Planetary embryo collisions and the wiggly nature of extreme debris discs

ABSTRACT In this paper, we present results from a multistage numerical campaign to begin to explain and determine why extreme debris disc detections are rare, what types of impacts will result in extreme debris discs and what we can learn about the parameters of the collision from the extreme debris discs. We begin by simulating many giant impacts using a smoothed particle hydrodynamical code with tabulated equations of state and track the escaping vapour from the collision. Using an N-body code, we simulate the spatial evolution of the vapour generated dust post-impact. We show that impacts release vapour anisotropically not isotropically as has been assumed previously and that the distribution of the resulting generated dust is dependent on the mass ratio and impact angle of the collision. In addition, we show that the anisotropic distribution of post-collision dust can cause the formation or lack of formation of the short-term variation in flux depending on the orientation of the collision with respect to the orbit around the central star. Finally, our results suggest that there is a narrow region of semimajor axis where a vapour generated disc would be observable for any significant amount of time implying that giant impacts where most of the escaping mass is in vapour would not be observed often but this does not mean that the collisions are not occurring.

Planet Formation in Transition Discs

<p>High-resolution imaging of protoplanetary discs has revealed their wealth of substructure. Perhaps the most striking observation has been the presence of large-scale crescent-shaped features, which have been interpreted as large quantities of dust trapped in anticyclonic vortices. Such regions of high dust-to-gas ratios are expected to promote planet formation processes, so understanding their formation and evolution is of primary interest.<br />Gas-only hydrodynamics simulations have demonstrated that the thermal feedback from a planetary embryo undergoing rapid formation by pebble accretion can trigger the generation a large-scale vortex. However, the ability for such a vortex to trap dust and the impact this has on the forming planet are yet to be investigated. I will present results from hydrodynamics simulations of a disc containing both gas and dust, showing the efficiency with which dust grains accumulate in a vortex, and discuss the consequences this has for the growth of the planetary embryo.</p>

A new and simple prescription for planet orbital migration and eccentricity damping by planet–disc interactions based on dynamical friction

ABSTRACT During planet formation, gravitational interaction between a planetary embryo and the protoplanetary gas disc causes orbital migration of the planetary embryo, which plays an important role in shaping the final planetary system. While migration sometimes occurs in the supersonic regime, wherein the relative velocity between the planetary embryo and the gas is higher than the sound speed, migration prescriptions proposed thus far describing the planet–disc interaction force and the time-scales of orbital change in the supersonic regime are inconsistent with one another. Here we discuss the details of existing prescriptions in the literature and derive a new simple and intuitive formulation for planet–disc interactions based on dynamical friction, which can be applied in both supersonic and subsonic cases. While the existing prescriptions assume particular disc models, ours include the explicit dependence on the disc parameters; hence, it can be applied to discs with any radial surface density and temperature dependence (except for the local variations with radial scales less than the disc scale height). Our prescription will reduce the uncertainty originating from different literature formulations of planet migration and will be an important tool to study planet accretion processes, especially when studying the formation of close-in low-mass planets that are commonly found in exoplanetary systems.

N-body simulations of terrestrial planet growth with resonant dynamical friction

ABSTRACT We investigate planetesimal accretion via a direct N-body simulation of an annulus at 1 au orbiting a 1 M$\odot$ star. The planetesimal ring, which initially contains N = 106 bodies is evolved into the oligarchic growth phase. Unlike previous lower resolution studies, we find that the mass distribution of planetesimals develops a bump at intermediate mass after the oligarchs form. This feature marks a boundary between growth modes. The smallest planetesimals are packed tightly enough together to populate mean motion resonances with the oligarchs, which heats the small bodies, enhancing their growth. If we depopulate most of the resonances by decreasing the width of the annulus, this effect becomes weaker. To clearly demonstrate the dynamics driving these growth modes, we also examine the evolution of a planetary embryo embedded in an annulus of collisionless planetesimals. In this case, we find that the resonances push planetesimals away from the embryo, decreasing the surface density of the bodies adjacent to the embryo. This effect only occurs when the annulus is wide enough and the mass resolution of the planetesimals is fine enough to populate the resonances. The bump we observe in the mass distribution resembles the 100 km power-law break seen in the size distribution of asteroid belt objects. Although the bump produced in our simulations occurs at a size larger than 100 km, we show that the bump location is sensitive to the initial planetesimal mass, which implies that this feature is potentially useful for constraining planetesimal formation models.

Delivery of carbon, nitrogen, and sulfur to the silicate Earth by a giant impact

Earth’s status as the only life-sustaining planet is a result of the timing and delivery mechanism of carbon (C), nitrogen (N), sulfur (S), and hydrogen (H). On the basis of their isotopic signatures, terrestrial volatiles are thought to have derived from carbonaceous chondrites, while the isotopic compositions of nonvolatile major and trace elements suggest that enstatite chondrite–like materials are the primary building blocks of Earth. However, the C/N ratio of the bulk silicate Earth (BSE) is superchondritic, which rules out volatile delivery by a chondritic late veneer. In addition, if delivered during the main phase of Earth’s accretion, then, owing to the greater siderophile (metal loving) nature of C relative to N, core formation should have left behind a subchondritic C/N ratio in the BSE. Here, we present high pressure-temperature experiments to constrain the fate of mixed C-N-S volatiles during core-mantle segregation in the planetary embryo magma oceans and show that C becomes much less siderophile in N-bearing and S-rich alloys, while the siderophile character of N remains largely unaffected in the presence of S. Using the new data and inverse Monte Carlo simulations, we show that the impact of a Mars-sized planet, having minimal contributions from carbonaceous chondrite-like material and coinciding with the Moon-forming event, can be the source of major volatiles in the BSE.