164 research outputs found
Turbulent Concentration of mm-Size Particles in the Protoplanetary Nebula: Scale-Dependent Cascades
The initial accretion of primitive bodies (here, asteroids in particular) from freely-floating nebula particles remains problematic. Traditional growth-by-sticking models encounter a formidable "meter-size barrier" (or even a mm-to-cm-size barrier) in turbulent nebulae, making the preconditions for so-called "streaming instabilities" difficult to achieve even for so-called "lucky" particles. Even if growth by sticking could somehow breach the meter size barrier, turbulent nebulae present further obstacles through the 1-10km size range. On the other hand, nonturbulent nebulae form large asteroids too quickly to explain long spreads in formation times, or the dearth of melted asteroids. Theoretical understanding of nebula turbulence is itself in flux; recent models of MRI (magnetically-driven) turbulence favor low-or- no-turbulence environments, but purely hydrodynamic turbulence is making a comeback, with two recently discovered mechanisms generating robust turbulence which do not rely on magnetic fields at all. An important clue regarding planetesimal formation is an apparent 100km diameter peak in the pre-depletion, pre-erosion mass distribution of asteroids; scenarios leading directly from independent nebula particulates to large objects of this size, which avoid the problematic m-km size range, could be called "leapfrog" scenarios. The leapfrog scenario we have studied in detail involves formation of dense clumps of aerodynamically selected, typically mm-size particles in turbulence, which can under certain conditions shrink inexorably on 100-1000 orbit timescales and form 10-100km diameter sandpile planetesimals. There is evidence that at least the ordinary chondrite parent bodies were initially composed entirely of a homogeneous mix of such particles. Thus, while they are arcane, turbulent concentration models acting directly on chondrule size particles are worthy of deeper study. The typical sizes of planetesimals and the rate of their formation can be estimated using a statistical model with properties inferred from large numerical simulations of turbulence. Nebula turbulence is described by its Reynolds number Re = (L/eta)(exp 4/3), where L = H alpha(exp 1/2) is the largest eddy scale, H is the nebula gas vertical scale height, alpha the turbulent viscosity parameter, and eta is the Kolmogorov or smallest scale in turbulence (typically about 1km), with eddy turnover time t(sub eta). In the nebula, Re is far larger than any numerical simulation can handle, so some physical arguments are needed to extend the results of numerical simulations to nebula conditions. In this paper, we report new physics to be incorporated into our statistical models
Planetesimal Initial Mass Functions and Creation Rates Under Turbulent Concentration Using Scale-Dependent Cascades
The initial accretion of primitive bodies from freely-floating nebula particles remains problematic. Traditional growth-by-sticking models in turbulent nebulae encounter a "meter-size barrier" due to both drift and destruction, or even a millimeter-to-centimeter-size "bouncing" barrier. Recent suggestions have been made that some "lucky" particles might be able to outgrow the collision and/or drift barriers, and lead to so-called "streaming instabilities" or SI. However, new full models of growth by sticking in the presence of radial drift show that lucky particles (the largest particles, at the tail of the size distribution, that grow beyond the nominal fragmentation and drift barriers) are far too rare to lead to any collective effects such as streaming or gravitational instabilities. Thus we need to focus on typical radii gamma(sub M) which contain most of the mass. Our models of disks with weak-to-moderate turbulence, which include all the most recent experimental constraints on collisional growth, erosion, bouncing, and fragmentation, as well as radial drift, find that growth stalls quite generally at sizes gamma(sub M) which are too small to settle into layers which are dense enough for any collective effects (streaming or gravitational instabilities) to arise. Even if growth by sticking could somehow breach the nominal barriers (perhaps if the actual sticking or strength is larger than current estimates for pure ice or pure silicate, with specific grain sizes), turbulent nebulae present subsequent formidable obstacles to incremental growth through the 1-10km size range. On the other hand, non-turbulent nebulae alpha is less than 10(Sup -4)
Charging and coagulation of dust in protoplanetary plasma environments
Combining a particle-particle, particle-cluster and cluster-cluster
agglomeration model with an aggregate charging model, the coagulation and
charging of dust particles in various plasma environments relevant for
proto-planetary disks have been investigated. The results show that charged
aggregates tend to grow by adding small particles and clusters to larger
particles and clusters, leading to greater sizes and masses as compared to
neutral aggregates, for the same number of monomers in the aggregate. In
addition, aggregates coagulating in a Lorentzian plasma (containing a larger
fraction of high-energy plasma particles) are more massive and larger than
aggregates coagulating in a Maxwellian plasma, for the same plasma densities
and characteristic temperature. Comparisons of the grain structure, utilizing
the compactness factor, {\phi}{\sigma}, demonstrate that a Lorentzian plasma
environment results in fluffier aggregates, with small {\phi}{\sigma}, which
exhibit a narrow compactness factor distribution. Neutral aggregates are more
compact, with larger {\phi}{\sigma}, and exhibit a larger variation in
fluffiness. Measurement of the compactness factor of large populations of
aggregates is shown to provide information on the disk parameters that were
present during aggregation
Material enhancement in protoplanetary nebulae by particle drift through evaporation fronts
