(ORDO NEWS) — Is our solar system comparable to other solar systems? What do other systems look like? We know from exoplanet studies that many other systems have hot Jupiters – massive gas giants orbiting very close to their stars. Is this normal, and is our solar system an exception?
One way to address these questions is to study the planet-forming disks around young stars to see how they evolve.
But studying a large sample of such systems is the only way to get an answer.
This is exactly what a group of astronomers did by studying 873 protoplanetary disks.
Mass is a critical ingredient in the new study of planet-forming disks. The mass of the disk determines how much matter is available to form planets.
By measuring the mass of the disks around young stars, astronomers can determine the total mass of planets that could form there, and move one step closer to understanding the architecture of the solar system.
The new study is “ALMA (SODA) Survey of Orion Disks: I. Cloud-Level Demographics of 873 Protoplanetary Disks”. It is published in the journal “Astronomy and Astrophysics”, the lead author is Serk van Terwisga, a scientist at the Max Planck Institute for Astronomy in Heidelberg, Germany.
“Until now, we didn’t know for sure which properties dominate the evolution of planet-forming disks around young stars,” van Terwisga said in a press release.
“Our new results show that in an environment without a corresponding external influence, the observed disk mass available for the formation of new planets depends only on the age of the star-disk system.”
The mass of dust not only tells astronomers the mass of planets that could form from the disk. Depending on the age of the disk, it can also tell astronomers which planets have already formed.
But there are other factors that affect disk mass, which vary from disk to disk. Things like stellar wind and radiation from nearby stars outside of the disk can also affect mass.
How were the researchers able to isolate these effects in such a large sample?
They focused on a well-known region of protoplanetary disks called Orion Cloud A, which is part of the Orion Molecular Cloud Complex (OMCC).
OMCC lies about 1,350 light-years away and is home to the well-studied Orion Nebula, which even backyard astronomers can see.
Alvaro Hakar is co-author of the study and a scientist at the University of Vienna, Austria. “Orion A has provided us with an unprecedentedly large sample of more than 870 disks around young stars,” Hakar said. “It was very important to be able to look for small variations in disk mass depending on age and even on the local environment within the cloud.”
This is a good sample because all disks belong to the same cloud. This means that their chemical composition is homogeneous and they all share the same history.
The nearby Orion Nebula Cluster (ONC) contains massive stars that could affect other disks, so the team rejected all disks in Orion A closer than 13 light-years from ONC.
Measuring the mass of all these disks was not easy. The team used the Atakama Large Millimeter/Submillimeter Array (ALMA) to observe the dust. ALMA can be tuned to different wavelengths, so the team observed young disks at 1.2mm.
At this wavelength, the dust is bright and the star is dim, helping to eliminate the influence of the star in each disk. Because observing at a wavelength of 1.2 mm makes observations insensitive to objects larger than a few millimeters – such as planets that have already formed – the team only measured the dust available to form new planets.
Measuring dust without interference from stars was one hurdle, but the researchers ran into another: data.
A detailed study of almost 900 protoplanetary disks creates a large amount of data, and all of this data must be processed before it acquires any collective meaning. If the team had relied on existing methods, processing all of this data would have taken about six months.
Instead, they developed their own method of processing data using parallel processing. What would have taken months took less than one day. “Our new approach has increased the data processing speed by 900 times,” said co-author Raymond Oonk.
After processing the data, the researchers found that most disks contain only 2.2 Earth masses of dust. Only 20 of the nearly 900 disks contained enough dust for 100 or more Earth masses.
“In order to find variations, we cut the Orion A cloud and analyzed these regions separately. With hundreds of disks, the subsamples were large enough to give statistically significant results,” van Terwisga explained.
The researchers found some variability in the mass of disk dust in different regions of Orion A, but these changes were minimal. According to the authors, these fluctuations can be explained by the effect of age. With age, the disk mass decreases, and clusters of disks of the same age have the same mass distribution.
“We must emphasize that the differences between these clusters, located far apart in the sky, are small and not very significant relative to each other and the field, even in the most extreme cases,” the authors write in their paper.
It is expected that as the disks age, the mass of dust in them decreases. Most of this decrease is due to the formation of planets: what was once dust becomes planets.
But other effects also contribute to dust loss. Dust can migrate towards the center of the disk, and radiation from the host star can evaporate the dust.
But this study strengthens the correlation between age and dust loss.
Can the results of this study be applied to other populations of young stellar disks? The authors compared their results for Orion A with several neighboring star-forming regions with young disks.
Most, but not all, match the age-related mass loss seen in Orion A.
“Overall, we think our study proves that, for at least the next 1,000 light-years or so, all populations of planet-forming disks show the same mass distribution at a certain age. And they seem to evolve more or less the same way.” , said van Terwisga.
The researchers have a lot more work they would like to do. They are going to study what effect smaller stars can have on a scale of several light years.
In this study, they avoided the influence that massive stars in ONCs can have on neighboring disks. But smaller background stars can still influence the disks, and they could explain some of the small variations in the age-mass correlation.
The age of the star and its disk, the chemical properties and dynamics of the parent cloud all combine with mass to give a clearer picture of the solar system emerging from the disk. Astronomers cannot take such data and predict what type of planets might form in which solar system.
But it is noteworthy that the correlation between the age of the disk and its mass is strong, even in large structures such as Orion A.
“The remarkably uniform properties of disk samples of the same age is an amazing finding,” the authors conclude, and their results support what previous studies and reviews have hinted at.
“Now, however, we can show that this applies to more YSOs and YSO clusters forming in well-separated parts of the same giant cloud. For the first time, the unprecedented sample size of SODA (Survey of Orion Disks with Alma) disks allows us to zoom in influence of age gradients and clustering in one star-forming region”.
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