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Structured ACCREtion Disks: initial conditions for planet formation in the time domain

Periodic Reporting for period 2 - SACCRED (Structured ACCREtion Disks: initial conditions for planet formation in the time domain)

Reporting period: 2019-01-01 to 2020-06-30

It is now within our reach to answer one of the oldest questions of humanity: how our Sun and planet Earth were born. The formation of distant stars and their planets is one of the main research directions of modern astrophysics. We can now study stars and planets currently being born in the Milky Way’s star forming regions. We already know that newborn stars (protostars) are surrounded by a circumstellar disk of dust and gas, which plays a fundamental role in the creation processes, feeding the protostar with matter, and being the birthplace of the planetary system. I carry out an ambitious research project to understand the physical principles that govern the evolution of these disks, with the goal to define the initial conditions for the formation of planets. In particular, I want to understand why some protostars suddenly increase their brightness by a 100 times, and how these violent outbursts of light and heat affect planetary embryos forming in the disks. My research is based on the current theory that eruptions of young stars happen when an unusually large amount of gas and dust falls onto the growing star in a short period of years or decades. This infalling material must originate from the outer part of the system, thus my first goal is to answer how gas and dust are transported through the uneven structure of a disk. The eruptions require that the inward moving material stop and pile up close to the star, until it suddenly falls onto the protostellar surface. It is still debated what physical mechanism could halt the mass flow and let it fall later. We study the details of what kind of instabilities can explain this. Finally, I am curious whether the eruptions impact the region where terrestrial planets could form. The novel approach of my project is to consider the infall of matter from the disk onto the star and the feedback of the outbursting star on the disk as two sides of the same coin: two processes that mutually affect one another.
Eruptions of young stars are rare, and we put a significant effort in discovering new cases or being the first to characterize the progenitor objects and the properties of the eruptions announced in the astronomical literature. We used modern machine learning methods to identify more than a million young stellar object candidates and cross-check this list daily with the alerts on unexpected brightenings of celestial sources from the Gaia space mission. A trophy of this work was the discovery of the eruption of a young star in the Cygnus constellation, adding a new member to the group of only about 20 known FU Orionis-type young outbursting stars. An other novel aspect of our research is to utilize the latest and most powerful astronomical instruments to obtain the sharpest images and most sensitive spectra of the disks around young stars, with special focus on the outbursting ones. We used a technology that connects the infrared light from four of the largest telescopes in the world to form a giant mirror. With this we studied circumstellar disks around currently forming stars with resolution similar to the Earth-Sun distance, but for stars that are hundreds of light years away from us. We measured where the warm dust grains are located and in one case we discovered the appearance of a new circumstellar ring structure. Focusing on the cold outskirts of the circumstellar material, we used millimeter observations to survey the distribution of gas molecules and dust grains in ten FU Orionis-type objects using the ALMA antenna array in Chile. In one complicated system, L1551 IRS 5 in the Taurus star forming region, we managed for the first time to make an image of all components: two circumstellar disks, a circumbinary ring, and streamers of material that may connect these structures (Fig. 1).

Rings, although of different physical background, also appeared in our computationally demanding hydrodynamic simulations of the flow of material in the circumstellar disk. Our numerical modeling revealed the inward motion of these rings, raising the possibility to give a new type of explanation for the origin of the infalling material causing the protostellar outbursts (Fig. 2). Observing the light variations and measuring the time scales of brightening and fading of young stars may reveal a lot about the circumstellar structure and the physics of the eruptions. We are monitoring many FU Orionis-type stars to determine these parameters, and were among the first ones to realize that the accretion of mass onto the protostar V346 Nor temporarily stopped for a short time a few years ago, posing difficult questions to all outburst theories. We studied the effect of accretion outbursts on the disk from chemical and mineralogical points of view. We participated in a numerical modeling to predict what chemical reactions are triggered by the outburst heat and on what time scale the disk returns to the quiescent equilibrium. Finally, we combined observations with model simulations to follow up a previous result of our group, where we witnessed the crystallization of amorphous dust particles during the outburst of EX Lup. Now we could convincingly demonstrate that the fresh crystals were transported outward where they could reach the cold part of the circumstellar disk. We speculate that some crystals might be mixed with ice and form comets, possibly solving the mystery of finding crystalline particles, created at high temperature, in icy comets in the Solar System.
My project includes both detailed studies of individual young eruptive stars and statistical, survey-like investigations of the class of eruptive stars as a whole. To progress beyond the state of the art, we use the newest observing technologies and instruments. We proposed young eruptive stars as targets for the science verification of two new instruments of the European Southern Observatory in Chile. Science verification is an integral part of the commissioning of any new instrument. We are proud that via our successful applications, young eruptive stars were among the very first targets observed with the new instruments and our results could demonstrate the excellent capabilities of these instruments to the astronomy community. Modern instruments, in particular the ALMA antenna array, produce such large amounts of data that conventional data processing and manipulating techniques fail. We pioneered at Konkoly Observatory in using cloud computation. We performed sophisticated global numerical simulations of protoplanetary disk formation and evolution in a self-consistent way starting from the collapse of a cloud core. Our hydrodynamic calculations revealed the formation of rings in the disk that become unstable and cause variable accretion.

Based on the results we already achieved, our ongoing projects, and the new ideas we are discussing, my group members and I are enthusiastic about obtaining a significantly clearer picture of the eruptive phenomenon, and make important steps toward defining a new paradigm for it. With the collection of more and more data on the disk structure of both eruptive and non-eruptive young stars, we will try to collect proofs whether eruptive objects follow an extreme evolutionary track or they represent an obligatory period that all young stars need to go through. By the end of the project we will gain more knowledge on the impact of outbursts on the planet forming region, and make a suggestion if eruptions should be taken into account when setting the initial conditions for planet formation models. Our ongoing theoretical study is expected to contribute to the field by proposing a new instability mechanism leading to outbursts, based on a more realistic treatment of disk structure and molecular opacities of the disk material, related to the presence of water molecules. Thus, the expected outcome of the ERC project is a conclusive demonstration of the ubiquity and profound impact of episodic accretion on disk structure, providing the initial physical conditions for disk evolution and planet formation models.
Gas surface density and luminosity curve in a protoplanetary disk simulation from Kadam et al. (2019
ALMA 1.3 mm dust continuum map of L1551 IRS 5 from Cruz-Sáenz de Miera et al. (2019)