394 research outputs found
Population III star formation in a Lambda CDM universe, II: Effects of a photodissociating background
We examine aspects of primordial star formation in the presence of a
molecular hydrogen-dissociating ultraviolet background. We compare a set of AMR
hydrodynamic cosmological simulations using a single cosmological realization
but with a range of ultraviolet background strengths in the Lyman-Werner band.
This allows us to study the effects of Lyman-Werner radiation on suppressing H2
cooling at low densities as well as the high-density evolution of the
collapsing core in a self-consistent cosmological framework. We find that the
addition of a photodissociating background results in a delay of the collapse
of high density gas at the center of the most massive halo in the simulation
and, as a result, an increase in the virial mass of this halo at the onset of
baryon collapse. We find that, contrary to previous results, Population III
star formation is not suppressed for J, but occurs even with
backgrounds as high as J. We find that H2 cooling leads to collapse
despite the depressed core molecular hydrogen fractions due to the elevated H2
cooling rates at K. We observe a relationship between the
strength of the photodissociating background and the rate of accretion onto the
evolving protostellar cloud core, with higher LW background fluxes resulting in
higher accretion rates. Finally, we find that the collapsing halo cores in our
simulations do not fragment at densities below cm
regardless of the strength of the LW background, suggesting that Population III
stars forming in halos with T K may still form in isolation.Comment: 46 pages, 14 figures (9 color). Accepted by the Astrophysical
Journal, some minor revision
Population III Star Formation in a Lambda WDM Universe
In this paper we examine aspects of primordial star formation in a gravitino
warm dark matter universe with a cosmological constant. We compare a set of
simulations using a single cosmological realization but with a wide range of
warm dark matter particle masses which have not yet been conclusively ruled out
by observations. The addition of a warm dark matter component to the initial
power spectrum results in a delay in the collapse of high density gas at the
center of the most massive halo in the simulation and, as a result, an increase
in the virial mass of this halo at the onset of baryon collapse. Both of these
effects become more pronounced as the warm dark matter particle mass becomes
smaller. A cosmology using a gravitino warm dark matter power spectrum assuming
a particle mass of m_{WDM} ~ 40keV is effectively indistinguishable from the
cold dark matter case, whereas the m_{WDM} ~ 15 keV case delays star formation
by approx. 10^8 years. There is remarkably little scatter between simulations
in the final properties of the primordial protostar which forms at the center
of the halo, possibly due to the overall low rate of halo mergers which is a
result of the WDM power spectrum. The detailed evolution of the collapsing halo
core in two representative WDM cosmologies is described. At low densities
(n_{b} <= 10^5 cm^{-3}), the evolution of the two calculations is qualitatively
similar, but occurs on significantly different timescales, with the halo in the
lower particle mass calculation taking much longer to evolve over the same
density range and reach runaway collapse. Once the gas in the center of the
halo reaches relatively high densities (n_{b} >= 10^5 cm^{-3}) the overall
evolution is essentially identical in the two calculations.Comment: 36 pages, 12 figures (3 color). Astrophysical Journal, accepte
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Foreign body responses in central nervous system mimic natural wound responses and alter biomaterial functions
Biomaterials hold promise for diverse therapeutic applications in the central nervous system (CNS). Little is known about molecular factors that determine CNS foreign body responses (FBRs) in vivo , or about how such responses influence biomaterial function. Here, we probed these factors using a platform of injectable hydrogels readily modified to present interfaces with different representative physiochemical properties to host cells. We show that biomaterial FBRs mimic specialized multicellular CNS wound responses not present in peripheral tissues, which serve to isolate damaged neural tissue and restore barrier functions. Moreover, we found that the nature and intensity of CNS FBRs are determined by definable properties. For example, cationic, anionic or nonionic interfaces with CNS cells elicit quantifiably different levels of stromal cell infiltration, inflammation, neural damage and amyloid production. The nature and intensity of FBRs significantly influenced hydrogel resorption and molecular delivery functions. These results characterize specific molecular mechanisms that drive FBRs in the CNS and have important implications for developing effective biomaterials for CNS applications
Suppression of nitric oxide (NO)-dependent behavior by double-stranded RNA-mediated silencing of a neuronal NO synthase gene
