We investigate the grain opacity Kappasubgr in the atmosphere (outer radiative zone) of forming planets. This is important for the observed planetary mass-radius relationship since Kappasubgr affects ...the primordial H/He envelope mass of low-mass planets and the critical core mass of giant planets. The goal of this study is to derive a simple analytical model for Kappasubgr and to explore its implications for the atmospheric structure and resulting gas accretion rate. In agreement with earlier results, we find that Kappasubgr is typically much lower than in the ISM. In retrospect, this suggests that classical giant planet formation models should have considered the grain-free case to be as equally meaningful as the full ISM opacity case. This corroborates the result that core accretion can explain the observed increase of the giant planet frequency with stellar Fe/H.
The intrinsic luminosity of young Jupiters is of high interest for planet formation theory. It is an observable quantity that is determined by important physical mechanisms during formation, namely ...the structure of the accretion shock and, even more fundamentally, the basic formation mechanism. Our aim is to study the impact of the core mass on the post-formation entropy and luminosity of young giant planets forming via core accretion with a supercritical accretion shock that radiates all accretion shock energy. We make no claims about whether such massive cores can actually form in giant planets especially at large orbital distances. Instead of invoking gravitational instability as the consequently necessary formation mode, the high luminosity can also be caused, at least in principle, simply by a more massive core.
Context. We investigate the grain opacity κgr in the atmosphere (outer radiative zone) of forming planets. This is important for the observed planetary mass-radius relationship since κgr affects the ...primordial H/He envelope mass of low-mass planets and the critical core mass of giant planets. Aims. The goal of this study is to derive a simple analytical model for κgr and to explore its implications for the atmospheric structure and resulting gas accretion rate. Methods. Our model is based on the comparison of the timescales of the most important microphysical processes. We consider grain settling in the Stokes and Epstein drag regime, growth by Brownian motion coagulation and differential settling, grain evaporation in hot layers, and grain advection due to the contraction of the envelope. With these timescales and the assumption of a radially constant grain flux, we derive the typical grain size, abundance, and opacity. Results. We find that the dominating growth process is differential settling. In this regime, κgr has a simple functional form; it is given as 27Q/ 8Hρ in the Epstein regime in the outer atmosphere and as 2Q/Hρ for Stokes drag in the deeper layers. Grain growth leads to a typical radial structure of κgr with high ISM-like values in the outer layers but a strong decrease towards the deeper parts where κgr becomes so low that the grain-free molecular opacities take over. Conclusions. In agreement with earlier results, we find that κgr is typically much lower than in the ISM. In retrospect, this suggests that classical giant planet formation models should have considered the grain-free case to be as equally meaningful as the full ISM opacity case. The equations also show that a higher dust input in the top layers does not strongly increase κgr. This has two important implications. First, for the formation of giant planet cores via pebbles, there could be the adverse effect that pebbles tend to increase the grain input high in the atmosphere because of ablation. This could in principle increase the opacity, making giant planet formation difficult. Our study indicates that this potentially adverse effect is not important. Second, it means that a higher stellar Fe/H which presumably leads to a higher surface density of planetesimals only favors giant planet formation without being detrimental to it because of an increased κgr. This corroborates the result that core accretion can explain the observed increase of the giant planet frequency with stellar Fe/H.
ABSTRACT The composition of a planet's atmosphere is determined by its formation, evolution, and present-day insolation. A planet's spectrum therefore may hold clues on its origins. We present a ..."chain" of models, linking the formation of a planet to its observable present-day spectrum. The chain links include (1) the planet's formation and migration, (2) its long-term thermodynamic evolution, (3) a variety of disk chemistry models, (4) a non-gray atmospheric model, and (5) a radiometric model to obtain simulated spectroscopic observations with James Webb Space Telescope and ARIEL. In our standard chemistry model the inner disk is depleted in refractory carbon as in the Solar System and in white dwarfs polluted by extrasolar planetesimals. Our main findings are: (1) envelope enrichment by planetesimal impacts during formation dominates the final planetary atmospheric composition of hot Jupiters. We investigate two, under this finding, prototypical formation pathways: a formation inside or outside the water iceline, called "dry" and "wet" planets, respectively. (2) Both the "dry" and "wet" planets are oxygen-rich (C/O < 1) due to the oxygen-rich nature of the solid building blocks. The "dry" planet's C/O ratio is <0.2 for standard carbon depletion, while the "wet" planet has typical C/O values between 0.1 and 0.5 depending mainly on the clathrate formation efficiency. Only non-standard disk chemistries without carbon depletion lead to carbon-rich C/O ratios >1 for the "dry" planet. (3) While we consistently find C/O ratios <1, they still vary significantly. To link a formation history to a specific C/O, a better understanding of the disk chemistry is thus needed.
ABSTRACT Many parameters constraining the spectral appearance of exoplanets are still poorly understood. We therefore study the properties of irradiated exoplanet atmospheres over a wide parameter ...range including metallicity, C/O ratio, and host spectral type. We calculate a grid of 1D radiative-convective atmospheres and emission spectra. We perform the calculations with our new Pressure-Temperature Iterator and Spectral Emission Calculator for Planetary Atmospheres (PETIT) code, assuming chemical equilibrium. The atmospheric structures and spectra are made available online. We find that atmospheres of planets with C/O ratios ∼1 and 1500 K can exhibit inversions due to heating by the alkalis because the main coolants CH4, H2O, and HCN are depleted. Therefore, temperature inversions possibly occur without the presence of additional absorbers like TiO and VO. At low temperatures we find that the pressure level of the photosphere strongly influences whether the atmospheric opacity is dominated by either water (for low C/O) or methane (for high C/O), or both (regardless of the C/O). For hot, carbon-rich objects this pressure level governs whether the atmosphere is dominated by methane or HCN. Further we find that host stars of late spectral type lead to planetary atmospheres which have shallower, more isothermal temperature profiles. In agreement with prior work we find that for planets with K the transition between water or methane dominated spectra occurs at C/O ∼ 0.7, instead of ∼1, because condensation preferentially removes oxygen.
