94 research outputs found
Orbital Evolution of Moons in Weakly Accreting Circumplanetary Disks
We investigate the formation of hot and massive circumplanetary disks (CPDs)
and the orbital evolution of satellites formed in these disks. Because of the
comparatively small size-scale of the sub-disk, quick magnetic diffusion
prevents the magnetorotational instability (MRI) from being well-developed at
ionization levels that would allow MRI in the parent protoplanetary disk. In
the absence of significant angular momentum transport, continuous mass supply
from the parental protoplanetary disk leads to the formation of a massive CPD.
We have developed an evolutionary model for this scenario and have estimated
the orbital evolution of satellites within the disk. We find, in a certain
temperature range, that inward migration of a satellite can be stopped by a
change in the structure due to the opacity transitions. Moreover, by capturing
second and third migrating satellites in mean motion resonances, a compact
system in Laplace resonance can be formed in our disk models.Comment: 10 pages, 9 figure
Cosmic-Ray Ionization Rate in Protoplanetary Disks with Sheared Magnetic Fields
We investigate the effects of magnetic-field configurations on the ionization rate by cosmic rays in protoplanetary disks. First, we consider cosmic-ray propagation from the interstellar medium (ISM) to the protoplanetary disks and showed that the cosmic-ray density around the disk should be 2 times lower than the ISM value. Then, we compute the attenuation of cosmic rays in protoplanetary disks. The magnetic fields in the disk are stretched to the azimuthal directions, and cosmic rays need to detour while propagating to the midplane. Our results show that the detouring effectively enhances the column density by about two orders of magnitude. We employ a typical ionization rate by cosmic rays in diffuse ISM, which is considered too high to be consistent with observations of protoplanetary disks, and find that the cosmic rays are significantly shielded at the midplane. In the case of the disk around IM Lup, the midplane ionization rate is very low for r ≲ 100 au, while the value is as large as a diffuse ISM in the outer radii. Our results are consistent with the recent Atacama Large Millimeter/submillimeter Array observation that indicates the radial gradient in the cosmic-ray ionization rate. The high ionization rate in the outer radii of disks may activate the magnetorotational instability that was thought to be suppressed due to ambipolar diffusion. These results will have a strong influence on the dynamical and chemical evolutions of protoplanetary disks
On the contribution of the horizontal sea-bed displacements into the tsunami generation process
The main reason for the generation of tsunamis is the deformation of the
bottom of the ocean caused by an underwater earthquake. Usually, only the
vertical bottom motion is taken into account while the horizontal co-seismic
displacements are neglected in the absence of landslides. In the present study
we propose a methodology based on the well-known Okada solution to reconstruct
in more details all components of the bottom coseismic displacements. Then, the
sea-bed motion is coupled with a three-dimensional weakly nonlinear water wave
solver which allows us to simulate a tsunami wave generation. We pay special
attention to the evolution of kinetic and potential energies of the resulting
wave while the contribution of the horizontal displacements into wave energy
balance is also quantified. Such contribution of horizontal displacements to
the tsunami generation has not been discussed before, and it is different from
the existing approaches. The methods proposed in this study are illustrated on
the July 17, 2006 Java tsunami and some more recent events.Comment: 30 pages; 14 figures. Accepted to Ocean Modelling. Other authors
papers can be downloaded at http://www.lama.univ-savoie.fr/~dutykh
<i>Spitzer</i> Microlensing Parallax Reveals Two Isolated Stars in the Galactic Bulge
We report the mass and distance measurements of two single-lens events from the 2017 microlensing campaign. The ground-based observations yield the detection of finite-source effects, and the microlens parallaxes are derived from the joint analysis of ground-based observations and observations. We find that the lens of OGLE-2017-BLG-1254 is a 0.60 ± 0.03 M ⊙ star with D LS = 0.53 ± 0.11 kpc, where D LS is the distance between the lens and the source. The second event, OGLE-2017-BLG-1161, is subject to the known satellite parallax degeneracy, and thus is either a star with D LS = 0.40 ± 0.12 kpc or a star with D LS = 0.53 ± 0.19 kpc. Both of the lenses are therefore isolated stars in the Galactic bulge. By comparing the mass and distance distributions of the eight published finite-source events with the expectations from a Galactic model, we find that the sample is in agreement with the probability of finite-source effects occurring in single-lens events
Spitzer Microlensing Parallax for OGLE-2017-BLG-0896 Reveals a Counter-rotating Low-mass Brown Dwarf
The kinematics of isolated brown dwarfs in the Galaxy, beyond the solar neighborhood, is virtually unknown. Microlensing has the potential to probe this hidden population, as it can measure both the mass and five of the six phase-space coordinates (all except the radial velocity) even of a dark isolated lens. However, the measurements of both the microlens-parallax and finite-source effects are needed in order to recover the full information. Here, we combine the Spitzer satellite parallax measurement with the ground-based light curve, which exhibits strong finite-source effects, of event OGLE-2017-BLG-0896. We find two degenerate solutions for the lens (due to the known satellite-parallax degeneracy), which are consistent with each other except for their proper motion. The lens is an isolated brown dwarf with a mass of either 18 ± 1 M J or 20 ± 1 M J . This is the lowest isolated-object mass measurement to date, only ~45% more massive than the theoretical deuterium-fusion boundary at solar metallicity, which is the common definition of a free-floating planet. The brown dwarf is located at either 3.9 ± 0.1 kpc or 4.1 ± 0.1 kpc toward the Galactic bulge, but with proper motion in the opposite direction of disk stars, with one solution suggesting it is moving within the Galactic plane. While it is possibly a halo brown dwarf, it might also represent a different, unknown population
A dehydrated space-weathered skin cloaking the hydrated interior of Ryugu
Without a protective atmosphere, space-exposed surfaces of airless Solar System bodies gradually experience an alteration in composition, structure and optical properties through a collective process called space weathering. The return of samples from near-Earth asteroid (162173) Ryugu by Hayabusa2 provides the first opportunity for laboratory study of space-weathering signatures on the most abundant type of inner solar system body: a C-type asteroid, composed of materials largely unchanged since the formation of the Solar System. Weathered Ryugu grains show areas of surface amorphization and partial melting of phyllosilicates, in which reduction from Fe3+ to Fe2+ and dehydration developed. Space weathering probably contributed to dehydration by dehydroxylation of Ryugu surface phyllosilicates that had already lost interlayer water molecules and to weakening of the 2.7 µm hydroxyl (–OH) band in reflectance spectra. For C-type asteroids in general, this indicates that a weak 2.7 µm band can signify space-weathering-induced surface dehydration, rather than bulk volatile loss
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