EXCAVACIÓN DE LA EXPLANADA

317.01 — Establishing the Diversity of Super-Earth Systems with a Continuum of Formation Condi- tions

Mariah MacDonald1; Sarah Morrison1; Rebekah Dawson-Rigas1

1 Astronomy & Astrophysics, Pennsylvania State University (State College, Pennsylvania, United States)

Multi-planet systems observed by Kepler that con- tain super-Earths exhibit a diversity of orbital and compositional properties. Here we investigate what planetary system outcomes arise from a range of pro- toplanetary disk solid surface densities and dissipa- tive conditions shortly before disk dispersal, through simulating the giant impact phase of planet forma- tion and subsequent dynamical evolution. We also compare the orbit distributions of these outcomes to the multi- transiting systems observed by the Kepler mission. For the same degree of dissipation from a gaseous disk and with no orbital migration, we find that larger solid surface densities lead to more tightly packed, flatter systems than smaller solid sur- face densities. We find that the spread in mass- radius relation observed in the Kepler population can also be explained with a wide range of solid sur- face densities, where small solid surface densities lead to rocky, dense planets and large solid surface densities lead to larger, gaseous planets. The distri- butions of the period ratios, spacings in mutual Hill radii, and transit duration ratios of adjacent planets — as well as the distribution of planet multiplicity — arising from these solid surface densities in con- junction with moderate gas damping (corresponding to a protoplanetary disk depleted by a factor of 100 in mass before disk dispersal) agree with the distri- butions of observed systems. These disk conditions can also produce super Earth systems with resonant chains, successive pairs near and in mean motion res- onances.

317.02 — Suppression of pebble accretion by planet-induced gas flow: The implication for the formation of super-Earths

Ayumu Kuwahara1; Hiroyuki Kurokawa2

1 Earth and Planetary Sciences, Tokyo Institute of Technology (Tokyo, Japan)

2 Earth-Life Science Institute, Tokyo Institute of Technology (Tokyo, Japan)

mystery on the planet formation, because these plan- ets are expected to become gas giants via runaway gas accretion within the lifetime of protoplanetary disk. Super-Earths cores should be formed in the late-stage of the disk evolution to avoid the runaway gas accretion. Previous studies have found the three- dimensional (3D) structure of the gas flow around embedded planets (e.g., Ormel et al. 2015). Disk gas enters at high latitude of the Bondi/Hill sphere of the planet and exits through the midplane region. This outward gas flow was considered to suppress the ac- cretion of∼mm—cm-sized particles, called pebbles, and delay the core growth (Kurokawa & Tanigawa 2018; Kuwahara et al. 2019).

We calculated the trajectories of pebbles accreting onto a planet in the 3D gas flow field obtained from non-isothermal hydrodynamical simulations.

The efficiency of pebble accretion is lower in the planet-induced flow than in the unperturbed Kep- lerian shear flow. In the midplane, pebbles coming from a window between the horseshoe and shear re- gions can accrete onto the core if the pebble size is sufficiently large. Otherwise the outflow prohibits pebble from accreting. Because the horseshoe flow extends well above the midplane, pebbles coming from high altitudes are also influenced. We analyti- cally derived that the pebble accretion is suppressed whenm≥ √(St), wheremis the dimensionless plane- tarty mass expressed by the ratio of the Bondi radius to the disk scale height and St is the Stokes number of pebbles (the stopping time times the Keplerian fre- quency). This means that, for a given St, a growing proto-core starts to suppress the accretion of those pebbles as the core reaches the massm= √(St).

We suggest a scenario for the formation of super- Earths as follows. 1. Proto-cores form in the proto- planetary disk under the influence of the flow field. Due to the planet-induced gas flow field, the growth of proto-cores may halt whenm∼√(St). 2. When the growth of the proto-cores halts, they begin to migrate inward. 3. Super-Earths are formed by giant impact during disk dispersal.

