Astrophysics

Recent ALMA molecular line observations have revealed 3-D gas velocity structure in protoplanetary disks, shedding light on mechanisms of disk accretion and structure formation. 1) By carrying out viscous simulations, we confirm that the disk's velocity structure differs dramatically using vertical stress profiles from different accretion mechanisms. Thus, kinematic observations tracing flows at different disk heights can potentially distinguish different accretion mechanisms. On the other hand, the disk surface density evolution is mostly determined by the vertically integrated stress. The sharp disk outer edge constrained by recent kinematic observations can be caused by a radially varying α in the disk. 2) We also study kinematic signatures of a young planet by carrying out 3-D planet-disk simulations. The relationship between the planet mass and the "kink" velocity is derived, showing a linear relationship with little dependence on disk viscosity, but some dependence on disk height when the planet is massive, e.g. 10 M J . We predict the "kink" velocities for the potential planets in DSHARP disks. At the gap edge, the azimuthally-averaged velocities at different disk heights deviate from the Keplerian velocity at similar amplitudes, and its relationship with the planet mass is consistent with that in 2-D simulations. After removing the planet, the azimuthally-averaged velocity barely changes within the viscous timescale, and thus the azimuthally-averaged velocity structure at the gap edge is due to the gap itself and not directly caused to the planet. Combining both axisymmetric kinematic observations and the residual "kink" velocity is needed to probe young planets in protoplanetary disks.

We investigate the origins of Kepler-419, a peculiar system hosting two nearly coplanar and highly eccentric gas giants with apsidal orientations librating around anti-alignment, and use this system to place constraints on the properties of their birth protoplanetary disk. We follow the proposal by Petrovich, Wu, & Ali-Dib (2019) that these planets have been placed on these orbits as a natural result of the precessional effects of a dissipating massive disk and extend it by using direct N-body simulations and models for the evolution of the gas disks, including photoevaporation. Based on a parameter space exploration, we find that in order to reproduce the system the initial disk mass had to be at least 95 M_Jup and dissipate on a timescale of at least 10^4 yr. This mass is consistent with the upper end of the observed disk masses distribution, and the dissipation timescale is consistent with photoevaporation models. We study the properties of such disks using simplified 1D thin disk models and show that they are gravitationally stable, indicating that the two planets must have formed via core accretion and thus prone to disk migration. We hence finally investigate the sensitivity of this mechanism to the outer planet's semi major axis, and find that the nearby 7:1, 8:1, and 9:1 mean-motion resonances can completely quench this mechanism, while even higher order resonances can also significantly affect the system. Assuming the two planets avoid these high order resonances and/or close encounters, the dynamics seems to be rather insensitive to planet c semi major axis, and thus orbital migration driven by the disk.

KH 15D is a system which consists of a young, eccentric binary, and a circumbinary disk which obscures the binary as the disk precesses. We develop a self-consistent model that provides a reasonable fit to the photometric variability that was observed in the KH 15D system over the past 60 years. Our model suggests that the circumbinary disk has an inner edge r in ≲1 au , an outer edge r out ∼a few au , and that the disk is misaligned relative to the stellar binary by ∼ 5-16 degrees, with the inner edge more inclined than the outer edge. The difference between the inclinations (warp) and longitude of ascending nodes (twist) at the inner and outer edges of the disk are of order ∼ 10 degrees and ∼ 15 degrees, respectively. We also provide constraints on other properties of the disk, such as the precession period and surface density profile. Our work demonstrates the power of photometric data in constraining the physical properties of planet-forming circumbinary disks.

