2,815 research outputs found
The formation of giant planets in wide orbits by photoevaporation-synchronised migration
The discovery of giant planets in wide orbits represents a major challenge
for planet formation theory. In the standard core accretion paradigm planets
are expected to form at radial distances au in order to form
massive cores (with masses ) able to trigger
the gaseous runaway growth before the dissipation of the disc. This has
encouraged authors to find modifications of the standard scenario as well as
alternative theories like the formation of planets by gravitational
instabilities in the disc to explain the existence of giant planets in wide
orbits. However, there is not yet consensus on how these systems are formed.
In this letter, we present a new natural mechanism for the formation of giant
planets in wide orbits within the core accretion paradigm. If photoevaporation
is considered, after a few Myr of viscous evolution a gap in the gaseous disc
is opened. We found that, under particular circumstances planet migration
becomes synchronised with the evolution of the gap, which results in an
efficient outward planet migration. This mechanism is found to allow the
formation of giant planets with masses in wide
stable orbits as large as 130 au from the central star.Comment: Accepted for publication in MNRAS Letters. Comments are welcom
Terrestrial-type planet formation: Comparing different types of initial conditions
To study the terrestrial-type planet formation during the post oligarchic
growth, the initial distributions of planetary embryos and planetesimals used
in N-body simulations play an important role. Most of these studies typically
use ad hoc initial distributions based on theoretical and numerical studies. We
analyze the formation of planetary systems without gas giants around solar-type
stars focusing on the sensitivity of the results to the particular initial
distributions of planetesimals and embryos. The formation of terrestrial
planets in the habitable zone (HZ) and their final water contents are topics of
interest. We developed two different sets of N-body simulations from the same
protoplanetary disk. The first set assumes ad hoc initial distributions for
embryos and planetesimals and the second set obtains these distributions from
the results of a semi-analytical model which simulates the evolution of the
gaseous phase of the disk. Both sets form planets in the HZ. Ad hoc initial
conditions form planets in the HZ with masses from to
. More realistic initial conditions obtained from a
semi-analytical model, form planets with masses between and
. Both sets form planets in the HZ with water contents between
4.5% and 39.48% by mass. Those planets with the highest water contents respect
to those with the lowest, present differences regarding the sources of water
supply. We suggest that the number of planets in the HZ is not sensitive to the
particular initial distribution of embryos and planetesimals and thus, the
results are globally similar between both sets. However, the main differences
are associated to the accretion history of the planets in the HZ. These
discrepancies have a direct impact in the accretion of water-rich material and
in the physical characteristics of the resulting planets.Comment: Accepted for publication in Astronomy and Astrophysics, 13 pages, 9
figure
Chemical composition of Earth-like planets
Models of planet formation are mainly focused on the accretion and dynamical
processes of the planets, neglecting their chemical composition. In this work,
we calculate the condensation sequence of the different chemical elements for a
low-mass protoplanetary disk around a solar-type star. We incorporate this
sequence of chemical elements (refractory and volatile elements) in our
semi-analytical model of planet formation which calculates the formation of a
planetary system during its gaseous phase. The results of the semi-analytical
model (final distributions of embryos and planetesimals) are used as initial
conditions to develope N-body simulations that compute the post-oligarchic
formation of terrestrial-type planets. The results of our simulations show that
the chemical composition of the planets that remain in the habitable zone has
similar characteristics to the chemical composition of the Earth. However,
exist differences that can be associated to the dynamical environment in which
they were formed.Comment: 3 pages, 4 figures - Accepted for publication in the Bolet\'in de la
Asociaci\'on Argentina de Astronom\'ia, vol.5
Opportunities from low-resolution modelling of river morphology in remote parts of the world
Abstract. River morphodynamics are the result of a variety of processes, ranging from the typical small-scale of fluid mechanics (e.g. flow turbulence dissipation) to the large-scale of landscape evolution (e.g. fan deposition). However, problems inherent in the long-term modelling of large rivers derive from limited computational resources and the high level of process detail (i.e. spatial and temporal resolution). These modelling results depend on processes parameterization and calibrations based on detailed field data (e.g. initial morphology). Thus, for these cases, simplified tools are attractive. In this paper, a simplified 1-D approach is presented that is suited for modelling very large rivers. A synthetic description of the variations of cross-sections shapes is implemented on the basis of satellite images, typically also available for remote parts of the world. The model's flexibility is highlighted here by presenting two applications. In the first case, the model is used for analysing the long-term evolution of the lower Zambezi River (Africa) as it relates to the construction of two reservoirs for hydropower exploitation. In the second case, the same model is applied to study the evolution of the middle and lower Paraná River (Argentina), particularly in the context of climate variability. In both cases, having only basic data for boundary and initial conditions, the 1-D model provides results that are in agreement with past studies and therefore shows potential to be used to assist sediment management at the watershed scale or at boundaries of more detailed models
Effect of mixtures of pesticides used in the direct seeding technique on nontarget plant seeds
