Periodic Reporting for period 4 - SACCRED (Structured ACCREtion Disks: initial conditions for planet formation in the time domain)
Reporting period: 2022-01-01 to 2023-06-30
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, driving the construction and operation of such high-tech instruments as the James Webb Space Telescope, holding the interest of the general public as well. With the latest astronomical instrumentation and most advanced numerical simulations, 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. In the ERC project, I carried out an ambitious research project to understand the physical principles that govern the evolution of these disks to define the initial conditions for planet formation. Our results helped us to understand why some protostars suddenly increase their brightness by 100 times, and how these violent outbursts affect the physical conditions under which planetary embryos may form in the disks. Eruptions of young stars occur 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, we studied 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 studied the details of what kind of instabilities can explain this. Finally, we investigated whether the eruptions impact the region where terrestrial planets could form. The novel approach of my project was 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.
We put significant effort into discovering and characterizing new eruptions or young stars based on alerts for unexpected brightenings from the Gaia space mission. We used the latest and most powerful astronomical instruments to obtain the sharpest images and most sensitive spectra of the disks around young stars. We used a technology that connects the infrared light from four of the largest telescopes in the world to form a giant mirror. We used millimeter-wave observations to survey the distribution of cold dust grains in ten FU Orionis-type objects using the ALMA antenna array and found that these disks are typically smaller but more massive than disks around normal, noneruptive young stars. In the L1551 IRS 5 system, we managed for the first time to make an image of all components: two circumstellar disks, a circumbinary ring, and streamers of material connecting these structures (Fig.1). In the double-burster Z CMa system, we detected another streamer that seems to point to a hitherto unknown third component, an intruder whose likely fly-by may explain the outbursts (Fig.2). In the disk around V960 Mon, we discovered large dusty clumps that could collapse to create giant planets (Fig. 3).
Rings also appeared in our hydrodynamic disk simulations. Our modeling revealed the inward motion of these rings, providing a new type of explanation for the origin of outbursts (Fig.4). Measuring the timescales of brightening and fading of young stars may reveal much about the circumstellar structure and the physics of the eruptions. We monitored many FU Orionis-type stars to determine these parameters and were among the first to realize that the accretion of mass onto the protostar V346 Nor stopped for a short time a few years ago, posing difficult questions to outburst theories. We studied the effect of accretion outbursts on the disk from chemical and mineralogical points of view. We participated in numerical studies to predict what chemical reactions are triggered by the outburst heat and how the outbursts impact the size of the dust grains in the disk.
We conducted several studies of EX Lup, the prototype of one class of young eruptive stars. We combined observations with model simulations to follow up our previous result where we witnessed the crystallization of amorphous dust particles during the outburst of EX Lup in 2008. Now, we can demonstrate that the new crystals were transported outward. With the exceptional sensitivity of the James Webb Space Telescope, we successfully rediscovered the crystals in the cold parts of the disk. Some of these crystals might be mixed with ice and become parts of forming comets and planets. We also detected molecules that are essential ingredients for the development of life, such as carbon monoxide and water vapor (Fig.5).
Rings also appeared in our hydrodynamic disk simulations. Our modeling revealed the inward motion of these rings, providing a new type of explanation for the origin of outbursts (Fig.4). Measuring the timescales of brightening and fading of young stars may reveal much about the circumstellar structure and the physics of the eruptions. We monitored many FU Orionis-type stars to determine these parameters and were among the first to realize that the accretion of mass onto the protostar V346 Nor stopped for a short time a few years ago, posing difficult questions to outburst theories. We studied the effect of accretion outbursts on the disk from chemical and mineralogical points of view. We participated in numerical studies to predict what chemical reactions are triggered by the outburst heat and how the outbursts impact the size of the dust grains in the disk.
We conducted several studies of EX Lup, the prototype of one class of young eruptive stars. We combined observations with model simulations to follow up our previous result where we witnessed the crystallization of amorphous dust particles during the outburst of EX Lup in 2008. Now, we can demonstrate that the new crystals were transported outward. With the exceptional sensitivity of the James Webb Space Telescope, we successfully rediscovered the crystals in the cold parts of the disk. Some of these crystals might be mixed with ice and become parts of forming comets and planets. We also detected molecules that are essential ingredients for the development of life, such as carbon monoxide and water vapor (Fig.5).
We studied individual young eruptive stars and also conducted statistical, survey-like investigations of the class of eruptive stars as a whole. To progress beyond the state of the art, we used the newest observing technologies and instruments. We proposed young eruptive stars as targets for the science verification (SV) of two new instruments of the European Southern Observatory (SEPIA, NAOMI). SV is an integral part of the commissioning of a new instrument. Young eruptive stars were among the first targets observed with these new instruments and our results could demonstrate the excellent capabilities of these instruments to the astronomy community. We were among the first to obtain data from the newly commissioned infrared interferometer MATISSE and the newly launched James Webb Space Telescope (JWST). We were the first to publish the infrared spectrum of a protoplanetary disk obtained by the JWST. Modern instruments, in particular the ALMA antenna array, produce large amounts of data that conventional data processing and manipulating techniques fail. We pioneered at Konkoly Observatory using cloud computation. We performed sophisticated global numerical simulations of protoplanetary disk formation and evolution in a self-consistent way. Our hydrodynamic calculations revealed the formation of rings in the disk that become unstable and cause variable accretion. Based on the results we achieved during the ERC project, we obtained a clearer picture of the eruptive phenomenon. With increasing amounts of data on the disk structure of both eruptive and noneruptive young stars, we found that the eruptive phenomenon is part of a larger continuum of variability with different amplitudes and time scales that seem that all young stars seem to go through. By the end of the project, we gained more knowledge on the impact of outbursts on the planet-forming region, and made it clear that eruptions should be taken into account when setting the initial conditions for planet formation models. Our theoretical studies contributed to the field by proposing a new instability mechanism leading to outbursts based on a more realistic treatment of the disk structure and molecular opacities. Thus, the outcome of the ERC project was 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.