protoplanets are forming.

Polarization involves observing the orientation of the light waves emitted by dust grains in the protoplanetary disk. While previous studies have mapped the polarization, the new study from Stephens, et al. has captured the highest-resolution dust polarization image ever taken toward a protoplanetary disk.

The image is based on 10x more polarization measurements compared to any other disk, and 100x more measurements than most disks. The image was captured at a resolution of 5 AU, which is about the distance from the Sun to the gas giant Jupiter, and is by far the deepest polarization image of any disk captured thus far.

The new findings suggest that these grains behave more like prolate grains, and they put strong constraints on the shape and size of dust grains within the disk.

The study found that there is more polarization within the gaps of the disk compared to the rings, even though there is more dust in the rings. The polarization within the gaps is more azimuthal, suggesting that the polarization comes from aligned dust grains within the gaps. On the other hand, the polarization of the rings is more uniform, which suggests the polarization largely comes from scattering. In general, the polarization comes from a mix of scattering and dust alignment.

"Based on the data, it is unclear what is causing the dust grains to align, but they are likely not aligned along the magnetic field of the disk, which is the case for most dust outside of protoplanetary disks," the researchers said.