Astrophysics

White dwarfs that accrete the debris of tidally disrupted asteroids provide the opportunity to measure the bulk composition of the building blocks, or fragments, of exoplanets. This technique has established a diversity in compositions comparable to what is observed in the solar system, suggesting that the formation of rocky planets is a generic process. Whereas the relative abundances of lithophile and siderophile elements within the planetary debris can be used to investigate whether exoplanets undergo differentiation, the composition studies carried out so far lack unambiguous tracers of planetary crusts. Here we report the detection of lithium in the atmospheres of four cool (<5,000 K) and old (cooling ages 5-10 Gyr) metal-polluted white dwarfs, where one also displays photospheric potassium. The relative abundances of these two elements with respect to sodium and calcium strongly suggest that all four white dwarfs have accreted fragments of planetary crusts. We detect an infrared excess in one of the systems, indicating that accretion from a circumstellar debris disk is on-going. The main-sequence progenitor mass of this star was 4.8±0.2 M ??, demonstrating that rocky, differentiated planets may form around short-lived B-type stars.

While it is well established that giant-planet occurrence rises rapidly with host star metallicity, it is not yet clear if small-planet occurrence around late-type dwarf stars depends on host star metallicity. Using the Kepler Data Release 25 planet candidate list and its completeness data products, we explore planet occurrence as a function of metallicity in the Kepler field's late-type dwarf stellar population. We find that planet occurrence increases with metallicity for all planet radii R p down to at least R p ≈2 R ⊕ and that in the range 2 R ⊕ ≲ R p ≲5 R ⊕ planet occurrence scales linearly with metallicity Z . Extrapolating our results, we predict that short-period planets with R p ≲2 R ⊕ should be rare around early M dwarf stars with [M/H]≲−0.5 or late M dwarf stars with [M/H]≲+0.0 . This dependence of planet occurrence on metallicity observed in the Kepler field emphasizes the need to control for metallicity in estimates of planet occurrence for late-type dwarf stars like those targeted by Kepler's K2 extension and the Transiting Exoplanet Survey Satellite (TESS). We confirm the theoretical expectation that the small planet occurrence--host star metallicity relation is stronger for low-mass stars than for solar-type stars. We establish that the expected solid mass in planets around late-type dwarfs in the Kepler field is comparable to the total amount of planet-making solids in their protoplanetary disks. We argue that this high efficiency of planet formation favors planetesimal accretion over pebble accretion as the origin of the small planets observed by Kepler around late-type dwarf stars.

It has been recently claimed that KOIs-268.01, 303.01, 1888.01, 1925.01, 2728.01 & 3320.01 are exomoon candidates, based on an analysis of their transit timing. Here, we perform an independent investigation, which is framed in terms of three questions: 1) Are there significant excess TTVs? 2) Is there a significant periodic TTV? 3) Is there evidence for a non-zero moon mass? We applied rigorous statistical methods to these questions alongside a re-analysis of the Kepler photometry and find that none of the KOIs satisfy these three tests. Specifically, KOIs-268.01 & 3220.01 pass none of the tests and KOIs-303.01, 1888.01 & 1925.01 pass a single test each. Only KOI-2728.01 satisfies two, but fails the cross-validation test for predictions. Further, detailed photodynamical modeling reveals that KOI-2728.01 favours a negative radius moon (as does KOI-268.01). We also note that we find a significant photoeccentric for KOI-1925.01 indicating an eccentric orbit of e>(0.62+/-0.06). For comparison, we applied the same tests to Kepler-1625b, which reveals that 1) and 3) are passed, but 2) cannot be checked with the cross-validation method used here, due to the limited number of available epochs. In conclusion, we find no compelling evidence for exomoons amongst the six KOIs. Despite this, we're able to derive exomoon mass upper limits versus semi-major axis, with KOI-3220.01 leading to particularly impressive constraints of Ms/Mp < 0.4% [2 sigma] at a similar relative semi-major to that of the Earth-Moon.

