4,901 research outputs found
A study to develop neutron activation for measuring bone calcium content
Neutron activation analysis for measuring calcium in monkey bone
The role of government venture capital funds: recent lessons from the U.K. experience
UK Government Venture Capital (GVC) - backed schemes have evolved to provide more effective, targeted, funding for high growth potential firms.
Public-private co-financing, is generating attributable, additional, positive employment and sales turnover impacts in beneficiary firms, but significant long horizon follow-on investment requirements need addressing.
Policy designers should be cognisant of the changing external financing ecosystem when designing co-investment schemes
Nearby Clumpy, Gas Rich, Star Forming Galaxies: Local Analogs of High Redshift Clumpy Galaxies
Luminous compact blue galaxies (LCBGs) have enhanced star formation rates and
compact morphologies. We combine Sloan Digital Sky Survey data with HI data of
29 LCBGs at redshift z~0 to understand their nature. We find that local LCBGs
have high atomic gas fractions (~50%) and star formation rates per stellar mass
consistent with some high redshift star forming galaxies. Many local LCBGs also
have clumpy morphologies, with clumps distributed across their disks. Although
rare, these galaxies appear to be similar to the clumpy star forming galaxies
commonly observed at z~1-3. Local LCBGs separate into three groups: 1.
Interacting galaxies (~20%); 2. Clumpy spirals (~40%); 3. Non-clumpy,
non-spirals with regular shapes and smaller effective radii and stellar masses
(~40%). It seems that the method of building up a high gas fraction, which then
triggers star formation, is not the same for all local LCBGs. This may lead to
a dichotomy in galaxy characteristics. We consider possible gas delivery
scenarios and suggest that clumpy spirals, preferentially located in clusters
and with companions, are smoothly accreting gas from tidally disrupted
companions and/or intracluster gas enriched by stripped satellites. Conversely,
as non-clumpy galaxies are preferentially located in the field and tend to be
isolated, we suggest clumpy, cold streams, which destroy galaxy disks and
prevent clump formation, as a likely gas delivery mechanism for these systems.
Other possibilities include smooth cold streams, a series of minor mergers, or
major interactions.Comment: 22 pages, 5 figure
Path Integral Method for DNA Denaturation
The statistical physics of homogeneous DNA is investigated by the imaginary
time path integral formalism. The base pair stretchings are described by an
ensemble of paths selected through a macroscopic constraint, the fulfillement
of the second law of thermodynamics. The number of paths contributing to the
partition function strongly increases around and above a specific temperature
whereas the fraction of unbound base pairs grows continuosly around and
above . The latter is identified with the denaturation temperature.
Thus, the separation of the two complementary strands appears as a highly
cooperative phenomenon displaying a smooth crossover versus . The
thermodynamical properties have been computed in a large temperature range by
varying the size of the path ensemble at the lower bound of the range. No
significant physical dependence on the system size has been envisaged. The
entropy grows continuosly versus while the specific heat displays a
remarkable peak at . The location of the peak versus varies with the
stiffness of the anharmonic stacking interaction along the strand. The
presented results suggest that denaturation in homogeneous DNA has the features
of a second order phase transition. The method accounts for the cooperative
behavior of a very large number of degrees of freedom while the computation
time is kept within a reasonable limit.Comment: Physical Review E 2009 in pres
Formation of Globular Clusters in Galaxy Mergers
We present a high-resolution simulation of globular cluster formation in a
galaxy merger. For the first time in such a simulation, individual star
clusters are directly identified and followed on their orbits. We
quantitatively compare star formation in the merger to that in the unperturbed
galaxies. The merging galaxies show a strong starburst, in sharp contrast to
their isolated progenitors. Most star clusters form in the tidal features. With
a mass range of --, they are
identified as globular clusters. The merger remnant is an elliptical galaxy.
