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Recent observations found close-in planets with significant atmospheres of hydrogen and helium in great abundance. These are the so-called super-Earths and mini-Neptunes. Their atmospheric composition suggests that they formed early during the gas-rich phase of the circumstellar disk and were able to avoid becoming hot Jupiters. As a possible explanation, recent studies explored the recycling hypothesis and showed that atmosphere-disk recycling is able to fully compensate for radiative cooling and thereby halt Kelvin-Helmholtz contraction to prevent runaway gas accretion. To understand the parameters that determine the efficiency of atmospheric recycling, we extend our earlier studies by exploring the effects of the core mass, the effect of circumstellar gas on sub-Keplerian orbits (headwind), and the optical depth of the surrounding gas on the recycling timescale. Additionally, we analyze their effects on the size and mass of the forming atmosphere. For the explored parameter space, all simulations eventually reach an equilibrium where heating due to hydrodynamic recycling fully compensates radiative cooling. In this equilibrium, the atmosphere-to-core mass ratio stays well below $10 \, \%$, preventing the atmosphere from becoming self-gravitating and entering runaway gas accretion. Higher core masses cause the atmosphere to become turbulent, which further enhances recycling. Even for our highest core mass of $10 \, M_\mathrm{Earth}$, atmosphere-disk recycling is efficient enough to fully compensate for radiative cooling and prevent the atmosphere from becoming self-gravitating. Hence, in-situ formation of hot Jupiters is very unlikely, and migration of gas giants is a key process required to explain their existence. Our findings imply that atmosphere-disk recycling is the most natural explanation for the prevalence of close-in super-Earths and mini-Neptunes., Comment: 18 pages, 13 figures, Accepted for publication by A&A on 2/7/2022

Accreting planets have been seen at Ha (H alpha), but targeted searches have not been fruitful. For planets, accretion tracers should come from the shock itself, exposing them to extinction by the accreting material. High-resolution (R>5e4) spectrographs at Ha should soon allow studying how the incoming material shapes the line profile. We calculate how much the gas and dust accreting onto a planet reduce the Ha flux from the shock at the planetary surface and how they affect line shapes. We also study the absorption-modified relationship between Ha luminosity and Mdot. We compute the high-resolution radiative transfer of the Ha line using a 1D velocity-density-temperature structure for the inflowing matter in three representative accretion geometries: spherical symmetry, polar inflow, and magnetospheric accretion. For each, we explore wide ranges of Mdot and planet mass M. We use detailed gas opacities and estimate dust opacities. At Mdot<3e-6 MJ/yr, gas extinction is negligible for spherical or polar inflow and at most A_Ha<0.5 mag for magnetospheric accretion. Up to Mdot~3e-4 MJ/yr, the gas has A_Ha<4 mag. This decreases with M. We estimate realistic dust opacities at Ha as ~0.01-10 cm^2/g, i.e., 10-1e4 times lower than in the ISM. Extinction flattens the L_Ha-Mdot relationship, which becomes non-monotonic with a maximum L_Ha~1e-4 LSun near Mdot~1e-4 MJ/yr for M~10 MJ. In magnetospheric accretion, the gas can introduce features in line profiles, but the velocity gradient smears them out in other geometries. For most of parameter space, extinction by the accreting matter should be negligible, simplifying interpretation of observations, especially for planets in gaps. At high Mdot, strong absorption reduces the Ha flux, and some measurements can be interpreted as two Mdot values. Line profiles at R~1e5 can provide complex constraints on the accretion flow's thermal-dynamical structure., Comment: 25 pages, 11 figures (main text) + 3 pages, 4 figures (three appendices). In press at A&A. Comments welcome

In recent years, space missions such as Kepler and TESS have discovered many close-in planets with significant atmospheres consisting of hydrogen and helium: mini-Neptunes. This indicates that these planets formed early in gas-rich disks while avoiding the runaway gas accretion that would otherwise have turned them into hot-Jupiters. A solution is to invoke a long Kelvin-Helmholtz contraction (or cooling) timescale, but it has also been suggested that thermodynamical cooling can be prevented by hydrodynamical planet atmosphere-disk recycling. We investigate the efficacy of the recycling hypothesis in preventing the collapse of the atmosphere, check for the existence of a steady state configuration, and determine the final atmospheric mass to core mass ratio. We use three-dimensional radiation-hydrodynamic simulations to model the formation of planetary proto-atmospheres. Equations are solved in a local frame centered on the planet. Ignoring small oscillations that average to zero over time, the simulations converge to a steady state where the velocity field of the gas becomes constant in time. In a steady state, the energy loss by radiative cooling is fully compensated by the recycling of the low entropy gas in the planetary atmosphere with high entropy gas from the circumstellar disk. For close-in planets, recycling naturally halts the cooling of planetary proto-atmospheres, preventing them from contracting toward the runaway regime and collapsing into gas giants., Comment: 5 pages, 4 figures, accepted to A&A

Radiative feedback from luminous, massive stars during their formation is a key process in moderating accretion onto the stellar object. In the prior papers in this series, we showed that one form such feedback takes is UV line-driven disc ablation. Extending on this study, we now constrain the strength of this effect in the parameter range of star and disc properties appropriate to forming massive stars. Simulations show that ablation rate depends strongly on stellar parameters, but that this dependence can be parameterized as a nearly constant, fixed enhancement over the wind mass loss rate, allowing us to predict the rate of disc ablation for massive (proto)stars as a function of stellar mass and metallicity. By comparing this to predicted accretion rates, we conclude that ablation is a strong feedback effect for very massive (proto)stars which should be considered in future studies of massive star formation., Comment: 9 pages, 6 figures