The objective of the present study was to assess the effects on germination and root elongation of seeds exposed to the Roundup Max formulation of the glyphosate herbicide and the formulations Shooter and Sherpa of tiie insecticides chlorpyrifos and cypermethrin, separately and all of then- possible mixtures in imbibition solutions. Effects of environmental samples from crop fields are also analyzed.Centro de Investigaciones del Medio Ambient
Rationale for Medium Cutoff Membranes in COVID-19 Patients Requiring Renal Replacement Therapy
The current pandemic of coronavirus disease 2019 (COVID-19) spotlighted the vulnerability of patients with chronic kidney disease stage 5 on maintenance hemodialysis (HD) to the viral infection. Social distancing is the most effective preventive measure to reduce the risk of infection. Nonetheless, the necessity to frequently reach the dialysis center and the inherent social gathering both impede social distancing and also self-quarantine for infected individuals. A baseline hyperinflammatory state driven by factors such as the retention of uremic toxins afflicts these patients. Concomitantly, a condition of relative immunosuppression is also attributed to similar factors. The use of high-flux (HF) dialyzers for HD is the standard of care. However, with HF membranes, the removal of large middle molecules is scant. Medium cutoff (MCO) dialyzers are a new class of membranes that allow substantial removal of large middle molecules with negligible albumin losses. Recent trials confirmed long-term safety and long-term sustained reduction in the concentration of large uremic toxins with MCO dialyzers. Herein, we discuss the rationale for applying MCO membranes in COVID-19 patients and its possible immunoadjuvant effects that could mitigate the burden of COVID-19 infection in dialysis patients. We also discuss the direct cytopathic effect of the virus on renal tissue and extracorporeal blood purification techniques that can prevent kidney damage or reduce acute kidney injury progression
Peritoneal Dialysis for Chronic Congestive Heart Failure
Maladaptive responses between a failing heart and the kidneys ultimately lead to permanent chronic kidney disease, referred to as cardiorenal syndrome type 2. In this narrative review, we discuss the pathophysiological pathways in the progression of cardiorenal failure and review the current evidence on peritoneal dialysis as a treatment strategy in cardiorenal syndrome type 2. A patient with heart failure can present with clinical symptoms related to venous congestion even in the absence of end-stage renal disease. Diuretics remain the cornerstone for the treatment of fluid overload related to heart failure. However, with chronic use, diuretic resistance can supervene. When medical therapy is no longer able to relieve congestive symptoms, ultrafiltration might be needed. Patients with heart failure tolerate well the gentle rate of fluid removal through peritoneal dialysis. Recent publications suggest a positive impact of starting peritoneal dialysis in patients with cardiorenal syndrome type 2 on the hospitalisation rate, functional status and quality of life
Comparative response of Lemnaceae clones to copper(II), chromium(VI), and cadmium(II) toxicity
Vascular plants such as aquatic macrophytes have been used as reference organisms in ecotoxicological assessments of environmental toxicants in aquatic systems for more than two decades (Lewis 1995; Wang and Freemark 1995; Lytle and Lytle 2001). Lemnaceae are the most extensively studied family (Wang 1990; Wang 1992; Mohan and Hosetti 1999) and were incorporated to standardized protocols by environmental protection agencies or organizations (USEPA 1996; Environment Canada 1999; OECD 2000). Lemna gibba and Lemna minor were the selected species among the Lemnaceae family for most of the standarized protocols. A limitation in the selection of these reference species is distribution; L. gibba is widely distributed in South America while L. minor is not found in the Neotropical region (Landolt 1986; Landolt 1996). The search for reference organisms to be used in ecotoxicological testing with bioassays in the Pampean Region of Buenos Aires Province (Argentina) comprises the use of fish, amphibians, crustaceans and algae from surface water bodies (Ronco et al. 2000a). Previous local reports using vascular plants aimed at assessing toxicity with seeds (Sobrero et al. 1996; Ronco et al. 2000b). The present study reports data on the comparative response of a local clone of L. gibba with two collection clones of L. gibba and L. minor to three environmentally relevant toxic metals using laboratory toxicity tests.Instituto de FisiologÃa Vegeta
Most super-Earths formed by dry pebble accretion are less massive than 5 Earth masses
We study the formation of rocky planets by dry pebble accretion from
self-consistent dust-growth models. In particular, we aim at computing the
maximum core mass of a rocky planet that can sustain a thin H-He atmosphere to
account for the second peak of the Kepler's size distribution. We simulate
planetary growth by pebble accretion inside the ice line. The pebble flux is
computed self-consistently from dust growth by solving the advection-diffusion
equation for a representative dust size. Dust coagulation, drift, fragmentation
and sublimation at the water iceline are included. The disc evolution is
computed for -discs with photoevaporation from the central star. The
planets grow from a moon-mass embryo by silicate pebble accretion and gas
accretion. We analyse the effect of a different initial disc mass,
-viscosity, disc metallicity and embryo location. Finally, we compute
atmospheric mass-loss due to evaporation. We find that inside the ice line, the
fragmentation barrier determines the size of pebbles, which leads to different
planetary growth patterns for different disc viscosities. Within the iceline
the pebble isolation mass typically decays to values below 5 M
within the first million years of disc evolution, limiting the core masses to
that value. After computing atmospheric-mass loss, we find that planets with
cores below 4 M get their atmospheres completely stripped, and
a few 4-5 M cores retain a thin atmosphere that places them in the
gap/second peak of the Kepler size distribution. Overall, we find that rocky
planets form only in low-viscosity discs (). When
, rocky objects do not grow beyond Mars-mass. The most
typical outcome of dry pebble accretion is terrestrial planets with masses
spanning from Mars to 4 M.Comment: Accepted for publication in A&
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