194 research outputs found
Growth of Dust as the Initial Step Toward Planet Formation
We discuss the results of laboratory measurements and theoretical models
concerning the aggregation of dust in protoplanetary disks, as the initial step
toward planet formation. Small particles easily stick when they collide and
form aggregates with an open, often fractal structure, depending on the growth
process. Larger particles are still expected to grow at collision velocities of
about 1m/s. Experiments also show that, after an intermezzo of destructive
velocities, high collision velocities above 10m/s on porous materials again
lead to net growth of the target. Considerations of dust-gas interactions show
that collision velocities for particles not too different in surface-to-mass
ratio remain limited up to sizes about 1m, and growth seems to be guaranteed to
reach these sizes quickly and easily. For meter sizes, coupling to nebula
turbulence makes destructive processes more likely. Global aggregation models
show that in a turbulent nebula, small particles are swept up too fast to be
consistent with observations of disks. An extended phase may therefore exist in
the nebula during which the small particle component is kept alive through
collisions driven by turbulence which frustrates growth to planetesimals until
conditions are more favorable for one or more reasons.Comment: Protostars and Planets V (PPV) review. 18 pages, 5 figure
Co-Accretion of Chondrules and Dust in the Solar Nebula
We present a mechanism for chondrules to stick together by means of
compaction of a porous dust rim they sweep up as they move through the dusty
nebula gas. It is shown that dust aggregates formed out of micron-sized grains
stick to chondrules, forming a porous dust rim. When chondrules collide, this
dust can be compacted by means of rolling motions within the porous dust layer.
This mechanism dissipates the collisional energy, compacting the rim and
allowing chondrules to stick. The structure of the obtained chondrule-dust
agglomerates (referred to as compounds) then consists of three phases:
chondrules, porous dust, and dust that has been compacted by collisions.
Subsequently, these compounds accrete their own dust and collide with other
compounds. The evolution of the compound size distribution and the relative
importance of the phases is calculated by a Monte Carlo code. Growth ends, and
a simulation is terminated when all the dust in the compounds has been
compacted. Numerous runs are performed, reflecting the uncertainty in the
physical conditions at the chondrule formation time. It is found that compounds
can grow by 1-2 orders of magnitudes in radius, upto dm-sizes when turbulence
levels are low. However, relative velocities associated with radial drift form
a barrier for further growth. Earlier findings that the dust sweep-up by
chondrules is proportional to their sizes are confirmed. We contrast two
scenarios regarding how this dust evolved further towards the densely packed
rims seen in chondrites.Comment: 23 pages, accepted for publication in Ap
Saturn's icy satellites and rings investigated by Cassini - VIMS. III. Radial compositional variability
In the last few years Cassini-VIMS, the Visible and Infared Mapping
Spectrometer, returned to us a comprehensive view of the Saturn's icy
satellites and rings. After having analyzed the satellites' spectral properties
(Filacchione et al. (2007a)) and their distribution across the satellites'
hemispheres (Filacchione et al. (2010)), we proceed in this paper to
investigate the radial variability of icy satellites (principal and minor) and
main rings average spectral properties. This analysis is done by using 2,264
disk-integrated observations of the satellites and a 12x700 pixels-wide rings
radial mosaic acquired with a spatial resolution of about 125 km/pixel. The
comparative analysis of these data allows us to retrieve the amount of both
water ice and red contaminant materials distributed across Saturn's system and
the typical surface regolith grain sizes. These measurements highlight very
striking differences in the population here analyzed, which vary from the
almost uncontaminated and water ice-rich surfaces of Enceladus and Calypso to
the metal/organic-rich and red surfaces of Iapetus' leading hemisphere and
Phoebe. Rings spectra appear more red than the icy satellites in the visible
range but show more intense 1.5-2.0 micron band depths. The correlations among
spectral slopes, band depths, visual albedo and phase permit us to cluster the
saturnian population in different spectral classes which are detected not only
among the principal satellites and rings but among co-orbital minor moons as
well. Finally, we have applied Hapke's theory to retrieve the best spectral
fits to Saturn's inner regular satellites using the same methodology applied
previously for Rhea data discussed in Ciarniello et al. (2011).Comment: 44 pages, 27 figures, 7 tables. Submitted to Icaru
Discovery of Oxygen Kalpha X-ray Emission from the Rings of Saturn
Using the Advanced CCD Imaging Spectrometer (ACIS), the Chandra X-ray
Observatory (CXO) observed the Saturnian system for one rotation of the planet
(~37 ks) on 20 January, 2004, and again on 26-27 January, 2004. In this letter
we report the detection of X-ray emission from the rings of Saturn. The X-ray
spectrum from the rings is dominated by emission in a narrow (~130 eV wide)
energy band centered on the atomic oxygen K-alpha fluorescence line at 0.53
keV. The X-ray power emitted from the rings in the 0.49-0.62 keV band is 84 MW,
which is about one-third of that emitted from Saturn disk in the photon energy
range 0.24-2.0 keV. Our analysis also finds a clear detection of X-ray emission
from the rings in the 0.49-0.62 keV band in an earlier (14-15 April, 2003)
Chandra ACIS observation of Saturn. Fluorescent scattering of solar X-rays from
oxygen atoms in the H2O icy ring material is the likely source mechanism for
ring X-rays, consistent with the scenario of solar photo-production of a
tenuous ring oxygen atmosphere and ionosphere recently discovered by Cassini.Comment: 14 pages, 5 figures Astrophys. J Lett., in pres
Meiotic segregation and interchromosomal effect in the sperm of a double translocation carrier: a case report
<p>Abstract</p> <p>Background</p> <p>Infertility is a natural mechanism of selection intended to prevent the delivery of a child with malformations or mental retardation. Male infertility is closely related to chromosomal abnormalities. This study was focused on the analysis of meiotic segregation involving a Robertsonian translocation, 45,XY,der(13;13) [56]/45,XY,der(13;14) [44] and the evaluation of possible interchromosomal effects.</p> <p>Results</p> <p>Hybridisation with LSI 13q14 and subtelomere 14q probes and WCP13 SpectrumGreen and WCP14 SpectrumOrange probes showed a high proportion of unbalanced gametes, corresponding to 71.2% of the spermatozoa. The disomic frequencies of the sexual chromosomes and chromosome 18 of the patient were higher (5.28% and 2.55%, respectively) than those of the control (0.6% and 0.59%, respectively).</p> <p>Conclusion</p> <p>Meiotic segregation studies in sperm are an important tool for genetic counselling of chromosomal aberrations, allowing for a prediction of the risks and consequent implications for the reproductive life. The patient with this rare translocation exhibited meiotic segregation fidelity, and a high rate of unbalanced gametes with disomic spermatozoa.</p
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
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)
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