NPF researchers study stellar encounters

The goal is to analyze if these encounters affect the disks around stars and the planetary formation that is triggered within them.

Planets are born and evolve in disks made of gas and dust, the so-called “Protoplanetary disks”, which form around young stars. These stars are surrounded by other companions that are created within the same molecular gas cloud, and their presence might affect the morphology and evolution of the disks. In order to study this and the impact that it might have for the formation of planets, an international group of astronomers, led by the NPF postdoctoral researcher Nicolás Cuello, made theoretical simulations of these stellar encounters – also known as a flyby. This study also counted with the participation of the research associate Jorge Cuadra, the postdoctoral researcher Matías Montesinos and the postgraduate student Pedro Poblete.

“The main result is that the dust from the protoplanetary disk reacts in a very different way with respect to the gas during the stellar encounter. In our simulations, we observe that certain structures of the disk (such as spirals, matter bridges and creases) are more prominent for certain dust grain sizes. This grants us extremely valuable information to understand how and where planets form in these perturbed disks”, says Nicolás Cuello, who is also a postdoctoral researcher at the Institute of Astrophysics in Pontificia Universidad Católica de Chile.

The astrophysicist also points out that the thermal emission from the dust can be observed with the Very Large Telescope (VLT) and ALMA, both located in Chile, which would allow a comparison between the results from simulations and observations.

This research – a result of a collaboration between Chile, the United Kingdom, Australia and France – was carried out using three-dimensional hydrodynamic simulations that tracked the evolution of the gas and dust in the protoplanetary disk. Cuello highlights that these disks are mainly composed of gas (around 99%), but it is the dust that forms the planets. “Due to aerodynamic friction effects, the dust doesn’t behave in the same way as the gas. Therefore, it is important to perform a detailed modeling of its evolution in the disk, instead of assuming that it evolves just like the gas. In this work, we studied the evolution of distributions of dust grains of different sizes, from micrometers to dozens of centimeters”, he comments.

 

Future work

The effect of these stellar encounters on the evolution of protoplanetary disks began to be studied in the nineties. Some years after, other teams studied how the disks are truncated and become eccentric or tilted. Nowadays, thanks to the images obtained with the VLT and ALMA, the astronomers suspect that in some of the observed disks, there have been such stellar encounters. Because of this, it is relevant to understand what is going on and how is the formation of planets affected in these perturbed systems, according to Cuello.

“We are now creating a catalog of synthetic observations based on the hydrodynamic simulations. This way, we can directly compare our simulated disks with recent observations (from SPHERE and ALMA, for example). In parallel, we are applying these results to some observed disks where we think there’s a stellar encounter currently taking place.”, stresses Nicolás Cuello. The scientist comments that these encounters can last from a couple of dozens of years to many centuries, so that they try to re-construct them to explain the observations in a coherent way.

“One way or another, we are playing a big riddle game: we have many observational signatures and we need to figure out what happened to the disk. It is extremely fun to be an astrophysics detective!”, he concludes.

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