Astrophysics

We report new photometric lightcurve observations of the Lucy Mission target (11351) Leucus acquired during the 2017, 2018 and 2019 apparitions. We use these data in combination with stellar occultations captured during five epochs (Buie et al. 2020) to determine the sidereal rotation period, the spin axis orientation, a convex shape model, the absolute scale of the object, its geometric albedo, and a model of the photometric properties of the target. We find that Leucus is a prograde rotator with a spin axis located within a sky-projected radius of 3° (1 σ ) from J2000 Ecliptic coordinates ( λ=208° , β=+77° ) or J2000 Equatorial Coordinates (RA=248 ° , Dec=+58 ° ). The sidereal period is refined to P sid =445.683±0.007 h. The convex shape model is irregular, with maximum dimensions of (60.8, 39.1, 27.8) km. The convex model accounts for global features of the occultation silhouettes, although minor deviations suggest that local and global concavities are present. We determine a geometric albedo p V =0.043±0.002 . The derived phase curve supports a D-type classification for Leucus.

The origin of Uranus and Neptune remains a challenge for planet formation models. A potential explanation is that the planets formed from a population of a few planetary embryos with masses of a few Earth masses which formed beyond Saturn's orbit and migrated inwards. These embryos can collide and merge to form Uranus and Neptune. In this work we revisit this formation scenario and study the outcomes of such collisions using 3D hydrodynamical simulations. We investigate under what conditions the perfect-merging assumption is appropriate, and infer the planets' final masses, obliquities and rotation periods, as well as the presence of proto-satellite disks. We find that the total bound mass and obliquities of the planets formed in our simulations generally agree with N-body simulations therefore validating the perfect-merging assumption. The inferred obliquities, however, are typically different from those of Uranus and Neptune, and can be roughly matched only in a few cases. In addition, we find that in most cases the planets formed in this scenario rotate faster than Uranus and Neptune, close to break-up speed, and have massive disks. We therefore conclude that forming Uranus and Neptune in this scenario is challenging, and further research is required. We suggest that future planet formation models should aim to explain the various physical properties of the planets such as their masses, compositions, obliquities, rotation rates and satellite systems.

The long-term evolution of hydrogen-dominated atmospheres of sub-Neptune-like planets is mostly controlled by two factors: a slow dissipation of the gravitational energy acquired at the formation (known as thermal evolution) and atmospheric mass loss. Here, we use MESA to self-consistently couple the thermal evolution model of lower atmospheres with a realistic hydrodynamical atmospheric evaporation prescription. To outline the main features of such coupling, we simulate planets with a range of core masses (5-20 Mearth) and initial atmospheric mass fractions (0.5-30%), orbiting a solar-like star at 0.1 au. In addition to our computed evolutionary tracks, we also study the stability of planetary atmospheres, showing that the atmospheres of light planets can be completely removed within 1 Gyr, and that compact atmospheres have a better survival rate. From a detailed comparison between our results and the output of the previous-generation models, we show that coupling between thermal evolution and atmospheric evaporation considerably affects the thermal state of atmospheres for low-mass planets and, consequently, changes the relationship between atmospheric mass fraction and planetary parameters. We, therefore, conclude that self-consistent consideration of the thermal evolution and atmospheric evaporation is of crucial importance for evolutionary modeling and a better characterization of planetary atmospheres. From our simulations, we derive an analytical expression between the planetary radius and atmospheric mass fraction at different ages. In particular, we find that, for a given observed planetary radius, the predicted atmospheric mass fraction changes as age^0.11.

In this work, we employ a soft-sphere discrete element method with a cohesion implementation to model the dynamical process of sub-km-sized cohesive rubble piles under continuous spinup. The dependencies of critical spin periods T c on several material parameters for oblate rubble piles with different bulk diameters D are explored. Our numerical simulations show that both the increase of interparticle cohesion and particle shape parameter in our model can strengthen the bodies, especially for the smaller ones. In addition, we find there exists some critical diameter D cri,ρ at which the variation trend of T c with the bulk density ρ reverses. Though a greater static friction coefficient μ S can strengthen the body, this effect attains a minimum at a critical diameter D cri,ϕ close to D cri,ρ . The continuum theory (analytical method) is used for comparison and two equivalent critical diameters are obtained. The numerical results were fitted with the analytical method and the ratio of the interparticle cohesion c to the bulk cohesion C is estimated to be roughly 88.3. We find this ratio keeps constant for different c and ρ , while it strongly depends on the friction angle ϕ . Also, our numerical results further show that the dependency of T c on ϕ is opposite from that predicted by the continuum theory when D < D cri,ϕ . Finally, we find that the two critical diameters happen to be close to the diameter when the mean normal stress of the body equals zero, which is the separation between the compressive regime and the tensile regime.