We present K-band interferometric observations of the PDS 70 protoplanets along with their host star using VLTI/GRAVITY. We obtained K-band spectra and 100 μ as precision astrometry of both PDS 70 b and c in two epochs, as well as spatially resolving the hot inner disk around the star. Rejecting unstable orbits, we found a nonzero eccentricity for PDS 70 b of 0.17±0.06 , a near-circular orbit for PDS 70 c, and an orbital configuration that is consistent with the planets migrating into a 2:1 mean motion resonance. Enforcing dynamical stability, we obtained a 95% upper limit on the mass of PDS 70 b of 10 M Jup , while the mass of PDS 70 c was unconstrained. The GRAVITY K-band spectra rules out pure blackbody models for the photospheres of both planets. Instead, the models with the most support from the data are planetary atmospheres that are dusty, but the nature of the dust is unclear. Any circumplanetary dust around these planets is not well constrained by the planets' 1-5 μ m spectral energy distributions (SEDs) and requires longer wavelength data to probe with SED analysis. However with VLTI/GRAVITY, we made the first observations of a circumplanetary environment with sub-au spatial resolution, placing an upper limit of 0.3~au on the size of a bright disk around PDS 70 b.

Recently K2 and TESS have discovered transiting planets with radii between ∼ 5-10 R ⊕ around stars with ages <100 Myr. These young planets are likely to be the progenitors of the ubiquitous super-earths/sub-neptunes, that are well studied around stars with ages ≳1 Gyr. The formation and early evolution of super-earths/sub-neptunes are poorly understood. Various planetary origin scenarios predict a wide range of possible formation entropies. We show how the formation entropies of young ( ∼ 20-60 Myr), highly irradiated planets can be constrained if their mass, radius and age are measured. This method works by determining how low-mass a H/He envelope a planet can retain against mass-loss. This lower bound on the H/He envelope mass can then be converted into an upper bound on the entropy. If planet mass measurements with errors ≲20 \% can be achieved for the discovered young planets around DS Tuc A and V1298 Tau, then insights into their origins can be obtained. For these planets, higher measured planet masses would be consistent with standard core-accretion theory. In contrast, lower planet masses ( ≲6−7 M ⊕ ) would require a "boil-off" phase during protoplanetary disc dispersal to explain.

Context. The orientation of the spin axis of a comet is defined by the values of its equatorial obliquity and its cometocentric longitude of the Sun. These parameters can be computed from the components of the nongravitational force caused by outgassing, if the cometary activity is well characterized. The trajectories of known interstellar bodies passing through the Solar System show nongravitational accelerations. Aims. The spin-axis orientation of 1I/2017 U1 (`Oumuamua) remains to be determined; for 2I/Borisov, the already released results are mutually exclusive. Here, we investigate the orientation of the spin axes of `Oumuamua and 2I/Borisov using public orbit determinations that consider outgassing. Methods. We applied a Monte Carlo simulation using the covariance matrix method together with Monte Carlo random search techniques to compute the distributions of equatorial obliquities and cometocentric longitudes of the Sun at perihelion of `Oumuamua and 2I/Borisov from the values of the nongravitational parameters. Results. We find that the equatorial obliquity of `Oumuamua could be about 93 deg, if it has a very prolate (fusiform) shape, or close to 16 deg, if it is very oblate (disk-like). Different orbit determinations of 2I/Borisov gave obliquity values of 59 deg and 90 deg. The distributions of cometocentric longitudes were in general multimodal. Conclusions. The most probable spin-axis direction of `Oumuamua in equatorial coordinates is (280 degrees, +46 degrees) if very prolate or (312 deg, -50 deg) if very oblate. For the orbit determinations of 2I/Borisov used here, we find most probable poles pointing near (275 deg, +65 deg) and (231 deg, +30 deg), respectively. Although our analysis favors an oblate shape for 2I/Borisov, a prolate one cannot be ruled out.