Solid material in a protoplanetary nebula is subject to vigorous
redistribution processes relative to the nebula gas. Meter-sized particles
drift rapidly inwards near the nebula midplane, and material evaporates when
the particles cross a condensation/evaporation boundary. The material cannot be
removed as fast in its vapor form as it is being supplied in solid form, so its
concentration increases locally by a large factor (more than an order of
magnitude under nominal conditions). As time goes on, the vapor phase
enhancement propagates for long distances inside the evaporation boundary
(potentially all the way in to the star). Meanwhile, material is enhanced in
its solid form over a characteristic lengthscale outside the evaporation
boundary. This effect is applicable to any condensible (water, silicates, {\it
etc.}). Three distinct radial enhancement/depletion regimes can be discerned by
use of a simple model. Meteoritics applications include oxygen fugacity and
isotopic variations, as well as isotopic homogenization in silicates. Planetary
system applications include more robust enhancement of solids in Jupiter's core
formation region than previously suggested. Astrophysical applications include
differential, time-dependent enhancement of vapor phase CO and HO in the
terrestrial planet regions of actively accreting protoplanetary disks.Comment: To appear in Astrophys. J., vol 614, Oct 10 2004 issu
Oral lesions in leprosy
BACKGROUND: Leprotic oral lesions are more common in the lepromatous
form of leprosy, indicate a late manifestation, and have a great
epidemiological importance as a source of infection. METHODS: Patients
with leprosy were examined searching for oral lesions. Biopsies of the
left buccal mucosa in all patients, and of oral lesions, were performed
and were stained with H&E and Wade. RESULTS: Oral lesions were
found in 26 patients, 11 lepromatous leprosy, 14 borderline leprosy,
and one tuberculoid leprosy. Clinically 5 patients had enanthem of the
anterior pillars, 3 of the uvula and 3 of the palate. Two had palatal
infiltration. Viable bacilli were found in two lepromatous patients.
Biopsies of the buccal mucosa showed no change or a nonspecific
inflammatory infiltrate. Oral clinical alterations were present in 69%
of the patients; of these 50% showed histopathological features in an
area without any lesion. DISCUSSION: Our clinical and histopathological
findings corroborate earlier reports that there is a reduced incidence
of oral changes, which is probably due to early treatment. The
maintenance of oral infection in this area can also lead to and
maintain lepra reactions, while they may also act as possible infection
sources. Attention should be given to oral disease in leprosy because
detection and treatment of oral lesions can prevent the spread of the
disease
Forming Planetesimals by Gravitational Instability: II. How Dust Settles to its Marginally Stable State
Dust at the midplane of a circumstellar disk can become gravitationally
unstable and fragment into planetesimals if the local dust-to-gas density ratio
mu is sufficiently high. We simulate how dust settles in passive disks and ask
how high mu can become. We settle the dust using a 1D code and test for
dynamical stability using a 3D shearing box code. This scheme allows us to
explore the behavior of small particles having short but non-zero stopping
times in gas: 0 < t_stop << the orbital period. The streaming instability is
thereby filtered out. Dust settles until shearing instabilities in the edges of
the dust layer threaten to overturn the entire layer. In this state of marginal
stability, mu=2.9 for a disk whose bulk (height-integrated) metallicity is
solar. For a disk whose bulk metallicity is 4x solar, mu reaches 26.4. These
maximum values of mu, which depend on the background radial pressure gradient,
are so large that gravitational instability of small particles is viable in
disks whose bulk metallicities are just a few (<4) times solar. Earlier studies
assumed that dust settles until the Richardson number Ri is spatially constant.
Our simulations are free of this assumption but provide support for it within
the dust layer's edges, with the proviso that Ri increases with bulk
metallicity in the same way that we found in Paper I. Only modest enhancements
in bulk metallicity are needed to spawn planetesimals directly from small
particles.Comment: Accepted to Ap
A Recent Impact Origin of Saturn’s Rings and Mid-sized Moons
We simulate the collision of precursor icy moons analogous to Dione and Rhea as a possible origin for Saturn's remarkably young rings. Such an event could have been triggered a few hundred million years ago by resonant instabilities in a previous satellite system. Using high-resolution smoothed particle hydrodynamics simulations, we find that this kind of impact can produce a wide distribution of massive objects and scatter material throughout the system. This includes the direct placement of pure-ice ejecta onto orbits that enter Saturn's Roche limit, which could form or rejuvenate rings. In addition, fragments and debris of rock and ice totaling more than the mass of Enceladus can be placed onto highly eccentric orbits that would intersect with any precursor moons orbiting in the vicinity of Mimas, Enceladus, or Tethys. This could prompt further disruption and facilitate a collisional cascade to distribute more debris for potential ring formation, the re-formation of the present-day moons, and evolution into an eventual cratering population of planetocentric impactors
Origin and Evolution of Saturn's Ring System
The origin and long-term evolution of Saturn's rings is still an unsolved
problem in modern planetary science. In this chapter we review the current
state of our knowledge on this long-standing question for the main rings (A,
Cassini Division, B, C), the F Ring, and the diffuse rings (E and G). During
the Voyager era, models of evolutionary processes affecting the rings on long
time scales (erosion, viscous spreading, accretion, ballistic transport, etc.)
