PEBBLES: Observing stardust

Research Article published on 13 October 2023 , Updated on 16 January 2024

The PEBBLES project (Exploring the pristine conditions for transforming interstellar dust into planetesimals), led by Anaëlle Maury of the Paris-Saclay Astrophysics Instrumentation Modelisation Laboratory (AIM — Université Paris-Saclay, CNRS, CEA, Université Paris Cité), takes an unprecedented look at interstellar dust to understand how stars and their planets form. The researcher and her team use the best international telescopes to carry out their investigations. Their goal is to discern the size of tiny grains of dust located light-years away from Earth, in areas where new stars are emerging.

The term "interstellar vacuum" is no longer used, but rather "interstellar medium". Contrary to popular belief, the space between stars is not completely empty: it contains gas and around 1% silicate, iron and carbon dust. The presence of this "matter" alone gives us reason to turn our telescopes on these dark zones, to learn more about them, and this is also where we can see stars being born. 

 

The magnetism of young stars

The birth of a star follows a very specific process. In the coldest and densest parts of the galaxy, interstellar matter sometimes takes the form of a compact cloud of gas and dust. Under the effect of gravity, it collapses in on itself and begins to spin. This is the start of a process that lasts several million years: the matter present heats up and begins to emit light, first in the infrared range, and then in the visible range. A young star, known as a protostar, is born. 

"I observe and study the first stage in the formation of solar-type stars, named after their similarities to our Sun," says Anaëlle Maury, of the Paris-Saclay Astrophysics, Instrumentation Modelisation Laboratory (AIM — Université Paris-Saclay, CNRS, CEA, Université Paris Cité). From 2015 to 2022, with the MagneticYSOs project, winner of a Starting Grant from the European Research Council (ERC), the team focused on the magnetic field surrounding newborn stars. The team has been centring its efforts on locating the magnetic forces that exist around these protostars. However, as the researcher points out, "the magnetic field is invisible and undetectable with spectral space observation instruments". 

From this point on, you can perceive the star's presence only by the effect it has on its surroundings. On Earth, for example, our planet's magnetic field pulls the needle of a compass towards the North. In protostellar zones, it's the dust grains that act as a compass, by emitting light. But the radiation emitted is not uniform as these grains are not spherical. Under certain conditions, the grains align with the magnetic field, which allows the researcher to detect and map this magnetic field. "Our reconstruction of the magnetic field is realistic, as it is similar to that produced by the numerical models of star formation produced by my colleague, Patrick Hennebelle, with the magnetohydrodynamic (MHD) code RAMSES," says Anaëlle Maury.

 

Serendipity at work

However, after spending thousands of hours examining interstellar dust, used as an indirect indicator of the magnetic field, an anomaly caught the attention of the AIM scientists. Observations of the polarisation of the radiation emitted by dust grains and their emissivity suggest that the dust found surrounding these young stars still in their infancy does not resemble that predicted by theory: the grains have the potential to be much larger than expected or of a different composition. This unexpected discovery almost undermines current theories setting out the formation of stars and the planets around them. "A few tens of thousands of years after the first star embryo began to form, the dust may have already agglomerated to form structures about the size of a grain of sand, even though their size was thought to be a hundred times smaller," Anaëlle Maury speculates enthusiastically. 

Dust, which was initially only needed to observe the magnetic field, became the main focus of the team's research. In April 2023, Anaëlle Maury landed an Advanced Grant from the European Research Council (ERC) for her PEBBLES project, which aims to explore the previous hypothesis by combining telescope observations and physical modelling. 

 

Stars sending out radio waves

First, the researcher must solve a methodological problem. "A star that's still forming is a cold object buried in a cocoon of matter. It's hard to see what's going on inside." At an early age, protostars are buried too deep for the light they produce to emerge. However, long-wave radiation ranging from mid-infrared to radio waves can still penetrate the dense cocoon of matter surrounding these stars. This is where the solution to the problem can be found. "We capture these waves using radio telescopes on Earth, such as the international ALMA (Atacama Large Millimeter Array) telescope, located in the Atacama Desert in Chile, or its European counterpart NOEMA (Northern Extended Millimeter Array), located on the Plateau de Bure in France. They consist of dozens of antennae pointing simultaneously at a young star of interest. Thanks to these interferometric techniques, we're able to get a signal with unrivalled resolution. It's possible to distinguish an object with a length equal to the distance between the Earth and the Sun, and located more than one hundred light-years away from Earth. That's the equivalent of seeing a €2 coin on the Moon." Thanks to these super-powerful telescopes it will be possible to work out the size of the dust found in young stars, using extrapolation, by analysing the properties of the electromagnetic waves emitted by the hundred or so protostars within the telescopes' range.

 

What is stardust actually like?

This analysis will also be carried out with infrared observations using the James Webb Space Telescope. "Thanks to this telescope, located outside the Earth's infrared-opaque atmosphere, we'll be able to study the absorption and re-emission of light by the hottest dust particles at the centre of the protostar, which will provide information on their chemical composition and size," says the researcher. As for the numerical physical models, they will be supplemented with new information and will soon be able to model dust evolution.

Improved observation and modelling techniques are opening up unprecedented prospects for discovery, and promise to lift the veil on the formation of stars and their surrounding planets. "The aim is to build a comprehensive view of the initial conditions during star formation," concludes Anaëlle Maury. 

 

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