317.03 — Collisional growth of organic-mantled grains and formation of rocky planetesimals

Kazuaki A. Homma1; Satoshi Okuzumi1; Taishi Nakamoto1; Yuta Ueda1,2

1 Earth and Planetary Sciences, Tokyo Institute of Technology (Me- guro, Japan)

2 Earth and Planetary Science, The University of Tokyo (Hongo, Tokyo, Japan)

It is believed that the collisional growth of silicate dust grains is restricted by their poor stickiness. In

this study, we explore the possibility that the sticki- ness of silicate grains in protoplanetary disks is en- hanced by organic mantles. Silicate grains coated by organics can be commonly found in interplanetary dust particles, and previous laboratory experiments (Kudo et al. 2002) showed that such organic-coated particles are sticky in warm environments. To study in more detail how the stickiness of organic-mantled grains depends on temperature and mantle thick- ness, we construct a simple grain adhesion model that gives the binding energy of core-mantle grains in contact. Our model shows that the stickiness of organic-mantled grains increases with temperature. This occurs because as the temperature increases, the elasticity of organic mantles decreases and the con- tact area increases. We find that aggregates made of organic-coated grains are able to break through the fragmentation barrier in the inner part of protoplan- etary disks where temperature is above > 200 K. We also simulate the growth and radial drift of organic- mantled grains in a disk, finding that they indeed grow into planetesimal-sized objects in a warm in- ner region of the disk. We will discuss a new scenario for terrestrial planet formation based on our results. Reference: Homma, A. K. et al. 2019, ApJ, 877, 128 (DOI: 10.3847/1538-4357/ab1de0)

317.04 — Pebble-driven planet formation around stars of different masses

Liu Beibei1_{; Anders Johansen}1_{; Michiel Lambrechts}1 1 Lund University (Lund, Sweden)

Observational breakthrough has been achieved in characterizing the properties of protoplanetary disks and extrasolar planets in the last decade. We thus gain a better understanding on both the birth condi- tions and the end products of planets. Meanwhile, more advanced theoretical and numerical models are required to establish the bridge between these two based on evolving planet formation theories. We develop the pebble-driven core accretion model to study the formation and evolution of planets around stars in the range of 0.08 MSunand 1 MSun. By Monte

Carlo sampling of their initial conditions, the growth and migration of a large number of individual pro- toplanetary embryos are simulated in a population synthesis mannar. Two hypothesis are proposed for the birth locations of embryos, at the water ice line or log-uniformly distributed over distance in proto- planetary disks. Two types of disks with different turbulent viscous parameters α_t of 10−3 _{and 10}−4

are also investigated. The forming planet popula- tion is statistically compared with the observed ex- oplanets in terms of mass, semimajor axis, metallic-

ity and water content. We find that massive planets are likely to form when the characteristic disk sizes are larger, the disk accretion rates are higher, the disks are more metal rich and/or their stellar hosts are more massive. Our model shows that 1) the char- acteristic planet mass is set by the pebble isolation mass. It increases linearly with the stellar mass, cor- responding to one Earth mass around a Trappist-1 star and 20 Earth mass around a solar-mass star. 2) The low-mass planets up to 20 M_Ecan form around stars with a wide range of metallicities, while mas- sive gas giant planets are preferred to grow around metal rich stars. 3) The super-Earth planets mainly composed of silicates with relatively low water frac- tions can form from the seeds at the water ice line in less turbulent disks. Altogether, the model succeeds in quantitatively reproducing several important ob- served properties and correlations among exoplan- ets.

317.06 — Unified model of formation and atmo- spheric evolution of super-Earths and Neptune- mass planets

Masahiro Ogihara1_{; Yasunori Hori}2

1 National Astronomical Observatory of Japan (Tokyo, Japan) 2 Astrobiology center (Mitaka, Tokyo, Japan)

According to the theoretical study of atmospheric accretion, super-Earths and Neptune-mass planets (SENs) accumulate massive H/He atmospheres in a runaway fashion within the lifetime of protoplan- etary disks. In contrast, several observational evi- dences suggest that most SENs avoided accretion of massive atmospheres. Many ideas have been pro- posed to solve this discrepancy. As a possible solu- tion, we focus on the heating of atmospheres by peb- ble accretion, which would suppress the accretion of atmospheres. In addition, the accreted atmosphere can be lost by collisional erosion during late-stage gi- ant impacts and by long-term photoevaporation due to stellar XUV. In this study, we perform unified nu- merical calculations for N-body simulations of the formation of SENs formation and their atmospheric evolution. In other words, using our N-body simula- tions, the planetary growth and atmospheric evolu- tion can be consistently calculated. In this presenta- tion, we will reveal whether the amount of accreted atmospheres can be limited by the above mecha- nisms (i.e., heating by pebble accretion, atmospheric escape). We also discuss a few other observed prop- erties (e.g., orbital distribution, sub-Neptune desert) of SENs that can be reproduced by the results of our simulations. We can theoretically predict observable

properties (e.g., orbital property, mass, and atmo- sphere) and their mutual correlation, which help un- derstand the results of ongoing and future observa- tional projects (e.g., TESS, CHEOPS).

