When a young star forms, a ring of gas and dust called a protoplanetary disk forms around it.
Together, the star and the disk are a young solar system, and peering into solar systems much younger than ours reveals clues about how planets form in protoplanetary disks.
Our most powerful tool for examining these young systems is ALMA, the Atacama Large Millimeter/submillimeter Array.
ALMA is a powerful system of radio telescopes that can sense the faint emissions coming from dust in the disks.
A new observing program named AGE-PRO used ALMA to track how protoplanetary disks evolve over time and how the gas to dust ratio changes, leading to insights into planet formation.
The research team behind AGE-PRO has released their first results. It’s titled “The ALMA Survey of Gas Evolution of PROtoplanetary Disks (AGE-PRO): I. Program Overview and Summary of First Results,” and it’ll be published in The Astrophysical Journal. The lead author is Ke Zhang, professor of astronomy at University of Wisconsin–Madison.
“AGE-PRO aims to systematically trace the evolution of gas disk mass and size throughout the lifetime of protoplanetary disks,” the authors write.
To achieve its aim, AGE-PRO observed a sample of 30 young disks in three star-forming regions close to our Solar System.
They’re Ophiuchus, which is 0.5-1 million years old, Lupus which is 1-3 million years old, and Upper Scorpius, which is 2-6 million years old. Each of the three are in different phases of disk evolution.
ALMA is an interferometer made of 66 separate radio dishes. They’re moveable and can be spread over distances of up to 16 km. ALMA can have a resolution that’s up to 10 times more powerful than Hubble’s, though not in the same wavelengths. It’s especially adept at observing cold objects and is well-known for its groundbreaking images of protoplanetary disks.
In this work, the researchers used ALMA to measure gases in protoplanetary disks and how their masses evolve as the disk and the star evolve. “The AGE-PRO program was designed to measure gas disk masses and sizes at three evolutionary phases: the embedded disk phase, the middle age, and the end of the gas-rich phase,” the authors explain in their paper.
Astronomers want to know how different types of planets—rocky planets, gas giants, and ice giants—form in the disks. “In order to know what type of planets and how many planets you can have in a system, the fundamental requirement is understanding the mass in the disk around the young star. We call those disks protoplanetary disks,” said lead author Zhang.
“Our solar system is about 4.5 billion years old,” lead author Zhang said in a press release. “So, these systems that are just a few million years old are really just babies.”
Despite ALMA’s power, measuring the gas masses in protoplanetary disks has been a challenge. The dominant gas is H2, molecular hydrogen, but its emissions are largely undetectable. “CO (carbon monoxide) is the most widely detected molecule in disks and is considered the most robust molecule to trace gas distribution, because of its low condensation temperature and stable chemistry,” the authors explain. “But the conversion from CO gas mass to the total gas disk mass suffers from the high uncertainty of CO-to H2 ratio,” they write.
Protoplanetary disks are about 99% gas, and only about 1% dust. Though hydrogen is by far the dominant gas, there are others that are more readily detected. Each of these gaseous molecules has its own “signature” that ALMA can detect. Carbon monoxide is easily seen by ALMA, but can’t be used as a proxy. In this research, the team compared the signatures of one particular molecule and compared it to carbon monoxide. As time passes in a disk, the carbon monoxide thins out and molecules of the ion N2H+ become more numerous. This allowed the researchers to understand how the gas content in the 30 young disks in the three star-forming regions changes over the first few million years. During this timeframe, the disk goes through the three phases.
The main finding is that the gas to dust ratio doesn’t stay the same over time.
“The gas masses measured from AGE-PRO enable the first comparison of gas and dust mass evolution in disks,” the authors write in their research. “The gas and dust appear to evolve on different timescales, and the gas-to-dust mass ratio does not monotonically decrease over time.”
“Assuming the three regions had similar initial conditions and evolutionary paths, we find the median gas disk mass appears to decrease with age,” the authors write.
“Now, we can see that gas mass is decreasing very quickly in the first million years of protoplanetary disks and then gas mass decreases slowly, while dust mass probably decreases steadily over time,” said Zhang.
“So, if you want to form any gas giants like Jupiter, you have to get to work on that while more of the gas is still around, and that’s only a few million years.” The research shows that as gas molecules cross magnetic field lines created by the young star, they can be accelerated and blown out of the disks.
Rocky planets take a more leisurely route, since the dust that they’re made of doesn’t dissipate as quickly. They can takes tens of millions of years to form, according to the research.
AGE-PRO is the first ALMA effort to understand how gas evolves in protoplanetary disks. It shows that there are significant differences in the evolutionary timescales of gas and dust. It suggests that gas giants and ice giants have to form relatively quickly, while Earth-like planets can take more time.
There’s still much more to learn, though. “To advance our understanding of these processes, future observations need to target a broader range of disk ages, stellar masses, and environmental conditions,” the authors write in their conclusions.
The JWST has a role to play in the effort to understand protoplanetary disk evolution. While ALMA specializes in observing emissions from colder objects, the JWST sees in the infrared. This means it can observe the inner, warmer regions of the disk where rocky planets form. Zhang and his colleagues recently acquired JWST data from the 30 disks studied with ALMA.
“With that data, we can look at the materials close-in in the disk, which is where we think rocky planets are forming,” Zhang says. “If we can look at the chemical composition of materials like water and organics, we can understand how those change over the evolution of the disks.”
Written by Evan Gough/Universe Today.
