Astrophysics Of Planet Formation Armitage Pdf

[Ilario Grijalva]

Thisis the course home-page for the Midlands Physics Alliance Graduate School (MPAGS) module AS8: Formation of Planetary Systems. This is a graduate level (PhD-student) course taught in Spring Term 2022; lectures are 11am-1pm on Thursdays, beginning on February 24th.

This module aims to give students a broad overview of how planets form. We will primarily consider planet formation from an astrophysical perspective (rather than a planetary science or cosmochemistry approach), and the course will cover both observational and theoretical research into planets and their origins. We will review observations of both the Solar System and exoplanets, and discuss observations and models of the structure and evolution of protoplanetary discs (which are the sites of planet formation). We will then consider the dynamics of solid bodies, and discuss how sub-micron-sized dust grains grow to form larger bodies. From this point our theory of planet formation remains incomplete, but we will discuss and critique the leading models for both terrestrial and giant planet formation. Finally we will discuss planet migration and the dynamics of young planetary systems, and how these processes shape the architectures of planetary systems.

The handouts will provide a list of key papers and articles for each lecture, but there are also several books that provide excellent background reading. By far the most relevant textbook for this course is Astrophysics of Planet Formation, by Phil Armitage. It covers most of the material in the course (usually in greater detail than we will), and is an invaluable and up-to-date summary of the field. The book itself should be available from your library, but Phil has also made his lecture notes (on which the book is based) available on-line here.

In addition, the Protostars & Planets series provides a large collection of review articles that cover all aspects of star and planet formation. The PP meetings have been held once every 7-8 years since the late 1970s, and the proceedings books (which usually exceed 1000 pages in length) serve as standard reference texts in the field. The most recent meeting, PPVI, was held in Heidelberg in 2013, and (high-quality) videos of all of the talks from the meeting can be viewed online (and on YouTube). (The PPVII meeting was originally scheduled to take place in Kyoto in March 2021, but was postponed due to the Covid-19 pandemic; current plans are for it to take place in autumn 2022.) For copyright reasons the proceedings chapters are no longer available the PPVI website, but all but one of the chapters can be found on astro-ph or NASA ADS.

Note: the review chapters for PP7 have now started to appear on arXiv - the first tranche were posted on March 21st.

CCA researchers work to characterize individual exoplanets, use advanced statistical techniques to model the large population of known systems, and develop new methods that will enable the detection of true Earth analogs around other stars. We develop and use numerical simulations to model the physics of planet formation, including the evolution of protoplanetary disks, and the growth of dust to planetesimals, rocky, and gas giant planets.

Planetary systems form from dust and gas in the protoplanetary disks that orbit young stars for the first few million years of their lives. Key steps along the way, including the growth of dust to pebbles and planetesimals, the accretion of rocky and giant planets, and the interaction of protoplanets with the protoplanetary disk and among themselves, present novel problems in hydrodynamics and gravitational dynamics. CCA members work to understand planet formation physics, its relation to the earlier phase of star formation, and how it can be tested with protoplanetary disk and exoplanet observations. We work to develop and apply multi-fluid simulations, radiation hydrodynamics and Machine Learning techniques to open problems in planet formation. For more information, please contact Phil Armitage.

Phil Armitage is professor of astrophysical and planetary sciences at the University of Colorado, Boulder. In 2018, he will take up a joint position at Stony Brook University and the Center for Computational Astrophysics. His research focuses on the formation and evolution of planetary systems.

Context. Recent high-resolution observations of protoplanetary disks have revealed ring-like structures that can be associated to pressure maxima. Pressure maxima are known to be dust collectors and planet migration traps. The great majority of planet formation studies are based either on the pebble accretion model or on the planetesimal accretion model. However, recent studies proposed hybrid accretion of pebbles and planetesimals as a possible formation mechanism for Jupiter.

Methods. We compute, through numerical simulations, the gas and dust evolution in a protoplanetary disk, including dust growth, fragmentation, radial drift, and particle accumulation at a pressure maximum. The pressure maximum appears due to an assumed viscosity transition at the water ice line. We also consider the formation of planetesimals by streaming instability and the formation of a moon-size embryo that grows into a giant planet by the hybrid accretion of pebbles and planetesimals, all within the pressure maximum.

