60,718 research outputs found
Search for new resonant states in 10C and 11C and their impact on the cosmological lithium problem
The observed primordial 7Li abundance in metal-poor halo stars is found to be
lower than its Big-Bang nucleosynthesis (BBN) calculated value by a factor of
approximately three. Some recent works suggested the possibility that this
discrepancy originates from missing resonant reactions which would destroy the
7Be, parent of 7Li. The most promising candidate resonances which were found
include a possibly missed 1- or 2- narrow state around 15 MeV in the compound
nucleus 10C formed by 7Be+3He and a state close to 7.8 MeV in the compound
nucleus 11C formed by 7Be+4He. In this work, we studied the high excitation
energy region of 10C and the low excitation energy region in 11C via the
reactions 10B(3He,t)10C and 11B(3He,t)11C, respectively, at the incident energy
of 35 MeV. Our results for 10C do not support 7Be+3He as a possible solution
for the 7Li problem. Concerning 11C results, the data show no new resonances in
the excitation energy region of interest and this excludes 7Be+4He reaction
channel as an explanation for the 7Li deficit.Comment: Accepted for publication in Phys. Rev. C (Rapid Communication
Stationarity, soft ergodicity, and entropy in relativistic systems
Recent molecular dynamics simulations show that a dilute relativistic gas
equilibrates to a Juettner velocity distribution if ensemble velocities are
measured simultaneously in the observer frame. The analysis of relativistic
Brownian motion processes, on the other hand, implies that stationary
one-particle distributions can differ depending on the underlying
time-parameterizations. Using molecular dynamics simulations, we demonstrate
how this relativistic phenomenon can be understood within a deterministic model
system. We show that, depending on the time-parameterization, one can
distinguish different types of soft ergodicity on the level of the one-particle
distributions. Our analysis further reveals a close connection between time
parameters and entropy in special relativity. A combination of different
time-parameterizations can potentially be useful in simulations that combine
molecular dynamics algorithms with randomized particle creation, annihilation,
or decay processes.Comment: 4 page
Generating potentials via difference equations
The condition for pressure isotropy, for spherically symmetric gravitational
fields with charged and uncharged matter, is reduced to a recurrence equation
with variable, rational coefficients. This difference equation is solved in
general using mathematical induction leading to an exact solution to the
Einstein field equations which extends the isotropic model of John and Maharaj.
The metric functions, energy density and pressure are well behaved which
suggests that this model could be used to describe a relativistic sphere. The
model admits a barotropic equation of state which approximates a polytrope
close to the stellar centre.Comment: 11 pages, To appear in Math. Meth. Appl. Sc
Missing linkers : an alternative pathway to UiO-66 electronic structure engineering
UiO-66 is a promising metal-organic framework for photocatalytic applications. However, the ligand-to-metal charge transfer of an excited electron is inefficient in the pristine material. Herein, we assess the influence of missing linker defects on the electronic structure of UiO-66 and discuss their ability to improve ligand-to-metal charge transfer. Using a new defect classification system, which is transparent and easily extendable, we identify the most promising photocatalysts by considering both relative stability and electronic structure. We find that the properties of UiO-66 defect structures largely depend on the coordination of the constituent nodes and that the nodes with the strongest local distortions alter the electronic structure most. Defects hence provide an alternative pathway to tune UiO-66 for photocatalytic purposes, besides linker modification and node metal substitution. In addition, the decomposition of MOF properties into node- and linker-based behavior is more generally valid, so we propose orthogonal electronic structure tuning as a paradigm in MOF design
Gas Metallicity in the Narrow-Line Regions of High-Redshift Active Galactic Nuclei
We analyze optical (UV rest-frame) spectra of X-ray selected narrow-line QSOs
at redshift 1.5 < z < 3.7 found in the Chandra Deep Field South and of
narrow-line radio galaxies at redshift 1.2 < z < 3.8 to investigate the gas
metallicity of the narrow-line regions and their evolution in this redshift
range. Such spectra are also compared with UV spectra of local Seyfert 2
galaxies. The observational data are inconsistent with the predictions of shock
models, suggesting that the narrow-line regions are mainly photoionized. The
photoionization models with dust grains predict line flux ratios which are also
in disagreement with most of the observed values, suggesting that the
high-ionization part of the narrow-line regions (which is sampled by the
available spectra) is dust-free. The photoionization dust-free models provide
two possible scenarios which are consistent with the observed data: low-density
gas clouds (n < 10^3 cm^-3) with a sub-solar metallicity (0.2 < Z/Z_sun < 1.0),
or high-density gas clouds (n ~ 10^5 cm^-3) with a wide range of gas
metallicity (0.2 < Z/Z_sun < 5.0). Regardless of the specific interpretation,
the observational data do not show any evidence for a significant evolution of
the gas metallicity in the narrow-line regions within the redshift range 1.2 <
z < 3.8. Instead, we find a trend for more luminous active galactic nuclei to
have more metal-rich gas clouds (luminosity-metallicity relation), which is in
agreement with the same finding in the studies of the broad-line regions. The
lack of evolution for the gas metallicity of the narrow-line regions implies
that the major epoch of star formation in the host galaxies of these active
galactic nuclei is at z > 4.Comment: 16 pages, 12 figures, submitted to Astronomy and Astrophysic
An Integrated Market for Electricity and Natural Gas Systems with Stochastic Power Producers
In energy systems with high shares of weather-driven renewable power sources,
gas-fired power plants can serve as a back-up technology to ensure security of
supply and provide short-term flexibility. Therefore, a tighter coordination
between electricity and natural gas networks is foreseen. In this work, we
examine different levels of coordination in terms of system integration and
time coupling of trading floors. We propose an integrated operational model for
electricity and natural gas systems under uncertain power supply by applying
two-stage stochastic programming. This formulation co-optimizes day-ahead and
real-time dispatch of both energy systems and aims at minimizing the total
expected cost. Additionally, two deterministic models, one of an integrated
energy system and one that treats the two systems independently, are presented.
We utilize a formulation that considers the linepack of the natural gas system,
while it results in a tractable mixed-integer linear programming (MILP) model.
Our analysis demonstrates the effectiveness of the proposed model in
accommodating high shares of renewables and the importance of proper natural
gas system modeling in short-term operations to reveal valuable flexibility of
the natural gas system. Moreover, we identify the coordination parameters
between the two markets and show their impact on the system's operation and
dispatch
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