Excitation of Planetary Obliquities Through Planet-Disk Interactions

The tilt of a planet's spin axis off its orbital axis ("obliquity") is a basic physical characteristic that plays a central role in determining the planet's global circulation and energy redistribution. Moreover, recent studies have also highlighted the importance of obliquities in sculpting not only the physical features of exoplanets but also their orbital architectures. It is therefore of key importance to identify and characterize the dominant processes of excitation of non-zero axial tilts. Here we highlight a simple mechanism that operates early on and is likely fundamental for many extrasolar planets and perhaps even Solar System planets. While planets are still forming in the protoplanetary disk, the gravitational potential of the disk induces nodal recession of the orbits. The frequency of this recession decreases as the disk dissipates, and when it crosses the frequency of a planet's spin axis precession, large planetary obliquities may be excited through capture into a secular spin-orbit resonance. We study the conditions for encountering this resonance and calculate the resulting obliquity excitation over a wide range of parameter space. Planets with semi-major axes in the range $0.3 \ \mathrm{AU} \lesssim a \lesssim 2 \ \mathrm{AU}$ are the most readily affected, but large-$a$ planets can also be impacted. We present a case study of Uranus and Neptune and show that this mechanism likely cannot help explain their high obliquities. While it could have played a role if finely tuned and envisioned to operate in isolation, large-scale obliquity excitation was likely inhibited by gravitational planet-planet perturbations.Comment: 12 pages, 8 figures, accepted to Ap

Simulating planet migration in globally evolving disks

Numerical simulations of planet-disk interactions are usually performed with hydro-codes that -- because they consider only an annulus of the disk, over a 2D grid -- can not take into account the global evolution of the disk. However, the latter governs planetary migration of type II, so that the accuracy of the planetary evolution can be questioned. To develop an algorithm that models the local planet-disk interactions together with the global viscous evolution of the disk, we surround the usual 2D grid with a 1D grid ranging over the real extension of the disk. The 1D and 2D grids are coupled at their common boundaries via ghost rings, paying particular attention to the fluxes at the interface, especially the flux of angular momentum carried by waves. The computation is done in the frame centered on the center of mass to ensure angular momentum conservation. The global evolution of the disk and the local planet-disk interactions are both well described and the feedback of one on the other can be studied with this algorithm, for a negligible additional computing cost with respect to usual algorithms.Comment: 12 pages, 11 figures, accepted for publication in A&

The Disk Substructures at High Angular Resolution Project (DSHARP). VII. The Planet–Disk Interactions Interpretation

The Disk Substructures at High Angular Resolution Project (DSHARP) provides a large sample of protoplanetary disks with substructures that could be induced by young forming planets. To explore the properties of planets that may be responsible for these substructures, we systematically carry out a grid of 2D hydrodynamical simulations, including both gas and dust components. We present the resulting gas structures, including the relationship between the planet mass, as well as (1) the gaseous gap depth/width and (2) the sub/super-Keplerian motion across the gap. We then compute dust continuum intensity maps at the frequency of the DSHARP observations. We provide the relationship between the planet mass, as well as (1) the depth/width of the gaps at millimeter intensity maps, (2) the gap edge ellipticity and asymmetry, and (3) the position of secondary gaps induced by the planet. With these relationships, we lay out the procedure to constrain the planet mass using gap properties, and study the potential planets in the DSHARP disks. We highlight the excellent agreement between observations and simulations for AS 209 and the detectability of the young solar system analog. Finally, under the assumption that the detected gaps are induced by young planets, we characterize the young planet population in the planet mass–semimajor axis diagram. We find that the occurrence rate for \u3e5 M J planets beyond 5–10 au is consistent with direct imaging constraints. Disk substructures allow us to probe a wide-orbit planet population (Neptune to Jupiter mass planets beyond 10 au) that is not accessible to other planet searching techniques

Kinematic signatures of planet-disk interactions in VSI-turbulent protoplanetary disks

Context. Planets are thought to form inside weakly ionized regions of protoplanetary disks, where turbulence creates ideal conditions for solid growth. However, the nature of this turbulence is still uncertain. In this zone, vertical shear instability (VSI) can operate, inducing a low level of gas turbulence and large-scale motions. Resolving kinematic signatures of VSI may reveal the origin of turbulence in planet-forming disks. However, an exploration of kinematic signatures of the interplay between VSI and forming planets is needed for a correct interpretation of radio interferometric observations. Robust detection of VSI would lead to a deeper understanding of the impact of gas turbulence on planet formation. Aims. The goal of this study is to explore the effect of VSI on the disk substructures triggered by an embedded massive planet. We focus on the impact of this interplay on CO kinematic observations with ALMA. Methods. We conduct global 3D hydrodynamical simulations of VSI-unstable disks with and without embedded massive planets, exploring Saturn- and Jupiter-mass cases. We study the effect of planets on the VSI gas dynamics, comparing with viscous disks. Post-processing the simulations with a radiative transfer code, we examine the kinematic signatures expected in CO molecular line emission, varying disk inclination. Further, we simulate ALMA high-resolution observations to test the observability of VSI and planetary signatures. Results. The embedded planet dampens the VSI along a radial region, most effective at the disk midplane. For the Saturn case, the VSI modes are distorted by the planet's spirals producing mixed kinematic signatures. For the Jupiter case, the planet's influence dominates the disk gas kinematics. Conclusions. The presence of massive embedded planets can weaken the VSI large-scale gas flows, limiting its observability in CO kinematic observations.Comment: Accepted for publication in Astronomy & Astrophysics. 27 pages, 17 figures and 2 table

Planet-disk interactions in non-isothermal disks

Protoplanetary disks control the formation and evolution of planets, in reaction the planets also influence the disk structure. Disk gas and dust are the building materials of planets. Tidal forces between planet and disk determine the radial movement of the planet (migration); the planets simultaneously influence the disk, possibly carving out a gap. The interplay between planets and disks is important to understand the variety of exoplanets observed and constrain planet formation theories. This thesis studies new aspects of planet-disk interactions in non-isothermal disks; an important step to a better description of this subject. To this end, radiative-hydrodynamical simulations of planet-disk interactions are performed. Firstly, a Newtonian cooling law is used to investigate the dynamics of vortices, which are generated by the planet and amplified by the convective overstability. The formation of density rings beyond the position of a planetary gap carved out by a high mass planet is studied, as well as the migration rates of low mass planets. Finally, radiative-hydrodynamical simulations are performed. The coupling of radiation transfer to hydrodynamics yields a more accurate determination of the disk temperature in the planet’s vicinity, which has a strong influence on the formation and properties of gaps in disks

K2 reveals pulsed accretion driven by the 2 Myr old hot Jupiter CI Tau b

CI Tau is a young (~2 Myr) classical T Tauri star located in the Taurus star forming region. Radial velocity observations indicate it hosts a Jupiter-sized planet with an orbital period of approximately 9 days. In this work, we analyze time series of CI Tau's photometric variability as seen by K2. The lightcurve reveals the stellar rotation period to be ~6.6 d. Although there is no evidence that CI Tau b transits the host star, a ~9 d signature is also present in the lightcurve. We believe this is most likely caused by planet-disk interactions which perturb the accretion flow onto the star, resulting in a periodic modulation of the brightness with the ~9 d period of the planet's orbit.Comment: Accepted for publication in ApJ Letter