had suggested that Saturn's rings are not older than 100 My. In addition,
Saturn's large system of diffuse rings has been thought to be the result of
material loss from one or more of Saturn's satellites. In the Cassini era, high
spatial and spectral resolution data have allowed progress to be made on some
of these questions. Discoveries such as the ''propellers'' in the A ring, the
shape of ring-embedded moonlets, the clumps in the F Ring, and Enceladus' plume
provide new constraints on evolutionary processes in Saturn's rings. At the
same time, advances in numerical simulations over the last 20 years have opened
the way to realistic models of the rings's fine scale structure, and progress
in our understanding of the formation of the Solar System provides a
better-defined historical context in which to understand ring formation. All
these elements have important implications for the origin and long-term
evolution of Saturn's rings. They strengthen the idea that Saturn's rings are
very dynamical and rapidly evolving, while new arguments suggest that the rings
could be older than previously believed, provided that they are regularly
renewed. Key evolutionary processes, timescales and possible scenarios for the
rings's origin are reviewed in the light of tComment: Chapter 17 of the book ''Saturn After Cassini-Huygens'' Saturn from
Cassini-Huygens, Dougherty, M.K.; Esposito, L.W.; Krimigis, S.M. (Ed.) (2009)
537-57
A simple model for the evolution of the dust population in protoplanetary disks
Context: The global size and spatial distribution of dust is an important
ingredient in the structure and evolution of protoplanetary disks and in the
formation of larger bodies, such as planetesimals. Aims: We aim to derive
simple equations that explain the global evolution of the dust surface density
profile and the upper limit of the grain size distribution and which can
readily be used for further modeling or for interpreting of observational data.
Methods: We have developed a simple model that follows the upper end of the
dust size distribution and the evolution of the dust surface density profile.
This model is calibrated with state-of-the-art simulations of dust evolution,
which treat dust growth, fragmentation, and transport in viscously evolving gas
disks. Results: We find very good agreement between the full dust-evolution
code and the toy model presented in this paper. We derive analytical profiles
that describe the dust-to-gas ratios and the dust surface density profiles well
in protoplanetary disks, as well as the radial flux by solid material "rain
out", which is crucial for triggering any gravity assisted formation of
planetesimals. We show that fragmentation is the dominating effect in the inner
regions of the disk leading to a dust surface density exponent of -1.5, while
the outer regions at later times can become drift-dominated, yielding a dust
surface density exponent of -0.75. Our results show that radial drift is not
efficient in fragmenting dust grains. This supports the theory that small dust
grains are resupplied by fragmentation due to the turbulent state of the disk.Comment: 12 pages, 10 figures, accepted to A&
Mycobacterium tuberculosis Immune Response in Patients With Immune-Mediated Inflammatory Disease
Subjects with immune-mediated inflammatory diseases (IMID), such as rheumatoid arthritis (RA), have an intrinsic higher probability to develop active-tuberculosis (TB) compared to the general population. The risk ranges from 2.0 to 8.9 in RA patients not receiving therapies. According to the WHO, the RA prevalence varies between 0.3% and 1% and is more common in women and in developed countries. Therefore, the identification and treatment of TB infection (TBI) in this fragile population is important to propose the TB preventive therapy. We aimed to study the M. tuberculosis (Mtb) specific T-cell response to find immune biomarkers of Mtb burden or Mtb clearance in patients with different TB status and different risk to develop active-TB disease. We enrolled TBI subjects as example of Mtb-containment, the active-TB as example of a replicating Mtb status, and the TBI-IMID as fragile population. To study the Mtb-specific response in a condition of possible Mtb sterilization, we longitudinally enrolled TBI subjects and active-TB patients before and after TB therapy. Peripheral blood mononuclear cells were stimulated overnight with Mtb peptides contained in TB1- and TB2-tubes of the Quantiferon-Plus kit. Then, we characterized by cytometry the Mtb-specific CD4 and CD8 T cells. In TBI-IMID, the TB therapy did not affect the ability of CD4 T cells to produce interferon-γ, tumor necrosis factor-α, and interleukin-2, their functional status, and their phenotype. The TB therapy determined a contraction of the triple functional CD4 T cells of the TBI subjects and active-TB patients. The CD45RA- CD27+ T cells stood out as a main subset of the Mtb-specific response in all groups. Before the TB-preventive therapy, the TBI subjects had higher proportion of Mtb-specific CD45RA-CD27+CD4+ T cells and the active-TB subjects had higher proportion of Mtb-specific CD45RA-CD27-CD4+ T cells compared to other groups. The TBI-IMID patients showed a phenotype similar to TBI, suggesting that the type of IMID and the IMID therapy did not affect the activation status of Mtb-specific CD4 T cells. Future studies on a larger and better-stratified TBI-IMID population will help to understand the change of the Mtb-specific immune response over time and to identify possible immune biomarkers of Mtb-containment or active replication
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