21,893 research outputs found
Synthesis of C-rich dust in CO nova ourbursts
Context. Classical novae are thermonuclear explosions that take place in the
envelopes of accreting white dwarfs in stellar binary systems. The material
transferred onto the white dwarf piles up under degenerate conditions, driving
a thermonuclear runaway. In those outbursts, about 10-7 - 10-3 Msun, enriched
in CNO and, sometimes, other intermediate-mass elements (e.g., Ne, Na, Mg, or
Al, for ONe novae) are ejected into the interstellar medium. The large
concentrations of metals spectroscopically inferred in the nova ejecta reveal
that the (solar-like) material transferred from the secondary mixes with the
outermost layers of the underlying white dwarf.
Aims. Most theoretical models of nova outbursts reported to date yield, on
average, outflows characterized by O > C, from which only oxidized condensates
(e.g, O-rich grains) would be expected, in principle.
Methods. To specifically address whether CO novae can actually produce C-rich
dust, six different hydrodynamic nova models have been evolved, from accretion
to the expansion and ejection stages, with different choices for the
composition of the substrate with which the solar-like accreted material mixes.
Updated chemical profiles inside the H-exhausted core have been used, based on
stellar evolution calculations for a progenitor of 8 Msun through H and
He-burning phases.
Results. We show that these profiles lead to C-rich ejecta after the nova
outburst. This extends the possible contribution of novae to the inventory of
presolar grains identified in meteorites, particularly in a number of
carbonaceous phases (i.e., nanodiamonds, silicon carbides and graphites).Comment: 5 pages, accepted for publication in Astronomy & Astrophysic
Statistical Geometry of Packing Defects of Lattice Chain Polymer from Enumeration and Sequential Monte Carlo Method
Voids exist in proteins as packing defects and are often associated with
protein functions. We study the statistical geometry of voids in
two-dimensional lattice chain polymers. We define voids as topological features
and develop a simple algorithm for their detection. For short chains, void
geometry is examined by enumerating all conformations. For long chains, the
space of void geometry is explored using sequential Monte Carlo importance
sampling and resampling techniques. We characterize the relationship of
geometric properties of voids with chain length, including probability of void
formation, expected number of voids, void size, and wall size of voids. We
formalize the concept of packing density for lattice polymers, and further
study the relationship between packing density and compactness, two parameters
frequently used to describe protein packing. We find that both fully extended
and maximally compact polymers have the highest packing density, but polymers
with intermediate compactness have low packing density. To study the
conformational entropic effects of void formation, we characterize the
conformation reduction factor of void formation and found that there are strong
end-effect. Voids are more likely to form at the chain end. The critical
exponent of end-effect is twice as large as that of self-contacting loop
formation when existence of voids is not required. We also briefly discuss the
sequential Monte Carlo sampling and resampling techniques used in this study.Comment: 29 pages, including 12 figure
The Star Formation Region NGC 6530: distance, ages and Initial Mass Function
We present astrometry and photometry, down to , of the very
young open cluster NGC6530, obtained from observations taken with the Wide
Field Imager camera at the MPG/ESO 2.2 m Telescope. Both the vs. and
the vs. color-magnitude diagrams (CMD) show the upper main sequence
dominated by very bright cluster stars, while, due to the high obscuration of
the giant molecular cloud surrounding the cluster, the blue envelopes of the
diagrams at are limited to the main sequence stars at the
distance of NGC6530. This particular structure of the NGC6530 CMD allows us to
conclude that its distance is about pc, significantly lower
than the previous determination of d=1800 pc. We have positionally matched our
optical catalog with the list of X-ray sources found in a Chandra-ACIS
observation, finding a total of 828 common stars, 90% of which are pre-main
sequence stars in NGC6530. Using evolutionary tracks of Siess et al. (2000)},
mass and age values are inferred for these stars. The median age of the cluster
is about 2.3 Myr; in the mass range (0.6--4.0), the Initial Mass
Function (IMF) shows a power law index , consistent with both
the Salpeter index (1.35), and with the index derived for other young clusters
; towards smaller masses the IMF shows a peak and then it starts to decrease.Comment: 32 pages, 13 ps figures, in press in Astronomy and Astrophysic
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SAS-4 is recruited to a dynamic structure in newly forming centrioles that is stabilized by the gamma-tubulin-mediated addition of centriolar microtubules.
Centrioles are surrounded by pericentriolar material (PCM), which is proposed to promote new centriole assembly by concentrating gamma-tubulin. Here, we quantitatively monitor new centriole assembly in living Caenorhabditis elegans embryos, focusing on the conserved components SAS-4 and SAS-6. We show that SAS-4 and SAS-6 are coordinately recruited to the site of new centriole assembly and reach their maximum levels during S phase. Centriolar SAS-6 is subsequently reduced by a mechanism intrinsic to the early assembly pathway that does not require progression into mitosis. Centriolar SAS-4 remains in dynamic equilibrium with the cytoplasmic pool until late prophase, when it is stably incorporated in a step that requires gamma-tubulin and microtubule assembly. These results indicate that gamma-tubulin in the PCM stabilizes the nascent daughter centriole by promoting microtubule addition to its outer wall. Such a mechanism may help restrict new centriole assembly to the vicinity of preexisting parent centrioles that recruit PCM
Tidal Barrier and the Asymptotic Mass of Proto Gas-Giant Planets
Extrasolar planets found with radial velocity surveys have masses ranging
from several Earth to several Jupiter masses. While mass accretion onto
protoplanetary cores in weak-line T-Tauri disks may eventually be quenched by a
global depletion of gas, such a mechanism is unlikely to have stalled the
growth of some known planetary systems which contain relatively low-mass and
close-in planets along with more massive and longer period companions. Here, we
suggest a potential solution for this conundrum. In general, supersonic infall
of surrounding gas onto a protoplanet is only possible interior to both of its
Bondi and Roche radii. At a critical mass, a protoplanet's Bondi and Roche
radii are equal to the disk thickness. Above this mass, the protoplanets' tidal
perturbation induces the formation of a gap. Although the disk gas may continue
to diffuse into the gap, the azimuthal flux across the protoplanets' Roche lobe
is quenched. Using two different schemes, we present the results of numerical
simulations and analysis to show that the accretion rate increases rapidly with
the ratio of the protoplanet's Roche to Bondi radii or equivalently to the disk
thickness. In regions with low geometric aspect ratios, gas accretion is
quenched with relatively low protoplanetary masses. This effect is important
for determining the gas-giant planets' mass function, the distribution of their
masses within multiple planet systems around solar type stars, and for
suppressing the emergence of gas-giants around low mass stars
Quantum Noise Correlation Experiments with Ultracold Atoms
Noise correlation analysis is a detection tool for spatial structures and
spatial correlations in the in-trap density distribution of ultracold atoms. In
this book chapter, we discuss the implementation, properties and limitations of
the method applied to ensembles of ultracold atoms in optical lattices, and
describe some instances where it has been applied.Comment: 26 pages, 14 figures - To appear as Chapter 8 in "Quantum gas
experiments - exploring many-body states," P. T\"orm\"a, K. Sengstock, eds.
(Imperial College Press, to be published 2014
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