by ESO’s Atacama Large Millimeter/submillimeter Array (ALMA) is located high in the Chilean Andes. ALMA is made up of 66 high-precision antennas that work together to observe light between radio and infrared. His specialty is cold objects, and in recent years he has taken some impressive and scientifically illuminating images of protoplanetary disks and the planets that form in them.
But his latest image replaces them all.
The formation of solar systems and planets and how they evolve is one of the main themes of ALMA. It gained a reputation for imaging young T Tauri stars and their protoplanetary disks. These images show the telltale gaps created, astronomers think, by young and still-forming planets.
In new research, a team of astronomers took a deeper look at a protoplanetary disk. They measure the polarity of light coming from dust grains in the disk. This is not the first time that ALMA has studied the polarity of a disk. But this picture is based on 10 times more polarization measurements than any other disk and 100 times more measurements than most disks.
The research paper is “Aligned Grains and Scattered Light Found in Planetary-Forming Disk Gaps.” It was published in Nature and the lead author is Ian Stephens. Stephens is an assistant professor in the Department of Earth, Environment and Physics, Worcester State University, Worcester, MA, USA.
What’s so useful about measuring the polarity of dust in a protoplanetary disk? It can reveal things like the size and shape of dust grains. These are its basic characteristics and, in some way, they affect the way dust behaves and eventually forms planets.
There’s a lot going on in protoplanetary disks, although it takes millions of years for it all to happen. Eventually, scientists think, young disks like this one around HL Tauri will mature and stabilize. Planets can resonate with each other, some planets can migrate, and eventually things will probably stabilize like our Solar System.
And it all starts with dust.
HL Tau is about 450 light-years away in the Taurus Molecular Cloud, a star-forming region that may be the closest to Earth. All of the TMC stars, including HL Tau, are only one or two million years old. At that age, the disks around stars should be just starting to form planets, which is why ALMA is studying them.
And this is not the first time. In fact, the sharpest image ever captured by ALMA was that of HL Tau.
In the new study, Stephens and his colleagues wanted to investigate HL Tau even more deeply. They focused on the polarity of the dust because there is a lot we don’t know about how planets form. Polarity can provide clues to the process that other observations cannot. The polarity of the dust can reveal things about the underlying structure of the HL Tau disk that cannot be revealed otherwise.
Over time, the disk’s dust grains begin to stick together. This process continues until planetesimals form and, eventually, planets. HL Tau and its disk have their own magnetic field, and scientists think the field may affect how dust grains align and how they accumulate into larger structures. However, polarity measurements show that the dust is not aligned with the magnetic fields.
Instead, the polarity comes from the shape of the grains. The grains do not have to be round; they may be prolate, like elongated spheres. And that means they can polarize light. This restricts the size and shape of the grains, which in turn should affect how they clump together.
The ALMA image also showed that one side of the protoplanetary disk is more polarized than the other. This is probably due to asymmetries in the dust distribution or the difference between the properties of the grains on one side. But there is still no clear answer to this.
The images revealed another surprise. The polarity of the dust inside the gaps is more azimuthal, even though there is less dust there. This suggests that the dust is more aligned in the gaps. Gaps are where planets form. Do dust properties reflect planetary formation? Or help explain this? The polarity in the rings themselves is more uniform, indicating that the polarity comes from scattering, increasing complexity.
In general, polarity has two causes: dust scattering and dust alignment. But it’s not clear from the images and data what is causing the dust to line up this way. Dust is unlikely to be aligned with magnetic fields, although, strangely, dust outside a protoplanetary disk often is. Current thinking is that the alignment has a mechanical rather than a magnetic cause. It could result from movement around the star, but there is still no clear consensus.
This investigation does not provide any definitive answers to our questions about planet formation in the disks around young stars. But HL Tau’s disc appears to be highly evolved for its age. It’s probably no more than a million years old, but it displays telltale rings and gaps that indicate planet formation.
A previous study, also led by Ian Stephens of Worcester State University, suggested that the rapid rate of accretion may be due to HL Tau’s complex magnetic fields. “The unexpected morphology suggests that the role of the magnetic field in the accretion of a T Tauri star is more complex than our current theoretical understanding,” Stephens and his colleagues wrote in that research.
Unfortunately, even with this exceptional ALMA image, our questions remain unanswered. But this is just one record. The results show that a high-resolution image of the polarization of a protoplanetary disk reveals details that would otherwise be hidden. We need more images of more disks around young T Tauri stars like HL Tau.
With a large sample, scientists can make more progress.