We analyze two sets of observations of Dione's co-orbital satellite Helene taken by Cassini's Composite Infrared Spectrometer (CIRS). The first observation was a CIRS FP3 (600 to 1100cm-1, 9.1 to 16.7 μ m) stare of Helene's trailing hemisphere, where two of the ten FP3 pixels were filled. The daytime surface temperatures derived from these observations were 83.3 ± 0.9 K and 88.8 ± 0.8 K at local times 223 ° to 288 ° and 180 ° to 238 ° respectively. When these temperatures were compared to a 1-D thermophysical model only albedos between 0.25 and 0.70 were able to fit the data, with a mean and standard deviation of 0.43 ± 0.12. All thermal inertias tested between 1 and 2000 J m ?? K ?? s ??/2 could fit the data (i.e. thermal inertia was not constrained). The second observation analyzed was a FP3 and FP4 (1100 to 1400cm-1, 7.1 to 9.1 μm) scan of Helene's leading hemisphere. Temperatures between 77 and 89 K were observed with FP3, with a typical error between 5 and 10 K. The surface temperatures derived from FP4 were higher, between 98 and 106 K, but with much larger errors (between 10 and 30 K) and thus the FP3- and FP4-derived temperature largely agree within their uncertainty. Dione's disk-integrated bolometric Bond albedos have been found to be between 0.63 ± 0.15 (Howett et al., 2010) and 0.44 ± 0.13 (Howett et al., 2014). Thus Helene may be darker than Dione, which is the opposite of the trend found at shorter wavelengths (c.f. Hedman et al., 2020; Royer et al., 2020). However few conclusions can be drawn since the albedos of Dione and Helene agree within their uncertainty.

The formation of giant planets is best studied through direct imaging by observing planets both during and after formation. Giant planets are expected to form either by core accretion, which is typically associated with low initial entropy (cold-start models) or by gravitational instability, which corresponds to a high initial entropy of the gas (hot-start models). Thus, constraining the initial entropy provides insight into the planet formation mechanism and determines the resultant brightness evolution. We find that, by observing planets in nearby moving groups of known age both through direct imaging and astrometry with Gaia, it will be possible to constrain the initial entropy of giant planets. We simulate a set of planetary systems in stars in nearby moving groups identified by BANYAN Σ and assume a model for planet distribution consistent with radial velocity detections. We find that Gaia should be able to detect approximately 50% of planets in nearby moving groups greater than ~0.3 M J . Using 5 ? contrast limits of current and future instruments, we calculate the flux uncertainty, and using models for the evolution of the planet brightness, we convert this to an initial entropy uncertainty. We find that, for future instruments such as MICADO and METIS on E-ELT and VIKiNG with VLTI, the entropy uncertainty is less than 0.5 k B /baryon, showing that these instruments should be able to distinguish between hot and cold-start models.

Images of Europa's surface taken by the Galileo Solid State Imager (SSI) show smooth features measuring a few kilometers, potentially resulting from eruptions of low-viscosity material such as liquid cryomagma. We estimated the volume of four of these smooth features by producing digital elevation models (DEMs) of four Galileo/SSI images. We used the shape-from-shading technique with special care to estimate the uncertainties on the produced DEMs and estimated feature volumes to be between ( 5.7∗ 10 7 m 3 and ( 2.7∗ 10 8 m 3 . We discussed the implications for putative sub-surface liquid reservoir dimensions in the case of eruptions induced from freezing reservoirs. Our previous cryovolcanic eruption model was improved by considering a cycle of cryomagma freezing and effusion and by estimating the vaporized cryolava fraction once cryolava spreads onto Europa's surface. Our results show that the cryomagma reservoirs would have to be relatively large to generate these smooth features (1 to 100 km 3 if the flow features result from a single eruption, and 0.4 to 60 km 3 for the full lifetime of a reservoir generating cyclic eruptions). The two future missions JUICE (ESA) and Europa Clipper (NASA) should reach Europa during the late 2020s. They shall give more information on those putative cryovolcanic regions which appear as interesting targets that could provide a better understanding of the material exchanges between the surface, sub-surface and ocean of Europa.

The observed zonal winds at Jupiter's cloud tops have been shown to be closely linked to the asymmetric part of the planet's measured gravity field. However, other measurements suggest that in some latitudinal regions the flow below the clouds might be somewhat different from the observed cloud-level winds. Here we show, using both the symmetric and asymmetric parts of the measured gravity field, that the observed cloud-level wind profile between 25 ??S and 25 ??N must extend unaltered to depths of thousands of kilometers. Poleward, the midlatitude deep jets also contribute to the gravity signal, but might differ somewhat from the cloud-level winds. We analyze the likelihood of this difference and give bounds to its strength. We also find that to match the gravity measurements, the winds must project inward in the direction parallel to Jupiter's spin axis, and that their decay inward should be in the radial direction.