Context.
Dynamical models of Solar System evolution have suggested that the so-called P- and D-type volatile-rich asteroids formed in the outer Solar System beyond Neptune’s orbit and may be ...genetically related to the Jupiter Trojans, comets, and small Kuiper belt objects (KBOs). Indeed, the spectral properties of P- and D-type asteroids resemble that of anhydrous cometary dust.
Aims.
We aim to gain insights into the above classes of bodies by characterizing the internal structure of a large P- and D-type asteroid.
Methods.
We report high-angular-resolution imaging observations of the P-type asteroid (87) Sylvia with the Very Large Telescope Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument. These images were used to reconstruct the 3D shape of Sylvia. Our images together with those obtained in the past with large ground-based telescopes were used to study the dynamics of its two satellites. We also modeled Sylvia’s thermal evolution.
Results.
The shape of Sylvia appears flattened and elongated (a/b ~1.45; a/c ~1.84). We derive a volume-equivalent diameter of 271 ± 5 km and a low density of 1378 ± 45 kg m
−3
. The two satellites orbit Sylvia on circular, equatorial orbits. The oblateness of Sylvia should imply a detectable nodal precession which contrasts with the fully-Keplerian dynamics of its two satellites. This reveals an inhomogeneous internal structure, suggesting that Sylvia is differentiated.
Conclusions.
Sylvia’s low density and differentiated interior can be explained by partial melting and mass redistribution through water percolation. The outer shell should be composed of material similar to interplanetary dust particles (IDPs) and the core should be similar to aqueously altered IDPs or carbonaceous chondrite meteorites such as the Tagish Lake meteorite. Numerical simulations of the thermal evolution of Sylvia show that for a body of such a size, partial melting was unavoidable due to the decay of long-lived radionuclides. In addition, we show that bodies as small as 130–150 km in diameter should have followed a similar thermal evolution, while smaller objects, such as comets and the KBO Arrokoth, must have remained pristine, which is in agreement with in situ observations of these bodies. NASA Lucy mission target (617) Patroclus (diameter ≈140 km) may, however, be differentiated.
Context. In order to improve our understanding of the kinetics of the cometary coma, theoretical studies of the major reactive collisions in these environments are needed. Deep in the collisional ...coma, inelastic collisions between thermal electrons and molecular ions result in recombination and vibrational excitation, the rates of these processes being particularly elevated due to the high charged particle densities in the inner region. Aims. This work addresses the dissociative recombination, vibrational excitation, and vibrational de-excitation of electrons with CO+ molecular cations. The aim of this study is to understand the importance of these reactive collisions in producing carbon and oxygen atoms in cometary activity. Methods. The cross-section calculations were based on multichannel quantum defect theory. The molecular data sets, used here to take into account the nuclear dynamics, were based on ab initio R-matrix approach. Results. The cross-sections for the dissociative recombination, vibrational excitation, and vibrational de-excitation processes, for the six lowest vibrational levels of CO+ – relevant for the electronic temperatures observed in comets – are computed, as well as their corresponding Maxwell rate coefficients. Moreover, final state distributions for different dissociation pathways are presented. Conclusions. Among all reactive collisions taking place between low-energy electrons and CO+, the dissociative recombination is the most important process at electronic temperatures characterizing the comets. We have shown that this process can be a major source of O(3P), O(1D), O(1S), C(3P) and C(1D) produced in the cometary coma at small cometocentric distances.
ABSTRACT
We present the discovery of three transiting planets from the WASP survey, two hot-Jupiters: WASP-177 b (∼0.5 MJup, ∼1.6 RJup) in a 3.07-d orbit of a V = 12.6 K2 star, WASP-183 b (∼0.5 MJup, ...∼1.5 RJup) in a 4.11-d orbit of a V = 12.8 G9/K0 star; and one hot-Saturn planet WASP-181 b (∼0.3 MJup, ∼1.2 RJup) in a 4.52-d orbit of a V = 12.9 G2 star. Each planet is close to the upper bound of mass–radius space and has a scaled semimajor axis, a/R*, between 9.6 and 12.1. These lie in the transition between systems that tend to be in orbits that are well aligned with their host-star’s spin and those that show a higher dispersion.
Aims.
Asteroid (31) Euphrosyne is one of the biggest objects in the asteroid main belt and it is also the largest member of its namesake family. The Euphrosyne family occupies a highly inclined ...region in the outer main belt and contains a remarkably large number of members, which is interpreted as an outcome of a disruptive cratering event.
Methods.
The goals of this adaptive-optics imaging study are threefold: to characterize the shape of Euphrosyne, to constrain its density, and to search for the large craters that may be associated with the family formation event.
Results.
