As the summer heat is slowly retreating in Beijing, a new academic year has begun! We welcome in particular incoming graduate student Yu Wang. In this semester, we are in charge of organizing the Tsinghua Planet Group Seminar.
Ormel and collaborators discover that pebble accretion naturally provides planets with a metallicity gradient
A protoplanetary disk consist of rotating dense gas and dust which continues to feed onto the newly formed star. Starting from the iconic HL tau image, the Atacama Large Millimeter/submillimeter Array (ALMA) in the Atacama Desert of Northern Chile has observed many protoplanetary disks at (sub)millimeter wavelengths. It has revealed that a large fraction of disks harbor annular dust structures, e.g., “rings”, typically seen at distances of tens of au. This reflects a concentration of pebble-sized particles, ~100um--mm in size with a total mass of at least ~10 Earth masses per ring. As of today, it is unclear how these rings come about, how (long) they survive, and what role they play for the formation of planets. Pressure maximum in the gas disk is a popular explanation for these rings, because particles get trapped in these locations. However, it is unclear what the nature of these pressure bumps is and whether they will survive for disk lifetimes (~Myr). Even though many previous study invoke that these pressure bumps are carved by planets, direct observational detection of these planets is often lacking. And it also brings up a chicken or egg question: are planets carving the rings or are these rings somehow assisting planet formation?
In this paper, we present an alternative explanation for the origin of these dust substructures, involving only dust-gas aerodynamics in a standard smooth gas disk devoid of local pressure maxima. We develop the Clumpy Ring Model (CRM), in which we postulate that ALMA rings are a manifestation of a dense, clumpy midplane which is actively forming planetesimals. The clumpy medium itself hardly experiences radial drift, because its mass is dominated by pebble-sized particles, but clumps lose mass by disintegration, vertical transport and planetesimal formation. Therefore, for its long-term survival clumps must be continuously fed by an influx of pebbles from the outer disk regions. To quantitatively compare our results to the observations, we write down the 1D transport equations and numerically solve for the density distribution with time.
The CRM well explains not only the location and separation of the rings, but also their intensity and relative contrasts. Because the rings of the CRM produce large quantities of planetesimals, they are natural sites for the formation of planets. These massive planetesimal belts can also explain the huge mass budget inferred for some debris disk. Future research about how these clumpy rings help with planet formation and the link between protoplanetary disk and debris disks are ongoing.
Survival of ALMA Rings in the Absence of Pressure Maxima
Jiang, Haochang; Ormel, Chris
Accepted for publication in MNRAS
Following our previous research efforts, we have conducted numerical calculations about the thermal evolution of the envelope of pebble-accreting protoplanets. These protoplanets emerge early, when the protoplanetary gas disk is still present. Due to the accretion of solids, their envelope becomes hot, sublimating the infalling pebbles and transforming mm-sized solid particles (“pebbles”) into a metal vapor (e.g, SiO2). This transformation greatly affects the thermodynamical evolution of the protoplanet. For example, planets end up with a small "core" but may still undergo runaway gas accretion due to the high molecular weight and small pressure scale height of the atmosphere.
Using a novel numerical scheme we can now simulate the thermal evolution of the envelope for the entire period during the growth of the planet by pebble accretion: from their humble beginnings as rocks to the point where they enter runaway gas accretion. In the animation you see how the profiles of total density (solid black), vapor density (after sublimation from pebbles; dashed black) and temperature (red). Initially, most pebbles end up in the core, but after it reaches slightly above 2 Earth masses pebbles completely evaporate. Thereafter, the vapor mass (MZ,env) of the envelope rapidly increases. But the contribution of H+He gas, accreted from the gas disk surrounding the planet, also increases rapidly (Mxy). If the gas disk by this time (~5 Myr) has not dispersed, the planet can accrete much greater amounts of H+He gas and will turn into a Jupiter planet (not simulated).
What we end up with is an envelope characterized by a composition gradient. At late times it can clearly be seen that the ratio ρ/ρZ is decreasing within the envelope (the space between the two black lines is increasing throughout). Interestingly, this feature is in striking resemblance to the inference of a so-called "dilute core" for Jupiter by the JUNO mission. Jupiter’s metallicity gradually increases throughout its deep interior, instead of a sudden increase due to a solid core. In future work we will further investigate the connection between the formation scenario (e.g., pebble accretion) and the inferred internal structure of the solar system’s giant planets.
How planets grow by pebble accretion. III. Emergence of an interior composition gradient
Ormel, Chris; Vazan, Allona; Brouwers, Marc
Published in Astronomy & Astrophysics
Hello! My name is Yu, which means "rain"! But do not worry, I have a very sunny attitude! I'm interested in doing fluid dynamical simulations. In my free time, I check out WeChat, write a poem, and take pictures of the full Moon.
This is Helong Huang, a fourth year undergraduate student at the best university on this planet! I am studying the (aero)dynamical properties of large pebbles and am also interested in astrochemistry! Above all, I like mathematical rigor. In my free time I make beautiful powerpoint presentations containing many elegant equations!
btw.: You will not get rid of me soon, as I also plan to do my graduate school in Tsinghua!
Hi! I'm a second-year PhD student working to understand the physics of protoplanet discs substructures, for example, annular rings. My research interests also involve planet formation and detection in protoplanetary disks.
Hello Earthlings! I'm Shuo Huang, a second year graduate student. I am studying the origin of the complex dynamical configuration of the TRAPPIST-1 planet system and other compact planetary systems.In my free time, I watch scary movies and invent new jokes.
I am Tian, a 3rd year undergraduate student at Tsinghua! I am new to this group!
Seongjoong Kim postdoctoral researcher, Ibaraki University
Yixian Chen PhD student, Princeton
Sebastiaan Krijt Lecturer, University of Exeter, U.K.
Rico Visser PhD student, University of Amsterdam
Marc Brouwers PhD student, University of Cambridge, U.K.
Allona Vazan Professor, The Open University, Israel
Carsten Dominik Professor, University of Amsterdam
Pebble accretion efficiencies python script for pebble accretion efficiencies