Protoplanetary disks are the places where planets form. In many of them, ring-like structures, such as bright rings, or central cavities in the dust distribution have been observed, structures that are commonly interpreted as indirect clues to the presence of planets.
An international team of astrophysicists led by Karina Maucó, a postdoctoral researcher at Millennium Nucleus for Planet Formation (NPF), observed with ALMA radio facility a young star surrounded by a dusty disk with a huge central cavity, a system called Sz91. The star is located in the Lupus molecular cloud. The disk is formed by a thin ring of dust and the main conclusion of the study is that planetesimal formation, a key step in planet formation, may be taking place in this thin ring.
Also participating in this research were Matthias R. Schreiber, deputy director of the NPF, Johan Olofsson, research associate, Amelia Bayo, director of the center, and Claudio Caceres, associate researcher. The paper was published in the prestigious Astrophysical Journal.
The scientists knew that showed clear signs of the presence of submillimeter dust grains in the ring, with the presence of smaller grains (on the order of microns) in the innermost regions. It therefore represents an excellent opportunity to test models of grain growth in disks around young stars. “This type of feature is normally attributed to the interaction of forming planets shaping the disk. This is why we decided to observe this source at 2.1 mm ALMA data at an unprecedented resolution for this source. The idea is to characterize in detail the dust grains in the ring to try to understand how planets form”, explains Maucó.
By observing the disk in different ALMA bands, scientists can study the brightness distribution, i.e., whether the object is brighter at higher or lower frequencies. This “brightness ratio” is called the spectral index, and is directly related to the dominant size of the dust grains responsible for the detected emission. The researchers compared this spectral index of Sz91 with the rest of the disk population in Lupus, as well as with other disks in different star-forming regions, to understand its evolutionary state.
“We obtained that the spectral index in the ring is almost constant throughout the ring and implies maximum grain sizes of about 0.61 mm, so we show that there is indeed grain growth in the dust ring. By comparing this spectral index with the population of disks in Lupus, we observed that Sz91 stands out as the star with the highest spectral index”, says Maucó. The astrophysicist explains that in disks like Sz91 we see outer parts of the “original” disk, more separated from the star, where the density of gas and dust is lower and we can see the full emission of the disk. In more compact disks, the emission produced in the inner parts of the disk is not able to “pass through” all the remaining disk material because it is much denser (a phenomenon known as being “optically thick”), so the emission we see is that produced by the material at the surface of the disk, which need not be representative of the emission from the disk as a whole.
On the other hand, the emission coming from the Sz91 dust ring is not optically thick, at ALMA wavelengths, so these data can be used to correctly characterize the dust in the system.
“Comparing these observational results with theoretical models of grain growth in dust rings, which include the processes of fragmentation and formation of planetesimals (km-sized objects that are the main “seeds” for planet formation), we find that our results are in very good agreement with the predictions of these models, concluding that planetesimal formation is most likely occurring in the dust ring around Sz91,” says Karina Maucó.
Future Work
Scientists will obtain new ALMA data at higher resolution and longer wavelengths, along with data taken with the VLA, to obtain information about the larger grains and corroborate the existence of centimeter-sized grains.
“The idea is to resolve the ring in the radial direction – studying the disk sections themselves – to be able to investigate in detail the properties of the dust at different radial distances as well as inhomogeneities in the disk. With these data the spectral index (and hence grain size) can be studied over a wider range of wavelengths and along the radial direction of the ring. This will allow us to study azimuthal accumulations or “clumps” – inhomogeneities along the ring, not in its section but in its length – observed in the ring (these may be regions where planets may form in the future)”, concludes Maucó.