We obtained disk-resolved images of Euphrosyne using SPHERE/ZIMPOL at the ESO 8.2 m VLT as part of our large program (ID: 199.C-0074, PI: Vernazza). We reconstructed its 3D shape via the
ADAM
shape modeling algorithm based on the SPHERE images and the available light curves of this asteroid. We analyzed the dynamics of the satellite with the
Genoid
meta-heuristic algorithm. Finally, we studied the shape of Euphrosyne using hydrostatic equilibrium models.
Conclusions.
Our SPHERE observations show that Euphrosyne has a nearly spherical shape with the sphericity index of 0.9888 and its surface lacks large impact craters. Euphrosyne’s diameter is 268 ± 6 km, making it one of the top ten largest main belt asteroids. We detected a satellite of Euphrosyne – S/2019 (31) 1 – that is about 4 km across, on a circular orbit. The mass determined from the orbit of the satellite together with the volume computed from the shape model imply a density of 1665 ± 242 kg m
−3
, suggesting that Euphrosyne probably contains a large fraction of water ice in its interior. We find that the spherical shape of Euphrosyne is a result of the reaccumulation process following the impact, as in the case of (10) Hygiea. However, our shape analysis reveals that, contrary to Hygiea, the axis ratios of Euphrosyne significantly differ from those suggested by fluid hydrostatic equilibrium following reaccumulation.
In order to investigate the time variations and the peculiarities of the optical spectra of the RV Tauri star R Sct, high-resolution spectroscopic observations in the wavelength interval around the
...line (6540–6580 Å) were obtained between 2014 and 2018. The phase dependence of the equivalent widths and radial velocities was measured for the
and Fe I
5669.224 Å lines, respectively.
shows considerable changes in its line profile. We observed double-peaked emission and an inverse P Cygni profile in this line. We detected a variation from inverse P Cygni to P Cygni profile for both Ti I
6554.239 Å and Ca I
6572.797 Å lines at phases where the shock wave is propagating through the atmospheric layer in which the H
α
lines are forming, particularly around
which is just after the deep minimum and
, just after shallow minimum. An increase in the shock wave velocity generates a decrease in the
intensity, which is explained by the ionization phenomenon. The detailed description of the variations in R Sct reveals its important properties. The observation of the P Cygni and the inverse P Cygni profile indicate that expanding and infalling layers are simultaneously present in the stellar atmosphere, pointing out the complicated nature of this object.
Context. Until 2006, helium emission lines had never been observed in RR Lyrae stars. For the first time, a pre-maximum helium emission in 11 RRab stars was observed during rising light (around the ...pulsation phase 0.92) and the reappearance of helium emission near maximum light (phase 0.0) in one RRab star: RV Oct. This post-maximum emission has been only observed in the He I λ5875.66 (D3) line. Its intensity is very weak, and its profile mimics a P-Cygni profile with the emission peak centered at the laboratory wavelength. The physical explanation for this unexpected line profile has not been proposed yet. Aims. Using new observations of RR Lyr, we investigate the physical origin of the presence of a P-Cygni profile in the He I λ5875.66 (D3) line. Methods. High-resolution spectra of RR Lyr, collected with a spectrograph eShel/C14 at the Oukaïmeden Observatory (Morocco) in 2013, were analyzed to understand the origin of the observed P-Cygni profile at D3. Results. When the shock intensity is moderate, helium emission cannot be produced in the shock wake, and consequently, the two consecutive helium emissions (pre- and post-maximum light emissions) are not observed. This is the most frequent case. When the shock intensity becomes high enough, a pre-maximum He I emission first occurs, which can be followed by the appearance of a P-Cygni profile if the shock intensity is still strong in the high atmosphere. The observation of a P-Cygni profile means that the shock wave is already detached from the photosphere. It is shown that the shock strongly first decelerates between the pulsation phases 0.90 and 1.04 from 130 km s-1 to 60 km s-1, probably before accelerating again to 80 km s-1 near phase 1.30. Conclusions. The presence of the P-Cygni profile seems to be a natural consequence of the large extension of the expanding atmosphere, which is induced by strong (radiative) shock waves propagating toward the high atmosphere. This kind of P-Cygni profile has already been observed in the Hα line of some RR Lyrae stars and long-period Cepheids.
In this paper, we have used the National Centers for Environmental Prediction (NCEP)/National Centers for Atmospheric Research (NCAR) Reanalysis data base to study, first, a comparison between ...balloon sounding made at different stations with coinciding model-based meteorological analysis. The comparison allows the assessment of reliability of the analysis in the studied period and to highlight NCEP/NCAR Reanalysis as an interesting data base for site characterization.