Context. The young T Tauri star PDS 70 has two gas accreting planets sharing one large gap in a pre-transitional disc. Dust continuum emission from PDS 70 c has been detected by Atacama Large ...Millimeter/submillimeter Array (ALMA) Band 7, considered as the evidence of a circumplanetary disc. However, there has been no detection of the dust emission from the CPD of PDS 70 b. Aims. We constrain the planet mass and the gas accretion rate of the planets by introducing a model of dust evolution in the CPDs and reproducing the detection and non-detection of the dust emission. Methods. We first develop a 1D steady gas disc model of the CPDs reflecting the planet properties. We then calculate the radial distribution of the dust profiles considering the dust evolution in the gas disc and calculate the total flux density of dust thermal emission from the CPDs. Results. We find positive correlations between the flux density of dust emission and three planet properties, the planet mass, gas accretion rate, and their product called ‘MMdot’. We then find that the MMdot of PDS 70 c is ≥4 × 10 −7 M J 2 yr −1 , corresponding to the planet mass of ≥5 M J and the gas accretion rate of ≥2 × 10 −8 M J yr −1 . This is the first case to succeed in obtaining constraints on planet properties from the flux density of dust continuum emission from a CPD. We also find some loose constraints on the properties of PDS 70 b from the non-detection of its dust emission. Conclusions. We propose possible scenarios for PDS 70 b and c explaining the non-detection respectively detection of the dust emission from their CPDs. The first explanation is that planet c has larger planet mass, larger gas accretion rate, or both than planet b. The other possibility is that the CPD of planet c has a larger amount of dust supply, weaker turbulence, or both than that of planet b. If the dust supply to planet c is larger than b due to its closeness to the outer dust ring, it is also quantitatively consistent with that planet c has weaker H α line emission than planet b considering the dust extinction effect.
Context.
Observations have revealed in the
Kepler
data a depleted region separating smaller super-Earths from larger sub-Neptunes. This can be explained as an evaporation valley between planets with ...and without H/He that is caused by atmospheric escape.
Aims.
We want to analytically derive the valley’s locus and understand how it depends on planetary properties and stellar X-ray and ultraviolet (XUV) luminosity. We also want to derive constraints for planet formation models.
Methods.
First, we conducted numerical simulations of the evolution of close-in low-mass planets with H/He undergoing escape. We performed parameter studies with grids in core mass and orbital separation, and we varied the postformation H/He mass, the strength of evaporation, and the atmospheric and core composition. Second, we developed an analytical model for the valley locus.
Results.
We find that the bottom of the valley quantified by the radius of the largest stripped core,
R
bare
, at a given orbital distance depends only weakly on postformation H/He mass. The reason is that a high initial H/He mass means that more gas needs to evaporate, but also that the planet density is lower, increasing mass loss. Regarding the stellar XUV-luminosity,
R
bare
is found to scale as
L
XUV
0.135
. The same weak dependency applies to the efficiency factor
ε
of energy-limited evaporation. As found numerically and analytically,
R
bare
varies a function of orbital period
P
for a constant
ε
as
P
−2
p
c
∕3
≈
P
−0.18
, where
M
c
∝
R
c
p
c
is the mass-radius relation of solid cores. We note that
R
bare
is about 1.7
R
⊕
at a ten-day orbital period for an Earth-like composition.
Conclusions.
The numerical results are explained very well with the analytical model where complete evaporation occurs if the temporal integral over the stellar XUV irradiation that is absorbed by the planet is larger than the binding energy of the envelope in the gravitational potential of the core. The weak dependency on the postformation H/He means that the valley does not strongly constrain gas accretion during formation. But the weak dependency on primordial H/He mass, stellar
L
XUV
, and
ε
could be the reason why the valley is so clearly visible observationally, and why various models find similar results theoretically. At the same time, given the large observed spread of
L
XUV
, the dependency on it is still strong enough to explain why the valley is not completely empty.
Abstract
The luminosity of young giant planets can inform about their formation and accretion history. The directly imaged planets detected so far are consistent with the “hot-start” scenario of high ...entropy and luminosity. If nebular gas passes through a shock front before being accreted into a protoplanet, the entropy can be substantially altered. To investigate this, we present high resolution, 3D radiative hydrodynamic simulations of accreting giant planets. The accreted gas is found to fall with supersonic speed in the gap from the circumstellar disk's upper layers onto the surface of the circumplanetary disk and polar region of the protoplanet. There it shocks, creating an extended hot supercritical shock surface. This shock front is optically thick, therefore, it can conceal the planet's intrinsic luminosity beneath. The gas in the vertical influx has high entropy which when passing through the shock front decreases significantly while the gas becomes part of the disk and protoplanet. This shows that circumplanetary disks play a key role in regulating a planet's thermodynamic state. Our simulations furthermore indicate that around the shock surface extended regions of atomic – sometimes ionized – hydrogen develop. Therefore circumplanetary disk shock surfaces could influence significantly the observational appearance of forming gas-giants.
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