317.07 — Formation of compact system of super- Earth via dynamical instabilities and giant impacts

Sanson Poon1_{; Richard Nelson}1

1 Astronomy Unit, Queen Mary University of London (London, United Kingdom)

NASA’s Kepler mission discovered∼700 planets that reside in multisystems containing 3 or more tran- siting planets, many of which are super-Earths and mini-Neptunes in compact configurations whose ori- gins are not yet understood. Using N-body simula- tions, we examine the final stage assembly of mul- tiplanet systems through the collisional accretion of protoplanets. Our initial conditions are constructed using a subset of the Kepler 5-planet systems as tem- plates, and apply to the epoch after gas disc disper- sal. Two different prescriptions for the outcomes of planetary collisions are adopted. The simulations address a number of questions: do the results de- pend on the accretion prescription?; do the resulting systems resemble the Kepler systems and do they re- produce the observed distribution of planetary mul- tiplicities when synthetically observed?; do colli- sions lead to significant modification of protoplanet compositions, or to stripping of gaseous envelopes?; do the eccentricity distributions agree with those in- ferred for the Kepler planets? We find the accretion prescription is unimportant in determining the out- comes. On average, the final planetary systems look similar to the Kepler templates we adopted, but the simulations do not reproduce the observed distribu- tions of planetary multiplicities or eccentricities, be- cause gravitational scattering does not dynamically excite the systems sufficiently. In addition, we find that approximately 1% of our final systems contain a co-orbital planet pair in horseshoe or tadpole or- bits. Post-processing the collision outcomes suggests they would not lead to significant changes of the wa- ter fractions of initially ice-rich protoplanets, but sig- nificant stripping of low mass gaseous atmospheres appears likely. Hence, it may be difficult to recon- cile the observation that many of the low mass Ke- pler planets appear to have H/He envelopes with a formation scenario that involves giant impacts after dispersal of the gas disc.

317.08 — Evolution and growth of dust grains in protoplanetary disks with magnetically driven disk wind

Tetsuo Taki1,2_{; Koh Kuwabara}2_{; Hiroshi Kobayashi}3_;

Takeru K. Suzuki2

1 National Astronomical Observatory of Japan (Mitaka, Tokyo, Japan)

2 University of Tokyo (Tokyo, Japan) 3 Nagoya University (Nagoya, Japan)

Magnetically driven disk winds (DWs) are one of the promising mechanism of dispersal processes of protoplanetary disks (Suzuki et al. 2010, Bai 2013). When the DWs play a key role, the gaseous compo- nent of protoplanetary disks evolves in a different manner from that of the classical viscous evolution. As a result, the subsequent planet formation is also affected by the DWs. In this work, we investigate the effects of the DWs on the radial drift of solid parti- cles with the size of 0.1μm - 1km. We propose that the DWs is a possible solution to the ”radial drift bar- rier” of collisionally growing dust grains, which is a severe obstacle to the planet formation (e.g., Naka- gawa et al.1986). In order to study the evolution of dust grains in the disks, we calculate the advection and the collisional growth of dust particles in evolv- ing protoplanetary disks under the 1+1 D (time + radial distance) approximation. We solve a coagu- lation equation of solid particles under a single-size approximation (Sato et al. 2016) for various condi- tions of turbulent viscosity, the mass loss by the DW, and the magnetic braking by the DW. We found that rapid grain growth occurs in the inner region of the protoplanetary disks. The DWs disperse the pro- toplanetary disk from inside to outside. On such the process, the region where a pressure gradient is smaller than the typical value of the protoplanetary disks appears. At the same time, the Stokes number of dust grains tend to be larger than the case without such region. The growth timescale of dust grains be- comes shorter than the radial drift timescale of them in such the flat and gas dispersed region. In addition, when the disk gas is mainly lost by the DWs rather than by the accretion, an outwardly moving pressure bump is formed. The pressure bump can halt the dust radial migration (e.g., Taki et al. 2016). We con- firmed that the dust grains trapped in the pressure bumps in our simulations. It is a potential advantage for the planetesimal formation.

