The 3D structure of disc-instability protoplanets (2402.01432v1)

Abstract: Context. The model of disc fragmentation due to gravitational instabilities offers an alternate formation mechanism for gas giant planets, especially those on wide orbits. Aims. Our goal is to determine the 3D structure of disc-instability protoplanets and to examine how this relates to the thermal physics of the fragmentation process. Methods. We modelled the fragmentation of gravitationally unstable discs using the SPH code PHANTOM, and followed the evolution of the protoplanets formed through the first and second-hydrostatic core phases (up to densities 1e-3 g/cm3). Results. We find that the 3D structure of disc-instability protoplanets is affected by the disc environment and the formation history of each protoplanet (e.g. interactions with spiral arms, mergers). The large majority of the protoplanets that form in the simulations are oblate spheroids rather than spherical, and they accrete faster from their poles. Conclusions. The 3D structure of disc-instability protoplanets is expected to affect their observed properties and should be taken into account when interpreting observations of protoplanets embedded in their parent discs.

• The paper demonstrates that disc-instability protoplanets form with oblate spheroidal shapes due to rotational influences.
• It uses PHANTOM SPH simulations to model gravitational instabilities, emphasizing temperature, opacity, cooling rates, and the Toomre criterion.
• The study highlights that polar accretion enhances observational signatures, informing future research on gas giant formation in wide orbits.

The 3D Structure of Disc-Instability Protoplanets

The paper conducted by Fenton and Stamatellos provides a detailed analysis of the formation and structural characteristics of gas giant planets formed via disc fragmentation due to gravitational instabilities. The research focuses on exploring the three-dimensional structures of these so-called disc-instability protoplanets and delineating the influences of the disc environment and the conditions during their formation.

The phenomenon of disc fragmentation presents an alternative mechanism to the traditional core accretion theory for the formation of gas giant planets. In this paper, the authors use Smoothed Particle Hydrodynamics (SPH) simulations—specifically, the PHANTOM code—to model the evolution of gravitational instabilities within protostellar discs. Such instabilities occur when specific thermal and density criteria are met. The Toomre parameter $Q \lesssim 1$, which involves the sound speed, epicyclic frequency, and surface density, is a key criterion for instability, and these conditions are further influenced by the rate of cooling and magnetic fields.

A significant finding from the simulations is the deviation from spherical symmetry in disc-instability protoplanets. The majority of these protoplanets exhibit oblate spheroidal shapes, indicating that rotation plays an influential role in their structure. This observation is important because the 3D shape may significantly impact the interpretation of observational data related to protoplanets still embedded in their parental discs.

Fenton and Stamatellos uncovered that disc conditions, such as initial temperature profiles and opacity parameters, have substantial effects on the formation and final characteristics of the protoplanets. High metallicity and opacity values relate to different cooling behaviors, impacting the collapse dynamics, and thus affecting the resultant shapes and kinematics of the forming protoplanets. Interestingly, the research also points out that accretion seems to occur more rapidly at the poles of these protoplanets, which could impart distinctive observational signatures related to their emission profiles.

Moreover, the paper elucidates the complex interplay between the initial conditions and the resulting morphology and dynamical properties of protoplanets. The research suggests that more rigid equations of state lead to fewer, yet more massive, protoplanets due to increased resistance to fragmentations. Through various mappings of temperature and density profiles, the authors highlight the dynamics of accretion shocks and the resulting stages of core formation that parallel the earlier processes observed in stellar formation from molecular cloud cores.

From a broader perspective, these findings have substantial implications for the theoretical understanding and subsequent observational identification of giant planet formation in wide orbits. With the ongoing advances in telescopic capabilities, accurate modeling of protoplanetary environments will be instrumental in distinguishing between different planet formation scenarios. Furthermore, while this paper does exclude magnetic effects, which could add another layer of complexity, it establishes a strong foundation for considering disc fragmentation as a viable mechanism alongside core accretion.

This work indicates promising directions for future research in protoplanetary formation and evolution. Expanded explorations that integrate magnetic fields or examine more varied disc environments could yield deeper insights into the potential diversity of planetary systems. Additionally, further simulations that incorporate direct comparisons with observational data could refine our understanding of the accretion and development processes that these emerging planets undergo.