Using the same system of Reanalysis, we present, secondly, the first complete characterization of main meteorological parameters at Oukaïmeden Observatory: wind speed, its direction, temperature and pressure. The statistical treatment of data will cover the years between 1990 and 2009. Monthly, seasonal and annual results are analysed.
The comprehensive and reliable statistics of tropospheric wind speeds at Oukaïmeden are presented. We found a clear annual periodicity of 200 mbar wind speed. This periodicity could be related to the seasonal dependence of seeing that is affected by the existence of cloud sea during the period around autumn-winter and by high wind speed regimes during spring. The connection of high- to low-altitude tropospheric winds has been explored. We found a high correlation comparable to the ones found at La Silla and La Palma sites. The local parameters in particular topography and stratocumulus formations might affect 700 mbar wind roses.
Richardson numbers
calculated for each month at Oukaïmeden and La Palma are presented. By analysing the
values, we found out that the periods and the regions of development of turbulence in relative terms of stability for the two locations are very similar. In addition, we present the first example of a
profile estimated from NCEP/NCAR Reanalysis. We found that this profile presents a tendency very similar to the same averaged profile measured by the balloon-born radiosondes.
► We present a compilation of lunar flashes detections for the period 1999–2007. ► Lunar flashes are characterized by temperatures ranging from 2000K to 3000K. ► The analysis reveals a correlation ...between lunar flashes duration and magnitude. ► Geminid and Sporadic impacts may be distinguished from Leonid meteors. ► Lunar flashes radiations are dominated by a cloud of silicate melt droplets.
Meteoroid falls on the Moon produce transient luminous events usually named impact flashes. These emissions have been reported by several independent observers using ground-based telescopes over the last decade. We present here a compendium of these observations for the period 1999–2007, including apparent flash magnitudes, durations and the origin of the bolide (meteor shower or sporadic impact). Impact flashes appear on 1–10 camera frames corresponding to durations ranging from ∼10ms to ∼1s. The analysis of these data reveals a correlation between duration and intensity, with the exception of Leonid meteors. The difference between Leonids and other meteoroids are likely explained by the higher velocity of this swarm. For the other events, the observed trend implies that impact flash detections are at present limited by the frame rate which is generally equal or less than 60 frames par second (f/s). The durations of these transient events are typically longer than predictions based on expanding plasma-gas clouds. We thus argue that these luminous events correspond to radiation emitted by a cloud composed of gas and small ejected melt droplets. A simple model considering the black body radiation of cooling droplets provides a time scale commensurable with the observations. In addition, such modeling is useful for optimizing the specifications of monitoring equipment. In particular, the inferred range of effective temperatures implies that near-infrared observations would efficiently increase the number of detections, whereas multi-spectral observations are essential to progress in the understanding of the nature of these luminous events.
Context.
Thanks to the
Gaia
mission, it will be possible to determine the masses of approximately hundreds of large main belt asteroids with very good precision. We currently have diameter estimates ...for all of them that can be used to compute their volume and hence their density. However, some of those diameters are still based on simple thermal models, which can occasionally lead to volume uncertainties as high as 20–30%.
Aims.
The aim of this paper is to determine the 3D shape models and compute the volumes for 13 main belt asteroids that were selected from those targets for which
Gaia
will provide the mass with an accuracy of better than 10%.
Methods.
We used the genetic Shaping Asteroids with Genetic Evolution (SAGE) algorithm to fit disk-integrated, dense photometric lightcurves and obtain detailed asteroid shape models. These models were scaled by fitting them to available stellar occultation and/or thermal infrared observations.
Results.
We determine the spin and shape models for 13 main belt asteroids using the SAGE algorithm. Occultation fitting enables us to confirm main shape features and the spin state, while thermophysical modeling leads to more precise diameters as well as estimates of thermal inertia values.
Conclusions.
We calculated the volume of our sample of main-belt asteroids for which the
Gaia
satellite will provide precise mass determinations. From our volumes, it will then be possible to more accurately compute the bulk density, which is a fundamental physical property needed to understand the formation and evolution processes of small Solar System bodies.
Using a numerical simulation, a new approach to determine the wave structure function, and therefore the astronomical seeing, is presented and discussed. This method is based on the study of the ...diffraction pattern produced by a double slit at the focus plane of a telescope. The phase screens are simulated using a fast Fourier transform (FFT) based method and Kolmogorov's law regarding atmospheric turbulence. From the scattered wave intensity, the wave structure function is calculated by taking into account both phase and amplitude fluctuations. This means that we can obtain a seeing value that is independent of the propagation distance between the turbulent layers and the ground level (Fresnel diffraction effect). Indeed, the seeing is related to the refractive-index structure constant (C
n
2) inside the turbulent layers and thus should be independent of the aforementioned propagation distance.
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