ALMA Takes Subsequent-Stage Photographs of a Protoplanetary Disk

The ESO’s Atacama Large Millimeter/submillimeter Array (ALMA) is perched high in the Chilean Andes. ALMA is made of 66 high-precision antennae that all work together to observe light just between radio and infrared. Its specialty is cold objects, and in recent years, it has taken some stunning and scientifically illuminating images of protoplanetary disks and the planets forming in them.

But its newest image supersedes them all.

The formation of solar systems and planets and how they evolve is one of ALMA’s primary subjects. It’s gained a reputation for imaging young T Tauri stars and their protoplanetary disks. These images show the tell-gale gaps created, astronomers think, by young, still-forming planets.

ALMA’s high-resolution images of nearby protoplanetary disks are from the Disk Substructures at High Angular Resolution Project (DSHARP). The observatory is often used to look for disks like these. Credit: ALMA (ESO/NAOJ/NRAO), S. Andrews et al.; NRAO/AUI/NSF, S. Dagnello

In new research, a team of astronomers took a deeper look at one protoplanetary disk. They measure the polarity of the light coming from the dust grains in the disk. This isn’t the first time ALMA has studied a disk’s polarity. But this image is based on 10x more polarization measurements than any other disk and 100x more measurements than most disks.

The research article is “Aligned grains and scattered light found in gaps of planet-forming disk.” It’s published in Nature, and the lead author is Ian Stephens. Stephens is an assistant professor at 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 their basic characteristics, and somehow, they affect how the dust behaves and eventually forms planets.

There’s a lot going on in protoplanetary disks, though it takes millions of years for it all to play out. Eventually, scientists think, young disks like this one around HL Tauri will mature and stabilize. Planets may enter into resonance with one another, some planets may migrate, and eventually, things will likely stabilize like our Solar System has.

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 one to Earth. All of the stars in the TMC, including HL Tau, are only about one or two million years old. At that age, the disks around the stars should just be starting to form planets, and that’s why ALMA is studying it.

And this isn’t the first time. In fact, the sharpest image ALMA ever captured was of HL Tau.

This is the sharpest image ever taken by ALMA — sharper than is routinely achieved in visible light with the NASA/ESA Hubble Space Telescope. It shows the protoplanetary disc surrounding the young star HL Tauri. With young stars like this one, observations reveal substructures within the disc that were never been seen before. They may show the possible positions of planets forming in the dark patches within the system. Credit: ALMA (ESO/NAOJ/NRAO)

In the new study, Stephens and his colleagues wanted to probe HL Tau even deeper. They focused on the polarity of the dust because there’s so much we don’t know about how planets form. Polarity may provide clues to the process that other observations can’t. Dust polarity could reveal things about the underlying structure of HL Tau’s disk that can’t be revealed in any other way.

Over time, the dust grains in the disk begin to stick together. This process goes on and on until planetesimals form, then eventually, planets. HL Tau and its disk have their own magnetic field, and scientists think that the field may affect how the dust grains align and how they accrete into larger structures. However, polarity measurements show that the dust isn’t aligned with the magnetic fields.

This figure from the research shows HL Tau’s polarization morphology. The polarity of the grains doesn’t line up with the system’s magnetic fields. Image Credit: Stephens et al. 2023

Instead, the polarity comes from the shape of the grains. Grains needn’t be round; they can be prolate, like elongated spheres. And that means they can polarize light. That constrains 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. That’s likely due to asymmetries in the distribution of the dust or how the properties of the grains are different on one side. But there’s no clear answer to it yet.

The images revealed another surprise. The polarity of the dust within the gaps is more azimuthal, even though there’s less dust there. That suggests that the dust is more aligned in the gaps. The gaps are where planets form. Do the properties of the dust reflect planet formation? Or does it help account for it? The polarity in the rings themselves is more uniform, indicating that the polarity comes from scattering, adding to the complexity.

This figure from the research shows the polarization fraction (L) and polarization intensity (R) of HL Tau’s disk. Polarization fractions are typically much higher in the gaps than in the rings. Even the polarized intensity is frequently higher in the gaps. Image Credit: Stephens et al. 2023.

Overall, the polarity has two causes: scattering and the alignment of the dust. But it’s not clear from the images and data what’s causing the dust to align the way it does. It’s unlikely that the dust is aligned with the magnetic fields, though strangely, dust outside of a protoplanetary disk usually is. The current thinking is that the alignment has a mechanical cause rather than a magnetic one. It could result from the movement around the star, but there’s no clear consensus yet.

This research doesn’t provide any definitive answers to our questions about planet formation in the disks around young stars. But HL Tau’s disk appears to be highly evolved for its age. It’s probably not more than one million years old, yet it displays the telltale rings and gaps that indicate planet formation.

A previous study, also led by Ian Stephens from Worcester State University, suggested that the rapid accretion rate might 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 disk. The results show that a high-resolution image of a protoplanetary disk’s polarization reveals details that are otherwise hidden. We need more of these images of more disks around young T Tauri stars like HL Tau.

With a large sample size, scientists might make more progress.

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