203 research outputs found
No self-shadowing instability in 2D radiation-hydrodynamical models of irradiated protoplanetary disks
Theoretical models of protoplanetary disks including stellar irradiation
often show a spontaneous amplification of scale height perturbations, produced
by the enhanced absorption of starlight in enlarged regions. In turn, such
regions cast shadows on adjacent zones that consequently cool down and shrink,
eventually leading to an alternating pattern of overheated and shadowed
regions. Previous investigations have proposed this to be a real self-sustained
process, the so-called self-shadowing or thermal wave instability, which could
naturally form frequently observed disk structures such as rings and gaps, and
even potentially enhance the formation of planetesimals. All of these, however,
have assumed in one way or another vertical hydrostatic equilibrium and
instantaneous radiative diffusion throughout the disk. In this work we present
the first study of the stability of accretion disks to self-shadowing that
relaxes these assumptions, relying instead on radiation-hydrodynamical
simulations. We first construct hydrostatic disk configurations by means of an
iterative procedure and show that the formation of a pattern of enlarged and
shadowed regions is a direct consequence of assuming instantaneous radiative
diffusion. We then let these solutions evolve in time, which leads to a fast
damping of the initial shadowing features in layers close to the disk surface.
These thermally relaxed layers grow towards the midplane until all temperature
extrema in the radial direction are erased in the entire disk. Our results
suggest that radiative cooling and gas advection at the disk surface prevent a
self-shadowing instability from forming, by damping temperature perturbations
before these reach lower, optically thick regions.Comment: 26 pages, 19 figures. Accepted for publication in ApJ. Complementary
videos can be found in https://youtu.be/RT8IFe8W13
Effect of diet, age and sex on the renal response to immune injury in the rat
Effect of diet, age and sex on the glomerular response to immune injury in the rat. We investigated the effect of three factors, namely dietary protein intake, age and sex, on the susceptibility of the renal glomerulus to the binding of antiglomerular basement membrane antibody (anti-GBM) in the early (heterologous) phase of anti-GBM nephritis, and the consequent reduction in glomerular filtration rate (GFR) as measured by inulin clearance (CIn). The effect of diet was examined in ≈ 8 week-old female Munich-Wistar rats fed a 40% high (HP) or a 6% low (LP) protein diet, and that of sex and age in male and female rats, 6 week or 10 month old. Following an intravenous dose (3 to 20 µg/g body wt) of radiolabeled nephritogenic anti-GBM, assessment of glomerular function was followed by quantitation of anti-GBM binding (values corrected for GBM surface area) in isolated glomeruli. At a given plasma level of antibody, the degree of binding of anti-GBM was slightly but significantly higher in HP than LP-fed rats; the decrease in GFR was significantly more pronounced in HP than LP-fed animals. The amount of anti-GBM binding was significantly greater in adult than young animals; however, the consequent decrease in GFR was more pronounced in the young than adult animals. Sex dependency was not discernible in anti-GBM binding or reduction in GFR. In all of the above experimental groups, the degree of anti-GBM binding was closely correlated with the plasma level of anti-GBM, but not with effective renal plasma flow rate, measured by PAH clearance. Separate groups of rats were subjected to experimental manipulation of single nephron GFR, glomerular capillary hydraulic pressure and glomerular plasma flow rate, by partial aortic constriction and saralasin administration. This set of experiments, using a tracer amount of non-nephritogenic anti-GBM, revealed that glomerular anti-GBM binding is independent of any of the above parameters. The studies indicate that dietary protein intake and age, but not sex, are among the factors determining the susceptibility of the glomerulus to acute immune injury. Since the binding of anti-GBM is determined by the affinity property of the glomerulus per se, and not by the prevailing hemodynamic pattern, the observed dependence of susceptibility to functional impairment on age and protein intake appears to also reflect a property of the glomerulus, which is influenced by age and the degree of dietary protein intake
How do Most Planets Form? -- Constraints on Disk Instability from Direct Imaging
Core accretion and disk instability have traditionally been regarded as the
two competing possible paths of planet formation. In recent years, evidence
have accumulated in favor of core accretion as the dominant mode, at least for
close-in planets. However, it might be hypothesized that a significant
population of wide planets formed by disk instabilities could exist at large
separations, forming an invisible majority. In previous work, we addressed this
issue through a direct imaging survey of B2--A0-type stars, and concluded that
<30% of such stars form and retain planets and brown dwarfs through disk
instability, leaving core accretion as the likely dominant mechanism. In this
paper, we extend this analysis to FGKM-type stars by applying a similar
analysis to the Gemini Deep Planet Survey (GDPS) sample. The results strengthen
the conclusion that substellar companions formed and retained around their
parent stars by disk instabilities are rare. Specifically, we find that the
frequency of such companions is <8% for FGKM-type stars under our most
conservative assumptions, for an outer disk radius of 300 AU, at 99%
confidence. Furthermore, we find that the frequency is always <10% at 99%
confidence independently of outer disk radius, for any radius from 5 to 500 AU.
We also simulate migration at a wide range of rates, and find that the
conclusions hold even if the companions move substantially after formation.
Hence, core accretion remains the likely dominant formation mechanism for the
total planet population, for every type of star from M-type through B-type.Comment: 10 pages, 4 figures, accepted for publication in Ap
Using a cognitive architecture to examine what develops
Different theories of development propose alternative mechanisms by which development occurs. Cognitive architectures can be used to examine the influence of each proposed mechanism of development while keeping all other mechanisms constant. An ACT-R computational model that matched adult behavior in solving a 21-block pyramid puzzle was created. The model was modified in three ways that corresponded to mechanisms of development proposed by developmental theories. The results showed that all the modifications (two of capacity and one of strategy choice) could approximate the behavior of 7-year-old children on the task. The strategy-choice modification provided the closest match on the two central measures of task behavior (time taken per layer, r = .99, and construction attempts per layer, r = .73). Modifying cognitive architectures is a fruitful way to compare and test potential developmental mechanisms, and can therefore help in specifying “what develops.
Planet formation: The case for large efforts on the computational side
Modern astronomy has finally been able to observe protoplanetary disks in
reasonable resolution and detail, unveiling the processes happening during
planet formation. These observed processes are understood under the framework
of disk-planet interaction, a process studied analytically and modeled
numerically for over 40 years. Long a theoreticians' game, the wealth of
observational data has been allowing for increasingly stringent tests of the
theoretical models. Modeling efforts are crucial to support the interpretation
of direct imaging analyses, not just for potential detections but also to put
meaningful upper limits on mass accretion rates and other physical quantities
in current and future large-scale surveys. This white paper addresses the
questions of what efforts on the computational side are required in the next
decade to advance our theoretical understanding, explain the observational
data, and guide new observations. We identified the nature of accretion, ab
initio planet formation, early evolution, and circumplanetary disks as major
fields of interest in computational planet formation. We recommend that
modelers relax the approximations of alpha-viscosity and isothermal equations
of state, on the grounds that these models use flawed assumptions, even if they
give good visual qualitative agreement with observations. We similarly
recommend that population synthesis move away from 1D hydrodynamics. The
computational resources to reach these goals should be developed during the
next decade, through improvements in algorithms and the hardware for hybrid
CPU/GPU clusters. Coupled with high angular resolution and great line
sensitivity in ground based interferometers, ELTs and JWST, these advances in
computational efforts should allow for large strides in the field in the next
decade.Comment: White paper submitted to the Astro2020 decadal surve
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