Periodic Reporting for period 1 - DiscEvol (Rebuilding the foundations of planet formation: proto-planetary disc evolution)
Berichtszeitraum: 2022-10-01 bis 2025-03-31
Planets form in proto-planetary discs, gas and dust structures that orbit young stars. It has long been known that these discs are accretion discs, i.e. that matter moves radially inwards in them and eventually falls onto the central star. The reason why this happens however is a long-standing problem. The traditional paradigm is to invoke turbulence. Indeed, high enough turbulence, which acts as an effective viscosity, can explain accretion. However, it remains difficult to explain why discs should be turbulent. In parallel, proving in observations that discs are indeed turbulent has turned out to be incredibly elusive, and most results of the last few years have shown that, if present, turbulence is actually low in proto-planetary discs, questioning whether it is strong enough to drive accretion.
The alternative is that discs instead evolve through a very different process, called magneto-hydrodynamics (MHD) winds. These winds, launched by the magnetic field, can remove angular momentum from the disc, causing the material left in the disc to spiral inwards and eventually accrete onto the star.
The two different mechanisms predict very different ways in which the disc will evolve and eventually disperse, leaving behind a fully formed planetary system like our own Solar System.
Understanding whether discs are turbulent or evolve because of winds is of fundamental importance not only for the consequences on disc evolution, but also because the levels of turbulence affect nearly every process in planet formation: for example the accretion of gas and solids on young, forming planets, as well as the way they migrate through the disc. Solving the problem of why discs accrete is therefore one of the major questions in planet formation and it is the main question that the DiscEvol project wishes to tackle.
The alternative is that discs instead evolve through a very different process, called magneto-hydrodynamics (MHD) winds. These winds, launched by the magnetic field, can remove angular momentum from the disc, causing the material left in the disc to spiral inwards and eventually accrete onto the star.
The two different mechanisms predict very different ways in which the disc will evolve and eventually disperse, leaving behind a fully formed planetary system like our own Solar System.
Understanding whether discs are turbulent or evolve because of winds is of fundamental importance not only for the consequences on disc evolution, but also because the levels of turbulence affect nearly every process in planet formation: for example the accretion of gas and solids on young, forming planets, as well as the way they migrate through the disc. Solving the problem of why discs accrete is therefore one of the major questions in planet formation and it is the main question that the DiscEvol project wishes to tackle.
In these two years we have worked on many aspects of the question of what drives disc evolution. For example, on one side we have analysed a large number of discs observations, measuring the vertical extent of dust grains, those that we detect in disc observations. This is an indirect way to measure turbulence: the stronger the turbulence, the more the grains are lifted up and the disc will appear vertically thick in the images. Measuring quantitively the thickness requires a significant modelling effort, so this project was a major undertaking. We found a few cases of very turbulent discs, but they are not the majority: most discs are relatively vertically thin, implying low levels of turbulence. In some cases we were able to exclude that accretion can be driven by these low levels of turbulence.
Our main results however have come by comparing with large disc surveys, i.e. observational campaigns that measure fundamental disc properties (such as the mass, radius and accretion rate) for large disc samples. We have compared the results of one of these campaigns, AGE-PRO, with theoretical models of discs evolving under the influence of winds and discs evolving because of turbulence. Our results indicate that the wind model is favoured. In particular, the turbulent model is unable to explain the relatively low masses and high accretion rates found by the program, which instead can be reproduced by the wind model.
During these two years we have also investigated external photo-evaporation, namely mass loss from the disc induced by UV photons from nearby, massive stars. We have shown that some proto-planetary discs have been sculpted by external photo-evaporation, which previously was thought to operate only much closer to massive stars. We thus highlighted the need to consider external photo-evaporation to understand how planets form.
Our main results however have come by comparing with large disc surveys, i.e. observational campaigns that measure fundamental disc properties (such as the mass, radius and accretion rate) for large disc samples. We have compared the results of one of these campaigns, AGE-PRO, with theoretical models of discs evolving under the influence of winds and discs evolving because of turbulence. Our results indicate that the wind model is favoured. In particular, the turbulent model is unable to explain the relatively low masses and high accretion rates found by the program, which instead can be reproduced by the wind model.
During these two years we have also investigated external photo-evaporation, namely mass loss from the disc induced by UV photons from nearby, massive stars. We have shown that some proto-planetary discs have been sculpted by external photo-evaporation, which previously was thought to operate only much closer to massive stars. We thus highlighted the need to consider external photo-evaporation to understand how planets form.
In these two years we have also made progress in terms of novel techinques beyond the state of the art. One example is that we are developing, in collaboration with other researchers, a new technique to better measure how large discs are. Traditionally this is done using images from telescopes such as ALMA in Chile, the most expensive ground based telescope ever built. These telescopes however, called interferometers, do not really see images in the same way conventional telescopes do;, images are the results of complex algorithms applied to the telescope data. The native quantities measured by the interferometer are the so-called visibilities, but working with visibilities requires advanced data modelling techniques. Using them however it is possible to constrain significantly better the disc size, even when the data is of relatively low quality.
Another significant result beyond the state of the art, concerning external photo-evaporation, is a new technique we developed to measure the UV field caused by massive stars. This is difficult to do because we only see stellar positions on the sky, but when we see a star we do not know how far it is from us. Even using the best available data from the ESA mission Gaia still carries significant uncertainty. We showed that, assuming that star forming regions are isotropic, we can significantly mitigate this problem and constrain the UV field much better than was possible before. Indeed, we have shown that traditional techniques over- or under-estimate the UV field even by orders of magnitude. The new method we have developed will allow us to study much more in depth the impact of external photo-evaporation on disc evolution.
Another significant result beyond the state of the art, concerning external photo-evaporation, is a new technique we developed to measure the UV field caused by massive stars. This is difficult to do because we only see stellar positions on the sky, but when we see a star we do not know how far it is from us. Even using the best available data from the ESA mission Gaia still carries significant uncertainty. We showed that, assuming that star forming regions are isotropic, we can significantly mitigate this problem and constrain the UV field much better than was possible before. Indeed, we have shown that traditional techniques over- or under-estimate the UV field even by orders of magnitude. The new method we have developed will allow us to study much more in depth the impact of external photo-evaporation on disc evolution.