We have used double-stranded RNA (dsRNA)-mediated RNA interference (RNAi) to disrupt neuronal nitric oxide (NO) synthase (nNOS) gene function in the snail Lymnaea stagnalis and have detected a specific behavioral phenotype. The injection of whole animals with synthetic dsRNA molecules targeted to the nNOS-encoding mRNA reduces feeding behavior in vivo and fictive feeding in vitro and interferes with NO synthesis by the CNS. By showing that synthetic dsRNA targeted to the nNOS mRNA causes a significant and long-lasting reduction in the levels of Lym-nNOS mRNA, we verify that specific RNAi has occurred. Importantly, our results establish that the expression of nNOS gene is essential for normal feeding behavior. They also show that dsRNA can be used in the investigation of functional gene expression in the context of whole animal behavior, regardless of the availability of targeted mutation technologies
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Injectable diblock copolypeptide hydrogel provides platform to maintain high local concentrations of taxol and local tumor control
Abstract Introduction Surgical resection and systemic chemotherapy with temozolomide remain the mainstay for treatment of glioblastoma. However, many patients are not candidates for surgical resection given inaccessible tumor location or poor health status. Furthermore, despite being first line treatment, temozolomide has only limited efficacy. Methods The development of injectable hydrogel-based carrier systems allows for the delivery of a wide range of chemotherapeutics that can achieve high local concentrations, thus potentially avoiding systemic side effects and wide-spread neurotoxicity. To test this modality in a realistic environment, we developed a diblock copolypeptide hydrogel (DCH) capable of carrying and releasing paclitaxel, a compound that we found to be highly potent against primary gliomasphere cells. Results The DCH produced minimal tissue reactivity and was well tolerated in the immune-competent mouse brain. Paclitaxel-loaded hydrogel induced less tissue damage, cellular inflammation and reactive astrocytes than cremaphor-taxol (typical taxol-carrier) or hydrogel alone. In a deep subcortical xenograft model, of glioblastoma in immunodeficient mice, injection of paclitaxel-loaded hydrogel led to a high local concentration of paclitaxel and led to local tumor control and improved survival. However, the tumor cells were highly migratory and were able to eventually escape the area of treatment. Conclusions These findings suggest this technology may be ultimately applicable to patients with deep-seated inoperable tumors, but as currently formulated, complete tumor eradication would be highly unlikely. Future studies should focus on targeting the migratory potential of surviving cells
The Santa Fe Light Cone Simulation Project: I. Confusion and the WHIM in Upcoming Sunyaev-Zel'dovich Effect Surveys
We present the first results from a new generation of simulated large sky
coverage (~100 square degrees) Sunyaev-Zeldovich effect (SZE) cluster surveys
using the cosmological adaptive mesh refinement N-body/hydro code Enzo. We have
simulated a very large (512^3h^{-3}Mpc^3) volume with unprecedented dynamic
range. We have generated simulated light cones to match the resolution and
sensitivity of current and future SZE instruments. Unlike many previous studies
of this type, our simulation includes unbound gas, where an appreciable
fraction of the baryons in the universe reside.
We have found that cluster line-of-sight overlap may be a significant issue
in upcoming single-dish SZE surveys. Smaller beam surveys (~1 arcmin) have more
than one massive cluster within a beam diameter 5-10% of the time, and a larger
beam experiment like Planck has multiple clusters per beam 60% of the time. We
explore the contribution of unresolved halos and unbound gas to the SZE
signature at the maximum decrement. We find that there is a contribution from
gas outside clusters of ~16% per object on average for upcoming surveys. This
adds both bias and scatter to the deduced value of the integrated SZE, adding
difficulty in accurately calibrating a cluster Y-M relationship.
Finally, we find that in images where objects with M > 5x10^{13} M_{\odot}
have had their SZE signatures removed, roughly a third of the total SZE flux
still remains. This gas exists at least partially in the Warm Hot Intergalactic
Medium (WHIM), and will possibly be detectable with the upcoming generation of
SZE surveys.Comment: 14 pages, 13 figures, version accepted to ApJ. Major revisions mad
Population III star formation in a Lambda CDM universe, I: The effect of formation redshift and environment on protostellar accretion rate
(abridged) We perform 12 extremely high resolution adaptive mesh refinement
cosmological hydrodynamic simulations of Population III star formation in a
Lambda CDM universe, varying the box size and large-scale structure, to
understand systematic effects in the formation of primordial protostellar
cores. We find results that are qualitatively similar to those observed
previously. We observe that the threshold halo mass for formation of a
Population III protostar does not evolve significantly with time in the
redshift range studied (33 > z > 19) but exhibits substantial scatter due to
different halo assembly histories: Halos which assembled more slowly develop
cooling cores at lower mass than those that assemble more rapidly, in agreement
with Yoshida et al. (2003). We do, however, observe significant evolution in
the accretion rates of Population III protostars with redshift, with objects
that form later having higher maximum accretion rates, with a variation of two
orders of magnitude (10^-4 - 10^-2 Msolar/year). This can be explained by