About one out of 200 Sun-like stars has a planet with an orbital period shorter than one day: an ultra-short-period planet (Sanchis-ojeda et al. 2014; Winn et al. 2018). All of the previously known ultra-short-period planets are either hot Jupiters, with sizes above 10 Earth radii (Re), or apparently rocky planets smaller than 2 Re. Such lack of planets of intermediate size (the "hot Neptune desert") has been interpreted as the inability of low-mass planets to retain any hydrogen/helium (H/He) envelope in the face of strong stellar irradiation. Here, we report the discovery of an ultra-short-period planet with a radius of 4.6 Re and a mass of 29 Me, firmly in the hot Neptune desert. Data from the Transiting Exoplanet Survey Satellite (Ricker et al. 2015) revealed transits of the bright Sun-like star \starname\, every 0.79 days. The planet's mean density is similar to that of Neptune, and according to thermal evolution models, it has a H/He-rich envelope constituting 9.0^(+2.7)_(-2.9)% of the total mass. With an equilibrium temperature around 2000 K, it is unclear how this "ultra-hot Neptune" managed to retain such an envelope. Follow-up observations of the planet's atmosphere to better understand its origin and physical nature will be facilitated by the star's brightness (Vmag=9.8).

Astrometric observations of the M9 dwarf TVLM 513 − 46546 taken with the VLBA reveal an astrometric signature consistent with a period of 221 ± 5 days. The orbital fit implies that the companion has a mass m p = 0.35 − 0.42 M J , a circular orbit ( e≃0 ), a semi-major axis a = 0.28 − 0.31 AU and an inclination angle i = 71 − 88 ∘ . The detected companion, TVLM~513 b , is one of the few giant-mass planets found associated to UCDs. The presence of a Saturn-like planet on a circular orbit, 0.3 AU from a 0.06 − 0.08 M ⊙ star, represents a challenge to planet formation theory. This is the first astrometric detection of a planet at radio wavelengths.

A system of four super-Jupiter planets around HR 8799 is the first multi-planet configuration discovered via the direct imaging technique. Despite over decade of research, the system's architecture remains not fully resolved. The main difficulty comes from still narrow observing window of ~20 years that covers small arcs of orbits with periods from roughly 50 to 500 years. Soon after the discovery it became clear that unconstrained best-fitting astrometric configurations self-disrupt rapidly, due to strong mutual gravitational interactions between ~10-Jupiter-mass companions. Recently, we showed that the HR 8799 system may be long term stable when locked in a generalized Laplace 8:4:2:1 mean motion resonance (MMR) chain, and we constrained its orbits through the planetary migration. Here we qualitatively improve this approach by considering the MMR in terms of an exactly periodic configuration. This assumption enables us to construct for the first time the self-consistent N -body model of the long-term stable orbital architecture, using only available astrometric positions of the planets relative to the star. We independently determine planetary masses, which are consistent with thermodynamic evolution, and the parallax overlapping to 1σ with the most recent GAIA DR2 value. We also determine the global structure of the inner and outer debris discs in the [8, 600] au range, consistent with the updated orbital solution.

Based on HARPS-N radial velocities (RVs) and TESS photometry, we present a full characterisation of the planetary system orbiting the late G dwarf TOI-561. After the identification of three transiting candidates by TESS, we discovered two additional external planets from RV analysis. RVs cannot confirm the outer TESS transiting candidate, which would also make the system dynamically unstable. We demonstrate that the two transits initially associated with this candidate are instead due to single transits of the two planets discovered using RVs. The four planets orbiting TOI-561 include an ultra-short period (USP) super-Earth (TOI-561 b) with period P b =0.45 d, mass M b =1.59±0.36 M ⊕ and radius R b =1.42±0.07 R ⊕ , and three mini-Neptunes: TOI-561 c, with P c =10.78 d, M c =5.40±0.98 M ⊕ , R c =2.88±0.09 R ⊕ ; TOI-561 d, with P d =25.6 d, M d =11.9±1.3 M ⊕ , R d =2.53±0.13 R ⊕ ; and TOI-561 e, with P e =77.2 d, M e =16.0±2.3 M ⊕ , R e =2.67±0.11 R ⊕ . Having a density of 3.0±0.8 g cm −3 , TOI-561 b is the lowest density USP planet known to date. Our N-body simulations confirm the stability of the system and predict a strong, anti-correlated, long-term transit time variation signal between planets d and e. The unusual density of the inner super-Earth and the dynamical interactions between the outer planets make TOI-561 an interesting follow-up target.

