289 research outputs found
Lunar accretion from a Roche-interior fluid disk
We use a hybrid numerical approach to simulate the formation of the Moon from
an impact-generated disk, consisting of a fluid model for the disk inside the
Roche limit and an N-body code to describe accretion outside the Roche limit.
As the inner disk spreads due to a thermally regulated viscosity, material is
delivered across the Roche limit and accretes into moonlets that are added to
the N-body simulation. Contrary to an accretion timescale of a few months
obtained with prior pure N-body codes, here the final stage of the Moon's
growth is controlled by the slow spreading of the inner disk, resulting in a
total lunar accretion timescale of ~10^2 years. It has been proposed that the
inner disk may compositionally equilibrate with the Earth through diffusive
mixing, which offers a potential explanation for the identical oxygen isotope
compositions of the Earth and Moon. However, the mass fraction of the final
Moon that is derived from the inner disk is limited by resonant torques between
the disk and exterior growing moons. For initial disks containing < 2.5 lunar
masses (ML), we find that a final Moon with mass > 0.8ML contains < 60%
material derived from the inner disk, with this material preferentially
delivered to the Moon at the end of its accretion.Comment: 42 pages, 10 figures, 5 tables. Accepted for publication in The
Astrophysical Journa
Long-term & large-scale viscous evolution of dense planetary rings
We investigate the long-term and large-scale viscous evolution of dense
planetary rings using a simple 1D numerical code. We use a physically realistic
viscosity model derived from N-body simulations (Daisaka et al., 2001), and
dependent on the disk's local properties (surface mass density, particle size,
distance to the planet). Particularly, we include the effects of gravitational
instabilities (wakes) that importantly enhance the disk's viscosity. We show
that common estimates of the disk's spreading time-scales with constant
viscosity significantly underestimate the rings' lifetime. With a realistic
viscosity model, an initially narrow ring undergoes two successive evolutionary
stages: (1) a transient rapid spreading when the disk is self-gravitating, with
the formation of a density peak inward and an outer region marginally
gravitationally stable, and with an emptying time-scale proportional to 1/M_0^2
(where M_0 is the disk's initial mass) (2) an asymptotic regime where the
spreading rate continuously slows down as larger parts of the disk become
not-self-gravitating due to the decrease of the surface density, until the disk
becomes completely not-self-gravitating. At this point its evolution
dramatically slows down, with an emptying time-scale proportional to 1/M_0,
which significantly increases the disk's lifetime compared to the case with
constant viscosity. We show also that the disk's width scales like t^{1/4} with
the realistic viscosity model, while it scales like t^{1/2} in the case of
constant viscosity, resulting in much larger evolutionary time-scales in our
model. We find however that the present shape of Saturn's rings looks like a
100 million-years old disk in our simulations. Concerning Jupiter's, Uranus'
and Neptune's rings that are faint today, it is not likely that they were much
more massive in the past and lost most of their mass due to viscous spreading
alone.Comment: 18 pages, 18 figures, 2 tables. Accepted for publication in Icaru
Universal electric-field-driven resistive transition in narrow-gap Mott insulators
One of today's most exciting research frontier and challenge in condensed
matter physics is known as Mottronics, whose goal is to incorporate strong
correlation effects into the realm of electronics. In fact, taming the Mott
insulator-to-metal transition (IMT), which is driven by strong electronic
correlation effects, holds the promise of a commutation speed set by a quantum
transition, and with negligible power dissipation. In this context, one
possible route to control the Mott transition is to electrostatically dope the
systems using strong dielectrics, in FET-like devices. Another possibility is
through resistive switching, that is, to induce the insulator-to-metal
transition by strong electric pulsing. This action brings the correlated system
far from equilibrium, rendering the exact treatment of the problem a difficult
challenge. Here, we show that existing theoretical predictions of the
off-equilibrium manybody problem err by orders of magnitudes, when compared to
experiments that we performed on three prototypical narrow gap Mott systems
V2-xCrxO3, NiS2-xSex and GaTa4Se8, and which also demonstrate a striking
universality of this Mott resistive transition (MRT). We then introduce and
numerically study a model based on key theoretically known physical features of
the Mott phenomenon in the Hubbard model. We find that our model predictions
are in very good agreement with the observed universal MRT and with a
non-trivial timedelay electric pulsing experiment, which we also report. Our
study demonstrates that the MRT can be associated to a dynamically directed
avalanche
Chemistry of the Protolunar Disk and Volatile Depletion of the Moon
In the giant impact theory for lunar origin, the Moon forms from a circumterrestrial disk of silicate debris produced by the collision of a planetary-sized impactor with the early Earth. Recent accretion models suggest that the final 10-60% of the lunar mass may be provided by the accretion of melt material spreading outward from the inner (Roche-interior) disk over the first ~102 years following the giant impact. The chemical and thermal evolution of the inner disk material is thus expected to strongly influence the bulk chemical composition of the Moon.