Conclusions. Pressure maxima generated by a viscosity transition at the water ice line are preferential locations for dust traps, planetesimal formation by streaming instability, and planet migration traps. All these conditions allow the fast formation of a giant planet by the hybrid accretion of pebbles and planetesimals.

In most planet formation simulations the most important physical properties of disks, such as for example the surface densities of gas and solids and the temperature, are characterized by a monotonically decreasing power-law profile. However, recent observations of protoplanetary disks by ALMA (Atacama Large Millimeter Array) reveal that disks are rich in substructures, such as for example rings (Andrews et al. 2018). Rings are dust concentrations that arise naturally by the presence of pressure bumps (Dullemond et al. 2018).

In order to perform a detailed study of the formation of a planet within a pressure maximum by concurrent solid and gaseous accretion, one must combine a model of pebble and planetesimal growth and evolution and their accretion onto a planetary embryo, coupled with the cooling of the gaseous envelope and gas supply from the disk. The accretion of solids by a protoplanet is typically studied under the simplifying assumption of a unique dominant particle size: either centimeter pebbles or kilometer planetesimals. Recently, Alibert et al. (2018) and Venturini & Helled (2020) proposed and studied the formation of Jupiter by a hybrid accretion of pebbles and planetesimals. In this scenario, the core is formed fast by pebble accretion, and after the planet reaches the pebble isolation mass, the heat released by the accretion of planetesimals delays the onset of the gaseous runaway.

The main goal of this study is to extend our previous work (Guilera & Sndor 2017), where we explored the formation of giant planets at the pressure maxima generated at the edges of a dead zone, incorporating a model of dust evolution, dust growth, and planetesimal formation together with a hybrid accretion of pebbles and planetesimals to study the formation of a giant planet at a pressure maximum in the disk. The paper is organized in the following way: in Sect. 2 we briefly describe our planet formation model with its new improvements, in Sect. 3 we present our results, in Sect. 4 we discuss our findings, and finally in Sect. 5 we draw our conclusions.

In order to compute the growth of the dust particles along the disk, we follow the approach derived by Drążkowska et al. (2016, hereafter D16) and Drążkowska & Alibert (2017) based on the results of Birnstiel et al. (2011, 2012). In this model, the maximum size of the dust particles at each radial bin is limited by the combined effects of the dust coagulation, radial drift, and fragmentation. As we consider a region of low-viscosity in the disk, and as was pointed out by D16, we also include a growth limitation due to the fragmentation induced by differential drift. Thus, the maximum size at a given time is given by(8)

where t is the temporal coordinate. Here, represents the sink term due to both planetesimal formation and planet accretion (see next sections). is the dust diffusivity (Youdin & Lithwick 2007), being the mass-weighted mean Stokes number of the dust size distribution (the dust distribution remains always in the Epstein drag regime). In this latter expression, represents the mass-weighted mean radius of the dust size distribution, given by(14)

Finally, Eq. (13) is solved using an implicit Donor cell algorithm considering zero density as boundary conditions, and the time-step is controlled by not allowing changes greater than 1% in the dust surface density for each radial bin between consecutive models.

The streaming instability (Youdin & Goodman 2005; Johansen et al. 2007) can be a possible mechanism for the spontaneous formation of km-sized planetesimals due to the clumping of radially drifting small particles. However, this process is not triggered along the entire protoplanetary disk, but in specific regions where dust pile-ups occur as a result of different mechanisms, such as for example dead zones, photoevaporation, and vortices (Drążkowska et al. 2016; Drążkowska & Alibert 2017; Ercolano et al. 2017).

Once the solid mass accumulated at the pressure bump in the form of planetesimals is enough to form a Moon-mass embryo, we put an embryo of this mass at the pressure bump location (Liu et al. 2019). The embryo subsequently grows by the concurrent hybrid accretion of pebbles and planetesimals, and the surrounding gas.

we consider that the pebble accretion rate is halted (aP represents the planet location). However, we note that pebble accretion has not yet been studied in a density maximum of a protoplanetary disk, and therefore the maximum size that a solid core can achieve by pebble accretion is uncertain. It may also happen that a core can grow larger than the isolation mass determined in disks where the surface density profile is described by a radial power-law profile (Sndor & Regly, in prep.). Therefore in Sect. 3.4 we also show a simulation in which the pebble isolation mass is increased. We note that in order to compute Eq. (26), we use the mass-weighted mean Stokes number of the dust-size distribution .

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