Clusters with different mass or age have different radial distributions in the
galaxy. Our results show that the high specific frequency and bimodal
distribution of metallicity observed in elliptical galaxies are natural
products of gas-rich mergers, supporting a merger origin for the ellipticals
and their globular cluster systems.Comment: ApJL accepted, version with high quality color images can be found in
http://research.amnh.org/~yuexing/astro-ph/0407248.pd
Flickering of 1.3 cm Sources in Sgr B2: Towards a Solution to the Ultracompact HII Region Lifetime Problem
Accretion flows onto massive stars must transfer mass so quickly that they
are themselves gravitationally unstable, forming dense clumps and filaments.
These density perturbations interact with young massive stars, emitting
ionizing radiation, alternately exposing and confining their HII regions. As a
result, the HII regions are predicted to flicker in flux density over periods
of decades to centuries rather than increasing monotonically in size as
predicted by simple Spitzer solutions. We have recently observed the Sgr B2
region at 1.3 cm with the VLA in its three hybrid configurations (DnC, CnB and
BnA) at a resolution of 0.25''. These observations were made to compare in
detail with matched continuum observations from 1989. At 0.25'' resolution, Sgr
B2 contains 41 UC HII regions, 6 of which are hypercompact. The new
observations of Sgr B2 allow comparison of relative peak flux densites for the
HII regions in Sgr B2 over a 23 year time baseline (1989-2012) in one of the
most source-rich massive star forming regions in the Milky Way. The new 1.3 cm
continuum images indicate that four of the 41 UC HII regions exhibit
significant changes in their peak flux density, with one source (K3) dropping
in peak flux density, and the other 3 sources (F10.303, F1 and F3) increasing
in peak flux density. The results are consistent with statistical predictions
from simulations of high mass star formation, suggesting that they offer a
solution to the lifetime problem for ultracompact HII regions.Comment: 12 pages, 3 figures, Accepted for publication in the Astrophysical
Journal Letter
Wolf-Rayet and LBV Nebulae as the Result of Variable and Non-Spherical Stellar Winds
The physical basis for interpreting observations of nebular morphology around
massive stars in terms of the evolution of the central stars is reviewed, and
examples are discussed, including NGC 6888, OMC-1, and eta Carinae.Comment: To be published in the Proceedings of IAU Colloquium 169 on Variable
and Non-Spherical Stellar Winds in Luminous Hot Stars, ed. B. Wolf
(Springer-Verlag, Berlin, Heidelberg). 7 pages, including 5 figures. A
full-resolution version of fig 4 is available in the version at
http://www.mpia-hd.mpg.de/theory/preprints.html#maclo
Mixing Time Scales in a Supernova-Driven Interstellar Medium
We study the mixing of chemical species in the interstellar medium (ISM).
Recent observations suggest that the distribution of species such as deuterium
in the ISM may be far from homogeneous. This raises the question of how long it
takes for inhomogeneities to be erased in the ISM, and how this depends on the
length scale of the inhomogeneities. We added a tracer field to the
three-dimensional, supernova-driven ISM model of Avillez (2000) to study mixing
and dispersal in kiloparsec-scale simulations of the ISM with different
supernova (SN) rates and different inhomogeneity length scales. We find several
surprising results. Classical mixing length theory fails to predict the very
weak dependence of mixing time on length scale that we find on scales of
25--500 pc. Derived diffusion coefficients increase exponentially with time,
rather than remaining constant. The variance of composition declines
exponentially, with a time constant of tens of Myr, so that large differences
fade faster than small ones. The time constant depends on the inverse square
root of the supernova rate. One major reason for these results is that even
with numerical diffusion exceeding physical values, gas does not mix quickly
between hot and cold regions.Comment: 23 pages, 14 figures that include 7 simulation images and 19 plots,
accepted for publication at Ap
Synthesis, characterization of a new carbonylated zirconium metallocene using a dichloro-zirconocene derived from partially alkylated s-indacene
Indexación: ScieloThis work describes the synthesis and characterization of new organometallic species, an unprecedented mononuclear zirconium complex bearing a tetraalkylated s-indacene ligand, and secondly, its respective dicarbonyl complex obtained by reduction with Mg/HgCl2. Theoretical calculations of these two compounds were carried out to gain further understanding of these novel molecular systems.http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0717-97072009000300014&lng=es&nrm=is
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