Striking resemblance to planet-forming discs
With the help of ALMA, the mass distribution of more than 870 planet-forming disks in the Orion A cloud could now be examined. It turned out that away from harsh environments like hot stars, the disk’s mass decline depends only on its age. This could mean that planet-forming disks and planetary systems evolve in a similar way.
This artist’s rendering illustrates what disks of planet formation around young stars often look like. They are initially made up of dust and gases which condense into rings of dense material. Over time, the solid components turn into rocks which can eventually evolve into planets.
Picture: MPIA Graphics Department [Großansicht]
Some of the most exciting questions in astronomical research today are: what are other planetary systems like and how does the solar system compare with other systems? A team of astronomers has now found crucial clues to solve this mystery. “Until now, we didn’t know exactly which properties dominate the development of planet-forming disks around young stars,” says Sierk van Terwisga, a scientist at the Max Planck Institute for Astronomy in Heidelberg. “Our new results now show that in environments without relevant external influences, the observed disk mass available for the formation of new planets depends solely on the age of the star-disk system.”
Disk mass is the key property when studying the evolution of planet-forming disks. This size determines the amount of material available for implementation in the planets. Depending on the age of the disc, it can also give clues to planets already present. External influences such as radiation and winds from nearby massive stars obviously affect the persistence of the disks. However, such environments are rare, and these processes don’t reveal much about the disks themselves.
Instead, experts are more interested in the internal properties of the disk such as the age, chemical composition or dynamics of the original cloud from which the young stars and their disks emerged. To disentangle the various contributions, the team chose a large, well-known region of young disc stars, the Orion A cloud. It lies about 1,350 light-years from Earth. “Orion A has provided us with an unprecedented sample of over 870 discs around young stars. This was crucial to be able to search for small variations in disk mass with age and even with the local environment in the cloud,” says team member Álvaro. Hacar, scientist at the University of Vienna.
The sample is based on previous observations with the Herschel Space Telescope, which were able to identify the discs. The combination of several wavelengths provided a criterion for estimating their age. Since they all belong to the same cloud, the researchers expected only minor influences from chemical and temporal changes in the cloud. They avoided any influence from massive stars in the neighboring Orion Nebula star cluster by excluding discs within 13 light-years.
To measure the mass of the disk, the team used the Atacama Large Millimeter/submillimeter Array (ALMA) located on the Chajnantor Plateau in the Atacama Desert in Chile. ALMA consists of 66 parabolic antennas that operate as a single telescope with adjustable angular resolution. The scientists used an observation mode that allowed them to effectively target each disk at a wavelength of about 1.2 millimeters. The cold disks are very bright in this spectral range, the contribution of the central stars, on the other hand, is negligible.
With this approach, the astronomers were able to determine the dust masses of the disks. However, observations are insensitive to objects much larger than a few millimeters, e.g. B. rocks and planets. The team therefore measured the mass of disc material from which planets can form. Before calculating the masses of the discs, the researchers combined and calibrated data from several dozen ALMA telescopes.
This task is quite difficult with large datasets. Using standard procedures, it would have taken months to process the data collected. Instead, the team developed a new method that relies on parallel computers: “Our new approach increased processing speed by a factor of 900,” points out Raymond Oonk of cooperating IT service provider SURF . The 3,000 CPU hours needed to complete the task and prepare the data for further analysis passed in less than a day.
In total, there are planet-forming disks in Orion A, each containing up to a few hundred Earth masses of dust. Of the 870 disks, however, only 20 of them contain dust of at least 100 Earth masses. In general, the number of disks decreases rapidly toward higher masses, with most containing less than 2.2 Earth masses of dust. “To look for discrepancies, we split the Orion A cloud and examined these regions separately. Thanks to the hundreds of slices, the subsets were still large enough to provide statistically significant results,” says van Terwisga.
Indeed, within Orion A, the team found small variations in the distribution of disk masses on the scale of tens of light-years, but these can all be explained by a d age, that is to say that in a few million years the masses of the discs tend to decrease towards an older population. However, within error tolerance, groups of planetary disks of the same age have the same mass distribution.
Not surprisingly, the dust mass in planet-forming disks decreases over time. After all, dust is one of the raw materials of the planets. Thus, the formation of planets undoubtedly reduces the amount of dust available. Other well-known processes are the migration of dust to the center of the disc and the evaporation of dust from radiation from the central star. Yet it is surprising that there is such a correlation between disc mass and age.
All of these disks formed from the same environment that forms the Orion A cloud today. How does this compare to other young populations of stellar disks? Astronomers investigated this question by comparing their results to several nearby star-forming regions with planet-forming disks. All but two fit well with the mass-age relationship found in Orion A. same mass distribution at a given age. And they seem to evolve in more or less the same way,” he said, van Terwisga concludes. The result could even be a clue to the formation of surprisingly similar planetary systems.
In a next step, the team will study the possible influences of nearby stars at distances smaller than a few light years. Although they avoided the powerful radiation field caused by the massive stars in the Orion Nebula, there may be other less powerful field stars that could affect the dust of nearby disks and alter the mass statistics of the disk. Such contributions could explain some of the discrepancies found in the relationship between disk mass and age. The results may help reinforce the overall picture of an age-dominated evolution of planet-forming disks.
The team reports the results in a specialist article published in the journal Astronomy & Astrophysics appeared.