In a previous study we explored the chemistry of the melt+vapor protolunar disk in order to examine the vapor pressure of the silicate magma and the chemistry of the protolunar disk atmosphere. Here we combine a chemical model for the disk with lunar accretion simulations and a thermal evolution model in order to explore the chemistry of the accreting lunar material and implications for the bulk lunar composition. A chemical equilibrium code is used to determine the partial pressure of each species in equilibrium with a BSE-composition melt. These vapor pressure results, along with the bulk elemental inventory of the disk, are used to estimate the relative fraction of each element in the melt vs. vapor phase as a function of the mass surface density and temperature of the disk.
The coupled chemistry + lunar accretion + thermal model suggests that the temperature of the melt in the inner disk remains above estimated 50% condensation temperatures for the volatile elements Zn, Na, and K until the Moon has nearly completed its accretion. We thus expect the portion of the lunar material derived from the inner disk to be depleted in these and similarly volatile elements, even in the absence of thermal escape
Protolunar Disk Evolution and the Depletion of Volatile Elements in the Moon
We explore how the evolution of the protolunar disk could lead to a depletion in K, Na, and Zn in the Moon relative to Earth even in the absence of escape
Migration of a moonlet in a ring of solid particles : Theory and application to Saturn's propellers
Hundred meter sized objects have been identified by the Cassini spacecraft in
Saturn's A ring through the so-called "propeller" features they create in the
ring. These moonlets should migrate, due to their gravitational interaction
with the ring ; in fact, some orbital variation have been detected. The
standard theory of type I migration of planets in protoplanetary disks can't be
applied to the ring system, as it is pressureless. Thus, we compute the
differential torque felt by a moonlet embedded in a two-dimensional disk of
solid particles, with flat surface density profile, both analytically and
numerically. We find that the corresponding migration rate is too small to
explain the observed variations of the propeller's orbit in Saturn's A-ring.
However, local density fluctuations (due to gravity wakes in the marginally
gravitationally stable A-ring) may exert a stochastic torque on a moonlet. Our
simulations show that this torque can be large enough to account for the
observations, depending on the parameters of the rings. We find that on time
scales of several years the migration of propellers is likely to be dominated
by stochastic effects (while the former, non-stochastic migration dominates
after ~ 10^{4-5} years). In that case, the migration rates provided by
observations so far suggests that the surface density of the A ring should be
of the order of 700 kg/m^2. The age of the propellers shouldn't exceed 1 to 100
million years, depending on the dominant migration regime.Comment: 17 pages, 5 figures, submitted to Astronomical Journal on february,
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Engaging HIV-HCV co-infected patients in HCV treatment: the roles played by the prescribing physician and patients' beliefs (ANRS CO13 HEPAVIH cohort, France)
<p>Abstract</p> <p>Background</p> <p>Treatment for the hepatitis C virus (HCV) may be delayed significantly in HIV/HCV co-infected patients. Our study aims at identifying the correlates of access to HCV treatment in this population.</p> <p>Methods</p> <p>We used 3-year follow-up data from the HEPAVIH ANRS-CO13 nationwide French cohort which enrolled patients living with HIV and HCV. We included pegylated interferon and ribavirin-naive patients (N = 600) at enrolment. Clinical/biological data were retrieved from medical records. Self-administered questionnaires were used for both physicians and their patients to collect data about experience and behaviors, respectively.</p> <p>Results</p> <p>Median [IQR] follow-up was 12[12-24] months and 124 patients (20.7%) had started HCV treatment. After multiple adjustment including patients' negative beliefs about HCV treatment, those followed up by a general practitioner working in a hospital setting were more likely to receive HCV treatment (OR[95%CI]: 1.71 [1.06-2.75]). Patients followed by general practitioners also reported significantly higher levels of alcohol use, severe depressive symptoms and poor social conditions than those followed up by other physicians.</p> <p>Conclusions</p> <p>Hospital-general practitioner networks can play a crucial role in engaging patients who are the most vulnerable and in reducing existing inequities in access to HCV care. Further operational research is needed to assess to what extent these models can be implemented in other settings and for patients who bear the burden of multiple co-morbidities.</p
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