Year: 2021
Type: article
Abstract: Axisymmetric dust rings are a ubiquitous feature of young protoplanetary disks. These rings are likely caused by pressure bumps in the gas profile; a small bump can induce a traffic-jam-like pattern in the dust density, while a large bump may halt radial dust drift entirely. The resulting increase in dust concentration may trigger planetesimal formation by the streaming instability (SI), as the SI itself requires some initial concentration of dust. Here we present the first 3D simulations of planetesimal formation in the presence of a pressure bump modeled specifically after those seen by Atacama Large Millimeter/submillimeter Array. We place a pressure bump at the center of a large 3D shearing box, along with an initial solid-to-gas ratio of Z = 0.01, and we include both particle back-reaction and particle self-gravity. We consider millimeter-sized and centimeter-sized particles separately. For simulations with centimeter-sized particles, we find that even a small pressure bump leads to the formation of planetesimals via the SI; a pressure bump does not need to fully halt radial particle drift for the SI to become efficient. Furthermore, pure gravitational collapse via concentration in pressure bumps (such as would occur at sufficiently high concentrations and without the SI) is not responsible for planetesimal formation. For millimeter-sized particles, we find tentative evidence that planetesimal formation does not occur. If this result is confirmed at higher resolution, it could put strong constraints on where planetesimals can form. Ultimately, our results show that for centimeter-sized particles planetesimal formation in pressure bumps is extremely robust.
Source: The Astronomical Journal
Authors: Daniel Carrera, Jacob B. Simon, Rixin Li, Katherine A. Kretke, Hubert Klahr
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: bronze
Awards: G6453454569, HOW 100 KM PLANETESIMALS FORM FROM SMALLER MM-CM SIZED PEBBLES IS ONE OF THE BIGGEST UNANSWERED QUESTIONS IN SOLAR SYSTEM FORMATION. WITH THE DISCOVERY OF THE STREAMING INSTABILITY A ROBUST CANDIDATE HAS EMERGED FOR CLUMPING THESE PEBBLES TO THE POINT OF GRAVITATIONAL COLLAPSE ALLOWING THEM TO FORM LARGER BODIES. HOWEVER THERE REMAIN TWO LARGELY UNRESOLVED ISSUES WITH THIS PROCESS. FIRST THE STREAMING INSTABILITY WILL ONLY TRIGGER CLUMPING FOR A SUFFICIENTLY LARGE CONCENTRATION OF PEBBLES A CONDITION THAT IS NOT ALWAYS EASILY MET IN DISKS WITH A SMOOTH MONOTONIC RADIAL STRUCTURE. ADDITIONALLY THE INITIAL PLANETESIMAL SIZE DISTRIBUTION THAT ARISES FROM NUMERICAL SIMULATIONS IS SIGNIFICANTLY MORE TOP-HEAVY COMPARED WITH OBSERVED SMALL BODY SOLAR SYSTEM POPULATIONS. ANOTHER ADVANCEMENT IN RECENT YEARS HAS BEEN THE REALIZATION THAT PRESSURE TRAPS I.E. LOCAL GAS PRESSURE ENHANCEMENTS ARE LIKELY PREVALENT IN