This artist’s impression of a planet-forming disc surrounding a young star shows a swirling ‘pancake’ of hot gas and dust from which planets form. Using the James Webb Space Telescope, the team obtained detailed images showing the layered, conical structure of disc winds – streams of gas blowing out into space. Image © National Astronomical Observatory of Japan (NAOJ).
Ilaria Pascucci’s research is directed towards understanding how planets form and evolve and how common are planetary systems like our own Solar System.
To this end, her group carries out observations aimed at characterising the physical and chemical evolution of gaseous dust discs around young stars, the birth sites of planets. In addition, the group uses exoplanet surveys to reconstruct the intrinsic frequency of planets around mature stars. By linking the birth sites of planets to the exoplanet populations, the group contributes to building a comprehensive and predictive planet formation theory, a necessary step in identifying which nearby stars most likely host a habitable planet like Earth.
Every second, more than 3,000 stars are born in the visible universe. Many are surrounded by what astronomers call a protoplanetary disc – a swirling ‘pancake’ of hot gas and dust from which planets form. The exact processes that give rise to stars and planetary systems, however, are still poorly understood.
A team of astronomers led by University of Arizona researchers used NASA’s James Webb Space Telescope (JWST) to obtain some of the most detailed insights into the forces that shape protoplanetary discs. The observations offer glimpses into what our solar system may have looked like 4.6 billion years ago.
Specifically, the team was able to trace so-called ‘disc winds’ in unprecedented detail. These winds are streams of gas blowing from the planet-forming disc out into space. Powered largely by magnetic fields, these winds can travel tens of miles in just one second.
ScienceVega spoke to lead researcher Professor Ilaria Pascucci to find out how.
ScienceVega: How has the James Webb Space Telescope changed the way that you can study young star systems?
IP: JWST is amazing because it is six times larger than its predecessor, which was the Spitzer Space Telescope working at the same wavelengths as Webb. This means you can now pick up signals that are 100 times fainter than you could with Spitzer. And the advantage of a big mirror is that you can also take much sharper images. For the specific study we published in Nature Astronomy, those sharper images were actually the key to understanding the origin of disc winds.
ScienceVega: Your research covers disc winds – can you explain what they are and how they exist in star systems?
IP: First of all we need to explain what a disc is. We know that planets form from the gas and dust around young stars. What we also know, because we have taken images for instance with the Hubble Space Telescope, is that this gas and dust are not uniformly distributed around the star. They are in a kind of pancake-like structure, which we call the disc, or sometimes the planet-forming disc. The disc wind is essentially flowing material from the surface of the disc, and it’s an important way to remove planet-forming material. There is a kind of competition between the material that ends up going into the planet and material that is removed through disc winds.
There are two main processes to form disc winds. The first way to drive disc winds is via high energy stellar photons that ionise and heat the gas in the disc. If the gas becomes very hot then that material moves away from the disc like steam rising from the surface of a cup of coffee.
The second way to drive the winds is through magnetic forces. If you do an experiment with a magnet and iron filings, you know that the metal aligns around the magnetic field of the magnet. In the instance of discs there is also a magnetic field and so the ionised gas gets pulled onto this magnetic line. At that point it also attracts and drags along neutral material, which is then removed from the discs. Those are the two main ways in which you can have disc winds.
This composite RGB image of one of the sources described in the paper (HH30) shows for the first time the nested morphology of disc winds with multiple tracers: Fe[II] emission (in blue) traces the fastest and most collimated component (the jet). The Fe[II] emission is nested inside a lower velocity hydrogen emission (green), which is itself nested inside an even slower carbon monoxide (J=2-1) emission (red). Fe[II] and hydrogen emissions were obtained with JWST/ NIRSpec, while the carbon monoxide emission was observed by ALMA (Atacama Large Millimeter Array) previously. Image © Ilaria Pascucci et al.
ScienceVega: How does the knowledge gained in this particular research help resolve questions around angular momentum?
IP: Angular momentum is a very important physical quantity that is associated with every rotating object. It is essentially proportional to the resistance of a body to move and the speed of that body. For instance, if you have an ice skater that spins, that skater has angular momentum, and we are all familiar that when the skater pulls in their arms, they spin faster. That is the law of the conservation of angular momentum, and that law applies to everybody on Earth as well as in space.
It also applies to these planet-forming discs. The discs are rotating because they inherit some of the angular momentum from the cloud where they formed. One critical question has always been if gas from the disc is accreted onto the star – as we know it is – how can this gas lose angular momentum? Magnetically driven winds are a way to remove a part of the angular momentum from the disc and enable part of the gas to be accreted.
Our study demonstrates that young accreting stars have those magnetically driven disc winds that can drive accretion.
ScienceVega: How has Webb helped to resolve our understanding of the magnetic field driven wind, the thermal wind and the X wind?
IP: The key results from Webb are spectacular, high-resolution images that we couldn’t take before. The instrument we have used, NIRSpec, has this capability of taking images at multiple wavelengths or different colours. The important thing is that with these different wavelengths or colours we can pick emissions from gas that is tracing these beautiful winds and so it is really through the morphology and the extent above the disc from where they come from that we can pin down which type of wind we are observing. We could say that in all four protoplanetary disc systems we studied and took images the wind is driven by magnetic forces and not by stellar photons.
This is important because those are the winds that can resolve the issue of angular momentum.
ScienceVega: Does that mean then that all young star systems are going to be similar and they’re going to be driven by the magnetic wind, or are there other systems that are being developed in different ways?
IP: This is a very important question because our study focuses on just four objects and it was meant to be a pilot study to show what James Webb can do. But now that we have demonstrated that Webb can distinguish between these types of winds, the next step is to see if they are common.
Recently we had a deadline to submit new proposals to make observations using the JWST and my team prepared a new proposal to expand on this study. We now want to do a larger sample and we also want to focus on smaller stars than our Sun because those are the most common stars in our Galaxy. This is how we hope to answer the question of whether there are different processes in different star systems.
ScienceVega: How do you decide on which astronomical research to undertake?
IP: This is an important question to answer in that people on the outside might think we simply point the telescope somewhere in the sky and find something cool to publish. That’s not the case. Most of the sky is empty and there is really a lot of study that you have to do in preparation to craft compelling proposals and observations. It’s very important to know the theory, in this specific case, of how the winds develop.
We also need to work together: those who do observations along with the theorists to craft an experiment. Essentially this is a kind of an experiment, so when you write the proposal, you need to know the ‘what if’ when you find something that is in agreement with the current understanding, but you also need to know the ‘what if’ when you find something different. You need to know what that is going to imply for our broader understanding.
Science is a collaborative effort and it’s really driven by teams of people from different countries and different backgrounds, all working together to advance our understanding on a specific science topic. Science is a unique and beautiful way to unite people and I want to encourage everybody to engage more in science in some way. Science fosters curiosity, connection, and overall a deeper understanding of the world around us.