considering the evolving virial properties of the halos with redshift and the
physics of molecular hydrogen formation at low densities. Our result implies
that the mass distribution of Population III stars inferred from their
accretion rates may be broader than previously thought, and may evolve with
redshift. Finally, we observe that our collapsing protostellar cloud cores do
not fragment, consistent with previous results, which suggests that Population
III stars which form in halos of mass 10^5 - 10^6 Msun always form in
isolation.Comment: Accepted by The Astrophysical Journal. Some minor changes. 65 pages,
3 tables, 21 figures (3 color). To appear in January 1, 2007 issu
Parallel HOP: A Scalable Halo Finder for Massive Cosmological Data Sets
Modern N-body cosmological simulations contain billions () of dark
matter particles. These simulations require hundreds to thousands of gigabytes
of memory, and employ hundreds to tens of thousands of processing cores on many
compute nodes. In order to study the distribution of dark matter in a
cosmological simulation, the dark matter halos must be identified using a halo
finder, which establishes the halo membership of every particle in the
simulation. The resources required for halo finding are similar to the
requirements for the simulation itself. In particular, simulations have become
too extensive to use commonly-employed halo finders, such that the
computational requirements to identify halos must now be spread across multiple
nodes and cores. Here we present a scalable-parallel halo finding method called
Parallel HOP for large-scale cosmological simulation data. Based on the halo
finder HOP, it utilizes MPI and domain decomposition to distribute the halo
finding workload across multiple compute nodes, enabling analysis of much
larger datasets than is possible with the strictly serial or previous parallel
implementations of HOP. We provide a reference implementation of this method as
a part of the toolkit yt, an analysis toolkit for Adaptive Mesh Refinement
(AMR) data that includes complementary analysis modules. Additionally, we
discuss a suite of benchmarks that demonstrate that this method scales well up
to several hundred tasks and datasets in excess of particles. The
Parallel HOP method and our implementation can be readily applied to any kind
of N-body simulation data and is therefore widely applicable.Comment: 29 pages, 11 figures, 2 table
The Nature of the Warm/Hot Intergalactic Medium I. Numerical Methods, Convergence, and OVI Absorption
We perform a series of cosmological simulations using Enzo, an Eulerian
adaptive-mesh refinement, N-body + hydrodynamical code, applied to study the
warm/hot intergalactic medium. The WHIM may be an important component of the
baryons missing observationally at low redshift. We investigate the dependence
of the global star formation rate and mass fraction in various baryonic phases
on spatial resolution and methods of incorporating stellar feedback. Although
both resolution and feedback significantly affect the total mass in the WHIM,
all of our simulations find that the WHIM fraction peaks at z ~ 0.5, declining
to 35-40% at z = 0. We construct samples of synthetic OVI absorption lines from
our highest-resolution simulations, using several models of oxygen ionization
balance. Models that include both collisional ionization and photoionization
provide excellent fits to the observed number density of absorbers per unit
redshift over the full range of column densities (10^13 cm-2 <= N_OVI <= 10^15
cm^-2). Models that include only collisional ionization provide better fits for
high column density absorbers (N_OVI > 10^14 cm^-2). The distribution of OVI in
density and temperature exhibits two populations: one at T ~ 10^5.5 K
(collisionally ionized, 55% of total OVI) and one at T ~ 10^4.5 K
(photoionized, 37%) with the remainder located in dense gas near galaxies.
While not a perfect tracer of hot gas, OVI provides an important tool for a
WHIM baryon census.Comment: 22 pages, 21 figures, emulateapj, accepted for publication in Ap
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Two-hit model of brain damage in the very preterm newborn: small for gestational age and postnatal systemic inflammation
Background: We sought to disentangle the contributions of perinatal systemic inflammation and small for gestational age (SGA) to the occurrence of low Bayley Mental Development Indices (MDIs) at age 2 years. Method We measured the concentration of 25 inflammation-related proteins in blood obtained during the first 2 postnatal weeks from 805 infants who were born before the 28th week of gestation and who had MDI measurements at age 2 years and were able to walk independently. Results: SGA newborns who did not have systemic inflammation (a concentration of an inflammation-related protein in the top quartile for gestational age on 2 days a week apart) were at greater risk of an MDI < 55, but not 55–69, than their peers who had neither SGA nor systemic inflammation. SGA infants who had elevated blood concentrations of IL-1beta, TNF-alpha, or IL-8 during the first two postnatal weeks were at even higher risk of an MDI < 55 than their SGA peers without systemic inflammation and of their non-SGA peers with systemic inflammation. Conclusion: SGA appears to place very preterm newborns at increased risk of a very low MDI. Systemic inflammation adds considerably to the increased risk
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