317.09 — Inertial concentration of dust particles in accretion disks

Pascale Garaud1_{; Sara Nasab}1

1 Applied Mathematics, UC Santa Cruz (Santa Cruz, California, United States)

Turbulence in protostellar disks can cause dense concentrations of dust particles to form through a process called inertial concentration. Using Direct Numerical Simulations in the two-fluid formalism (where the particles are treated as a continuum cou- pled with the gas through a linear drag term), we demonstrate the existence of a scaling law relating the maximum particle concentration observed at any given time to the particle Stokes number, the particle diffusion coefficient, and the rms velocity of the tur- bulent fluid. This law can be explained using simple dimensional arguments. We apply our findings to dusty disks, to predict what the largest possible dust concentrations may be at any given point in the disk.

317.10 — Fragmentation favours protoplanetary discs around high mass stars

James Cadman1

1 School of Physics and Astronomy, University of Edinburgh (Edin- burgh, United Kingdom)

Recent observations suggest that, when we observe wide-orbit gas giants, they are primarily found around high mass stars. A possible explanation to this may be provided by planet formation through fragmentaion. Fragmentation most likely occurs in the outer regions of protoplanetary discs where their cooling times are smallest, whilst also likely only forming massive gas giant planets and brown dwarfs rather than terrestrial planets, thus preferentially forming wide-orbit gas giants. The work done in this project aims to show that the conditions neces- sary for a disc to be unstable against fragmentation are more readily satisfied around higher mass stars, therefore potentially providing explaination of this observed planet population.

317.11 — Pebble-driven planet formation for TRAPPIST-1 and other compact systems

Djoeke Schoonenberg1; Beibei Liu2; Chris W. Ormel1; Caroline Dorn3

1 Anton Pannekoek Institute, University of Amsterdam (Amster- dam, Netherlands)

2 Department of Astronomy and Theoretical Physics, Lund Obser- vatory (Lund, Sweden)

3 University of Zurich (Zürich, Switzerland)

Two years ago, a spectacular planetary system was discovered around the M-dwarf star TRAPPIST-1. Seven Earth-sized planets are circling this star with

very short periods: their orbits would all fit well within Mercury’s orbit in the Solar System. Three out of the seven planets are located in the habit- able zone; the temperate conditions may support the presence of liquid water. Thanks to transit-timing variations, the planets’ masses and therefore compo- sitions have been constrained. Planet internal mod- elling suggests that the TRAPPIST-1 planets have moderate water fractions of a few to tens of mass percent, which is much more than that of the Earth. These values suggest that the TRAPPIST-1 planets formed outside the snowline and migrated inward while they were still growing. We have connected a model of the evolution of dust and pebbles and planetesimal formation with a model of the growth from planetesimals to planets. This strategy enables us to self-consistently model the assembly of the TRAPPIST-1 planets, keeping track of their compo- sition. The model may also be applied to other com- pact planetary systems. One of our key predictions is that the planet water fraction shows a V-shaped trend with planet order. Future (more precise) ob- servational measurements of planet water fractions in compact systems could therefore be used to con- strain our planet formation model.

317.12 — Photodissociation-Driven Mass Loss from Young and Highly-Irradiated Exoplanets

Alex Howe1_{; Fred Adams}2,3_{; Michael Meyer}4 1 Goddard Space Flight Center (Ann Arbor, Michigan, United States)

2 Department of Physics, University of Michigan (Ann Arbor, Michigan, United States)

3 Department of Astronomy, University of Michigan (Ann Arbor, Michigan, United States)

4 Department of Astronomy, The University of Michigan (Ann Arbor, Michigan, United States)

The most widely-studied mechanism of mass loss from irradiated exoplanets is photoevaporation via XUV ionization. However, lower-energy FUV disso- ciation of hydrogen molecules can also theoretically drive atmospheric evaporation on low-mass planets because the dissociation energy of hydrogen is an or- der of magnitude greater than the escape energy per proton from the gravity well of an Earth-sized planet. This implies that a significant fraction of a star’s blackbody flux can contribute to photoevaporation, potentially to a greater degree than ionizing radia- tion. For temperate planets such as the early Earth, impact erosion is expected to dominate over photoe- vaporation in most formation models, but for highly irradiated planets such as those near the “evapora- tion valley” observed inKepler planets, or for peb-

ble accretion formation models, they could plausibly be sculpted primarily by photodissociation. I present results of a survey of various mass loss processes and their relative contributions to mass loss from an early Earth-like planet. In particular, we find that pho-