The cyano radical (CN) is one of the most frequently remotely observed species in space, also in comets. Data from the high-resolution Double Focusing Mass Spectrometer (DFMS) on board the Rosetta orbiter, collected in the inner coma of comet 67P/Churyumov-Gerasimenko, revealed an unexpected chemical complexity, and, recently, also more CN than expected from photodissociation of its most likely parent hydrogen cyanide (HCN). This work is dedicated to the derivation of abundances relative to HCN of three cometary nitriles (including structural isomers) from DFMS data. Mass spectrometry of complex mixtures does not always allow distinction of structural isomers. We assumed the most stable and most abundant (in similar environments) structure in our analysis, that is HCN for CHN, CH3CN for C2H3N, HC3N for C3HN, and NCCN for C2N2. For cyanoacetylene (HC3N) and acetonitrile (CH3CN) the complete mission timeline was evaluated, while cyanogen (NCCN) often was below detection limit. By carefully selecting periods where cyanogen was above detection limit, we were able to follow the abundance ratio between NCCN and HCN from 3.16 au inbound to 3.42 au outbound. These are the first measurements of NCCN in a comet.We find that neither NCCN, nor any of the other two nitriles, is sufficiently abundant to be a relevant alternative parent to CN.

Here we aim to explore the origin of the strong C2H lines to reimagine the chemistry of protoplanetary disks. There are a few key aspects that drive our analysis. First, C2H is detected in young and old systems, hinting at a long-lived chemistry. Second, as a radical, C2H is rapidly destroyed, within <1000 yr. These two statements hint that the chemistry responsible for C2H emission must be predominantly in the gas-phase and must be in equilibrium. Combining new and published chemical models we find that elevating the total volatile (gas and ice) C/O ratio is the only natural way to create a long lived, high C2H abundance. Most of the \ce{C2H} resides in gas with a Fuv/n-gas ~ 10^-7 G0 cm^3. To elevate the volatile C/O ratio, additional carbon has to be released into the gas to enable an equilibrium chemistry under oxygen-poor conditions. Photo-ablation of carbon-rich grains seems the most straightforward way to elevate the C/O ratio above 1.5, powering a long-lived equilibrium cycle. The regions at which the conditions are optimal for the presence of high C/O ratio and elevated C2H abundances in the gas disk set by the Fuv/n-gas ~ 10^-7 G0 cm^3 condition lie just outside the pebble disk as well as possibly in disk gaps. This process can thus also explain the (hints of) structure seen in C2H observations.

Motivated by the development of high-dispersion spectrographs in the mid-infrared (MIR) range, we study their application to the atmospheric characterization of nearby non-transiting temperate terrestrial planets around M-type stars. We examine the detectability of CO 2 , H 2 O, N 2 O, and O 3 in high-resolution planetary thermal emission spectra at 12-18 μ m assuming an Earth-like profile and a simplified thermal structure. The molecular line width of such planets can be comparable to or broader than the Doppler shift due to the planetary orbital motion. Given the likely difficulty in knowing the high-resolution MIR spectrum of the host star with sufficient accuracy, we propose to observe the target system at two quadrature phases and extract the differential spectra as the planetary signal. In this case, the signals can be substantially suppressed compared with the case where the host star spectrum is perfectly known, as some parts of the spectral features do not remain in the differential spectra. Despite this self-subtraction, the CO 2 and H 2 O features of nearby ( ??5~pc) systems with mid-/late-M host stars would be practical with a 6.5-meter-class cryogenic telescope, and orbital inclination could also be constrained for some of them. For CO 2 and N 2 O in a 1~bar Earth-like atmosphere, this method would be sensitive when the mixing ratio is 1-10 3 ppm. The detectability of molecules except O 3 is not significantly improved when the spectral resolution is higher than R??0,000 , although the constraint on the orbital inclination is improved. This study provides some benchmark cases useful for assessing the value of MIR high-resolution spectroscopy in terms of characterization of potentially habitable planets.

We describe the discovery of a satellite of the Trojan asteroid (3548) Eurybates in images obtained with the Hubble Space Telescope. The satellite was detected on three separate epochs, two in September 2018 and one in January 2020. The satellite has a brightness in all three epochs consistent with an effective diameter of d2 =1.2+/-0.4 km. The projected separation from Eurybates was s~1700-2300 km and varied in position, consistent with a large range of possible orbits. Eurybates is a target of the Lucy Discovery mission and the early detection of a satellite provides an opportunity for a significant expansion of the scientific return from this encounter.

The UVS instrument on the Juno mission recorded transient bright emission from a point source in Jupiter's atmosphere. The spectrum shows that the emission is consistent with a 9600-K blackbody located 225 km above the 1-bar level and the duration of the emission was between 17 ms and 150 s. These characteristics are consistent with a bolide in Jupiter's atmosphere. Based on the energy emitted, we estimate that the impactor had a mass of 250-5000 kg, which corresponds to a diameter of 1-4 m. By considering all observations made with Juno UVS over the first 27 perijoves of the mission, we estimate an impact flux rate of 24,000 per year for impactors with masses greater than 250-5000 kg.

55 Cancri e is an ultra-short period transiting Super-Earth observed by TESS in Sector 21. Using this photometry, we measure the occultation depth in the TESS bandpass, leveraging the precise transit light curve and comparing multiple detrending methods. We measure the occultation depth to be (15.0±4.8) ppm - a staggeringly small change in brightness, yet one detected by TESS in just a single sector of data. This implies a brightness temperature of 2800 +130 −160 K, which is around 1.5 σ greater than expected given the mean depth measured with Spitzer. This is not a formally significant difference, and may be accounted for by the known variability, or by an albedo of ~0.5. In any case, future TESS observations of this system will provide an exciting opportunity to further study this diminutive world's atmosphere.