The spectrum of dicarbon (C2) is important in astrophysics and for spectroscopic studies of plasmas and flames. The C2 spectrum is characterized by many band systems with new ones still being actively identified; astronomical observations involve eight of these bands. Recently, Furtenbacher et al. (2016, Astrophys. J. Suppl., 224, 44) presented a set of 5699 empirical energy levels for 12C2, distributed among 11 electronic states and 98 vibronic bands, derived from 42 experimental studies and obtained using the MARVEL (Measured Active Rotational-Vibrational Energy Levels) procedure. Here, we add data from 13 new sources and update data from 5 sources. Many of these data sources characterize high-lying electronic states, including the newly detected 3Pig state. Older studies have been included following improvements in the MARVEL procedure which allow their uncertainties to be estimated. These older works in particular determine levels in the C1Pig state, the upper state of the insufficiently characterized Deslandres-d'Azambuja (C1Pig-A1Piu) band. The new compilation considers a total of 31323 transitions and derives 7047 empirical(MARVEL) energy levels spanning 20 electronic and 142 vibronic states. These new empirical energy levels are used here to update the 8states C2 ExoMol line list. This updated line list is highly suitable for high-resolution cross-correlation studies in astronomical spectroscopy of, for example, exoplanets, as 99.4% of the transitions with intensities over 10^(-18) cm/molecule at 1000 K have frequencies determined by empirical energy levels.

The ESA's JUICE mission will provide a thorough investigation of the Jupiter system and the Galilean moons during its nominal tour, comprising flybys of Europa and Callisto, and an orbital phase about Ganymede at the end of the mission. The 3GM experiment will exploit accurate Doppler and range measurements to determine the moons' orbits and gravity fields (both static and tidal) and infer their interior structure. This paper presents the attainable accuracies of the Callisto geodesy experiment and addresses the effect of different flybys mean anomaly distribution and geometry on the estimation of the tidal Love number k2.

The Venus Express Radio Science Experiment was part of the scientific payload of the Venus Express spacecraft and was targeted at the investigation of Venus' atmosphere, surface, and gravity field as well as the interplanetary medium. This paper describes the methods and the required calibrations applied to VEX-VeRa raw radio occultation selected data used to retrieve vertical profiles of Venus' ionosphere and neutral atmosphere. In this work we perform an independent analysis of a set of 25 VEX, single-frequency (X-band), occultations carried out in 2014, recorded in open-loop at the NASA Deep Space Network. The calibrations are performed to correct the observed frequency for the major noise sources and errors, since any uncalibrated effects will bias the retrieval of atmospheric properties. We present a study on the influence of the relativistic effects on radio occultation of Venus. The temperature differences between the relativistic and non-relativistic Doppler shift solutions are lower than 0.5 K at 50 km altitude, and our error analysis shows that, within this investigation, the relativity effects can safely be neglected. Our temperature, pressure and electron density vertical profiles are in agreement with previous studies available in the literature. Furthermore, our analysis shows that Venus' ionosphere is more influenced by the day/night condition than the latitude variations, while the neutral atmosphere experiences the opposite. Our scientific interpretation of these results is based on two major responsible effects: Venus' high thermal inertia and the zonal winds. Their presence within Venus' neutral atmosphere determine why in these regions a latitude dependence is predominant on the day/night condition. On the contrary, at higher altitudes the two aforementioned effects are less important or null [...]