Flux-limited Diffusion Approximation Models of Giant Planet Formation by Disk Instability. II. Quadrupled Spatial Resolution

While collisional accumulation is nearly universally accepted as the formation mechanism of rock and ice worlds, the situation regarding gas giant planet formation is more nuanced. Gas accretion by solid cores formed by collisional accumulation is the generally favored mechanism, but observations increasingly suggest that gas disk gravitational instability might explain the formation of at least the massive or wide-orbit gas giant exoplanets. This paper continues a series aimed at refining three-dimensional (3D) hydrodynamical models of disk instabilities, where the handling of the gas thermodynamics is a crucial factor. Boss (2017, 2021) used the β cooling approximation to calculate 3D models of disks with initial masses of 0.091 M ⊙ extending from 4 to 20 au around 1 M ⊙ protostars. Here we employ 3D flux-limited diffusion (FLD) approximation models of the same disks, in order to provide a superior treatment of disk gas thermodynamics. The new models have quadrupled spatial resolution compared to previous 3D FLD models, in both the radial and azimuthal spherical coordinates, resulting in the highest spatial resolution 3D FLD models to date. The new models continue to support the hypothesis that such disks can form self-gravitating, dense clumps capable of contracting to form gas giant protoplanets, and suggest that the FLD models yield similar numbers of clumps as β cooling models with β ∼ 1 to ∼10, including the critical value of β = 3 for fragmentation proposed by Gammie.

Color Dependence of Planetary Transit Depths due to Large Starspots and Dust Clouds: Application to PTFO 8-8695

Although thousands of exoplanets have now been discovered, there is still a significant lack of observations of young planets only a few Myr old. Thus there is little direct evidence available to differentiate between various models of planet formation. The detection of planets of this age would provide much-needed data that could help constrain the planet formation process. To explore what transit observations of such planets may look like, we model the effects of large starspots and dust clouds on the depths of exoplanet transits across multiple wavelengths. We apply this model to the candidate planet PTFO 8-8695b, whose depths vary significantly across optical and infrared wavelengths. Our model shows that, while large starspots can significantly increase the color dependence of planetary transits, a combination of starspots and a large cloud surrounding the planet is required to reproduce the observed transit depths across four wavelengths.

How Complete Are Surveys for Nearby Transiting Hot Jupiters?

Hot Jupiters are a rare and interesting outcome of planet formation. Although more than 500 hot Jupiters (HJs) are known, most of them were discovered by a heterogeneous collection of surveys with selection biases that are difficult to quantify. Currently, our best knowledge of HJ demographics around FGK stars comes from the sample of ≈40 objects detected by the Kepler mission, which have a well-quantified selection function. Using the Kepler results, we simulate the characteristics of the population of nearby transiting HJs. A comparison between the known sample of nearby HJs and simulated magnitude-limited samples leads to four conclusions. (1) The known sample of HJs appears to be ≈75% complete for stars brighter than Gaia G ≤ 10.5, falling to ≲50% for G ≤ 12. (2) There are probably a few undiscovered HJs with host stars brighter than G ≈ 10 located within 10° of the Galactic plane. (3) The period and radius distributions of HJs may differ for F-type hosts (which dominate the nearby sample) and G-type hosts (which dominate the Kepler sample). (4) To obtain a magnitude-limited sample of HJs that is larger than the Kepler sample by an order of magnitude, the limiting magnitude should be approximately G ≈ 12.5. This magnitude limit is within the range for which NASA’s Transiting Exoplanet Survey Satellite can easily detect HJs, presenting the opportunity to greatly expand our knowledge of hot-Jupiter demographics.

Disk Masses and Dust Evolution of Protoplanetary Disks around Brown Dwarfs

We present the largest sample of brown dwarf (BD) protoplanetary disk spectral energy distributions modeled to date. We compile 49 objects with ALMA observations from four star-forming regions: ρ Ophiuchus, Taurus, Lupus, and Upper Scorpius. Studying multiple regions with various ages enables us to probe disk evolution over time. Specifically, from our models, we obtain values for dust grain sizes, dust settling, and disk mass; we compare how each of these parameters vary between the regions. We find that disk mass decreases with age. We also find evidence of disk evolution (i.e., grain growth and significant dust settling) in all four regions, indicating that planet formation and disk evolution may begin to occur at earlier stages. We generally find that these disks contain too little mass to form planetary companions, though we cannot rule out that planet formation may have already occurred. Finally, we examine the disk mass–host mass relationship and find that BD disks are largely consistent with previously determined relationships for disks around T Tauri stars.

The Obliquity of HIP 67522 b: A 17 Myr Old Transiting Hot, Jupiter-sized Planet

HIP 67522 b is a 17 Myr old, close-in (P orb = 6.96 days), Jupiter-sized (R = 10 R ⊕) transiting planet orbiting a Sun-like star in the Sco–Cen OB association. We present our measurement of the system’s projected orbital obliquity via two spectroscopic transit observations using the CHIRON spectroscopic facility. We present a global model that accounts for large surface brightness features typical of such young stars during spectroscopic transit observations. With a value of ∣ λ ∣ = 5.8 − 5.7 + 2.8 ° it is unlikely that this well-aligned system is the result of a high-eccentricity-driven migration history. By being the youngest planet with a known obliquity, HIP 67522 b holds a special place in contributing to our understanding of giant planet formation and evolution. Our analysis shows the feasibility of such measurements for young and very active stars.