Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe Preservation of Super Earths and the Emergence of Gas Giants after Their Progenitor Cores have Entered the Pebble Isolation Phase
87
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
The omnipresence of super-Earths suggests that they are able to be retained in natal disks around low-mass stars, whereas exoplanets mass distribution indicates that some cores have transformed into gas giants through runaway gas accretion at 1AU from solar-type stars. In this paper, we show that transition to runaway gas accretion by cores may be self-impeded by an increase of the grain opacity in their envelope after they have acquired sufficient mass (typically 10Mearth) to enter a pebble-isolation phase. The accumulation of mm-m size pebbles in their migration barriers enhances their local fragmentation rates. The freshly produced sub-mm grains pass through the barrier, elevate the effective dust opacity and reduce the radiative flux in the cores envelope. These effects alone are adequate to suppress the transition to runaway accretion and preserve super-Earths in the stellar proximity (0.1 AU), albeit entropy advection between the envelope and the disk can further reduce the accretion rate. At intermediate distance (1AU) from their host stars, the escalation in the dust opacity dominates over entropy advection in stalling the transition to runaway accretion for marginally pebble-isolated cores. Beyond a few AU, the transformation of more massive cores to gas giants is reachable before severe depletion of disk gas. This requirement can be satisfied either in extended disks with large scale height via orderly accretion of migrating pebbles or through the mergers of oligarchic protoplanetary embryos, and can account for the correlated occurrence of long-period gas giants and close-in super-Earths.
rate research
Read More
The physical state and properties of silicates at conditions encountered in the cores of gas giants, ice giants and of Earth like exoplanets now discovered with masses up to several times the mass of the Earth remains mostly unknown. Here, we report on theoretical predictions of the properties of silica, SiO$_2$, up to 4 TPa and about 20,000K using first principle molecular dynamics simulations based on density functional theory. For conditions found in the Super-Earths and in ice giants, we show that silica remains a poor electrical conductor up to 10 Mbar due to an increase in the Si-O coordination with pressure. For Jupiter and Saturn cores, we find that MgSiO$_3$ silicate has not only dissociated into MgO and SiO$_2$, as shown in previous studies, but that these two phases have likely differentiated to lead to a core made of liquid SiO$_2$ and solid (Mg,Fe)O.
Observations of the population of cold Jupiter planets ($r>$1 AU) show that nearly all of these planets orbit their host star on eccentric orbits. For planets up to a few Jupiter masses, eccentric orbits are thought to be the outcome of planet-planet scattering events taking place after gas dispersal. We simulate the growth of planets via pebble and gas accretion as well as the migration of multiple planetary embryos in their gas disc. We then follow the long-term dynamical evolution of our formed planetary system up to 100 Myr after gas disc dispersal. We investigate the importance of the initial number of protoplanetary embryos and different damping rates of eccentricity and inclination during the gas phase for the final configuration of our planetary systems. We constrain our model by comparing the final dynamical structure of our simulated planetary systems to that of observed exoplanet systems. Our results show that the initial number of planetary embryos has only a minor impact on the final orbital eccentricity distribution of the giant planets, as long as damping of eccentricity and inclination is efficient. If damping is inefficient (slow), systems with a larger initial number of embryos harbor larger average eccentricities. In addition, for slow damping rates, we observe that scattering events already during the gas disc phase are common and that the giant planets formed in these simulations match the observed giant planet eccentricity distribution best. These simulations also show that massive giant planets (above Jupiter mass) on eccentric orbits are less likely to host inner super-Earths as these get lost during the scattering phase, while systems with less massive giant planets on nearly circular orbits should harbor systems of inner super-Earths. Finally, our simulations predict that giant planets are on average not single, but live in multi-planet systems.
We present empirical evidence, supported by a planet formation model, to show that the curve $R/R_oplus = 1.05,(F/F_oplus)^{0.11}$ approximates the location of the so-called photo-evaporation valley. Planets below that curve are likely to have experienced complete photo-evaporation, and planets just above it appear to have inflated radii; thus we identify a new population of inflated super-Earths and mini-Neptunes. Our N-body simulations are set within an evolving protoplanetary disk and include prescriptions for orbital migration, gas accretion, and atmospheric loss due to giant impacts. Our simulated systems broadly match the sizes and periods of super-Earths in the Kepler catalog. They also reproduce the relative sizes of adjacent planets in the same system, with the exception of planet pairs that straddle the photo-evaporation valley. This latter group is populated by planet pairs with either very large or very small size ratios ($R_{rm out} / R_{rm in} gg 1$ or $R_{rm out} / R_{rm in} ll 1$) and a dearth of size ratios near unity. It appears that this feature could be reproduced if the planet outside the photo-evaporation valley (typically the outer planet, but some times not) has its atmosphere significantly expanded by stellar irradiation. This new population of planets may be ideal targets for future transit spectroscopy observations with the upcoming James Webb Space Telescope.
We explore whether close-in super-Earths were formed as rocky bodies that failed to grow fast enough to become the cores of gas giants before the natal protostellar disk dispersed. We model the failed cores inward orbital migration in the low-mass or type I regime, to stopping points at distances where the tidal interaction with the protostellar disk applies zero net torque. The three kinds of migration traps considered are those due to the dead zones outer edge, the ice line, and the transition from accretion to starlight as the disks main heat source. As the disk disperses, the traps move toward final positions near or just outside 1~au. Planets at this location exceeding about 3~M$_oplus$ open a gap, decouple from their host trap, and migrate inward in the high-mass or type II regime to reach the vicinity of the star. We synthesize the population of planets formed in this scenario, finding that some fraction of the observed super-Earths can be failed cores. Most super-Earths formed this way have more than 4~M$_oplus$, so their orbits when the disk disperses are governed by type II migration. These planets have solid cores surrounded by gaseous envelopes. Their subsequent photoevaporative mass loss is most effective for masses originally below about 6 M$_oplus$. The failed core scenario suggests a division of the observed super-Earth mass-radius diagram into five zones according to the inferred formation history.
We investigate equilibrium chemistry between a metal-core, a silicate-mantle, and a hydrogen-rich atmosphere (reactive core model) using 18 independent reactions among 25 phase components for sub-Neptune-like exoplanets. We find hydrogen and oxygen typically comprise 1-2% and ~10% by weight of the metal-core, respectively, leading to under-dense cores and thereby offering a possible alternative explanation for the densities of the Trappist-1 planets. In addition, hydrogen occurs at about 0.1% by mass in the silicate mantle, setting a maximum limit to the hydrogen-budget for out-gassing by future super-Earths. The total hydrogen-budget of most sub-Neptunes can be, to first order, well estimated from their atmospheres alone, as more than ~93% of all H resides in their atmospheres. However, reactions with the magma ocean produce significant amounts of SiO and H_2O in the atmospheres which increase the mean molecular weight averaged over the whole atmosphere, by about a factor of two, to ~4 amu. We also investigated the case where metal is excluded from the equilibrium chemistry (unreactive core model). In this case, we find most noticeably that, as the hydrogen mass fraction is reduced from 2% to 1%, the atmosphere becomes water dominated and large fractions of H are absorbed by the magma. As water dominated atmospheres appear inconsistent with observations, we conclude that either the unreactive core model does not apply to sub-Neptunes and that their evolution is better described by a reactive core, or that in-gassing of hydrogen into the mantle is much less efficient than permitted by equilibrium chemistry.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