PROTOPLANETARY DISKS ARISING FROM A VARIETY OF PROCESSES E.G. THE OUTER EDGE OF PLANET-INDUCED GAPS. WHILE THE IMPORTANCE OF THESE TRAPS HAS BEEN REALIZED IN THE CONTEXT OF PLANET FORMATION AS A WHOLE THEIR ROLE IN PLANETESIMAL FORMATION VIA THE STREAMING INSTABILITY HAS ONLY BARELY BEEN EXPLORED. THUS STUDYING THE INTERACTION BETWEEN THESE PRESSURE TRAPS AND STREAMING-INDUCED PLANETESIMAL FORMATION IS ESSENTIAL FOR SEVERAL REASONS. FIRST AS MENTIONED ABOVE THESE TRAPS ARE MORE LIKELY THAN NOT TO EXIST IN PROTOPLANETARY DISKS. SECOND WHILE THESE TRAPS MAY LOCALLY ENHANCE THE CONCENTRATION OF PEBBLES TO STREAMING-UNSTABLE VALUES THESE EFFECTS MUST COMPETE WITH MOMENTUM FEEDBACK FROM THE PEBBLES ONTO THE PRESSURE TRAP WHICH COULD DEFORM THE TRAP AND ACTUALLY REDUCE CLUMPING. FINALLY IF THESE TRAPS CAN SURVIVE THEY COULD POTENTIALLY CHANGE THE INITIAL PLANETESIMAL SIZES (SINCE STREAMING-UNSTABLE WAVELENGTHS WILL DECREASE NEAR THE TRAP PEAK) AND/OR THE SIZE DISTRIBUTION (IF THE TRAPS MAINTAIN A PEBBLE CONCENTRATION CLOSE TO GRAVITATIONALLY INSTABILITY PLANETESIMALS MAY FORM ONE-BY-ONE EACH OF ROUGHLY THE SAME SIZE EQUATING TO A MORE BOTTOM HEAVY DISTRIBUTION). THUS WE AIM TO TEST WHETHER PRESSURE TRAPS WILL (A) SURVIVE THE MOMENTUM FEEDBACK FROM PEBBLES TO PRODUCE SIGNIFICANT CLUMPING AND (B) CHANGE THE RESULTING PLANETESIMAL SIZES AND/OR SIZE DISTRIBUTION. PRESSURE TRAPS COULD ALSO PLAY A CRUCIAL ROLE IN PLANET FORMATION. FOR EXAMPLE AS JUPITER IS BEING FORMED IT WILL OPEN A GAP THAT GENERATES A PRESSURE TRAP EXTERIOR TO ITS ORBIT. PLANETESIMALS FORMED FROM THIS TRAP COULD CRITICALLY INFLUENCE THE EVOLUTION OF THE SOLAR SYSTEM IN ANY COMBINATION OF THREE POSSIBILITIES: THESE PLANETESIMALS MAY MERGE AND/OR ACCRETE PEBBLES TO EVENTUALLY FORM SATURN BE SCATTERED AWAY FROM THE PLANET SOME OF WHICH MAY BE DEPOSITED AS VOLATILE RICH ASTEROIDS IN THE MAIN BELT AND/OR BE ACCRETED BY JUPITER PROVIDING THE SOURCE OF JUPITER'S OBSERVED SUPER-SOLAR METALLICITY. IN THIS PROPOSAL WE WILL USE STATE-OF-THE-ART CODES ATHENA (A GAS+PARTICLE DYNAMICS CODE) AND LIPAD (A PLANET FORMATION CODE) TO CARRY OUT THE MOST IN-DEPTH STUDY TO DATE TO TEST THESE CRITICAL IDEAS. SPECIFICALLY OUR PROPOSED WORK IS OUTLINED INTO THREE TASKS: TASK 1: USE ATHENA TO TEST WHETHER PRESSURE TRAPS WILL SURVIVE FEEDBACK FORCES TO ENHANCE CLUMPING. TASK 2: USE ATHENA TO TEST WHETHER PRESSURE TRAPS WILL CHANGE THE SIZES AND/OR SIZE DISTRIBUTION OF PLANETESIMALS. TASK 3: USE LIPAD TO DETERMINE WHETHER PLANETESIMALS PRODUCED IN A PRESSURE TRAP INDUCED BY PROTO-JUPITER WILL LEAD TO THE FORMATION OF SATURN'S CORE BE SCATTERED INTO THE ASTEROID BELT AND/OR ENRICH JUPITER'S ATMOSPHERE., I PROPOSE TO PERFORM THREE-DIMENSIONAL (3D) PROTOPLANETARY DISK SIMULATIONS TO BETTER UNDERSTAND THE FORMATION OF PLANETESIMALS, THE SUPER-KILOMETER
OpenAlex ID: W3047472883