3,324 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
Photon Conserving Radiative Transfer around Point Sources in multi-dimensional Numerical Cosmology
Many questions in physical cosmology regarding the thermal and ionization
history of the intergalactic medium are now successfully studied with the help
of cosmological hydrodynamical simulations. Here we present a numerical method
that solves the radiative transfer around point sources within a three
dimensional cartesian grid. The method is energy conserving independently of
resolution: this ensures the correct propagation speeds of ionization fronts.
We describe the details of the algorithm, and compute as first numerical
application the ionized region surrounding a mini-quasar in a cosmological
density field at z=7.Comment: 5 pages, 4 figures, submitted to ApJ
Adiabatic Quantum Search in Open Systems
Adiabatic quantum algorithms represent a promising approach to universal
quantum computation. Whilst in a closed system these algorithms are limited by
avoided level crossings, where the gap becomes exponentially small in the
system size, their robustness in open systems remains unresolved. We study the
dynamics in the proximity of such an avoided level crossing associated with the
adiabatic quantum search algorithm in a quantum system that is coupled to a
generic environment. At zero temperature, we find that the algorithm remains
scalable provided the noise spectral density of the environment decays
sufficiently fast at low frequencies. At finite temperature, however,
scattering processes render the algorithm inefficient and no quantum speedup
can be achieved. Owing to the generic nature of our model, we expect our
results to be widely applicable to other adiabatic quantum algorithms.Comment: Accepted version. 6 pages, 2 figures, 10 pages supplemental material
Relative importance of auditing to the accounting profession: Is auditing a profit center?
https://egrove.olemiss.edu/dl_proceedings/1030/thumbnail.jp
On the Performance of Trellis Coded Modulation in a Concatenated Reed-Bolomon System
Concatenated systems combining the error correcting power of trellis codes and Reed-Solomon (RS) codes have been used to achieve error rates between 10-10 and 10-20 on satelite communications links [l]. The need for spectral efficiency has motivated consideration of of non-binary signalling with Ungerboeck [2,3] codes or the Pragmatic standard recently proposed by Viterbi [4], in place of quaternary signalling with rate 1/2 coding. In this paper we investigates the use of 8-PSK trellis-coded modulation (TCM) in a concatenated system using a (255,233) RS code. The use of the 8-PSK codes doubles the spectral efficiency with a performance cost of approximately 2 dB
Real-time observation of epitaxial graphene domain reorientation.
Graphene films grown by vapour deposition tend to be polycrystalline due to the nucleation and growth of islands with different in-plane orientations. Here, using low-energy electron microscopy, we find that micron-sized graphene islands on Ir(111) rotate to a preferred orientation during thermal annealing. We observe three alignment mechanisms: the simultaneous growth of aligned domains and dissolution of rotated domains, that is, 'ripening'; domain boundary motion within islands; and continuous lattice rotation of entire domains. By measuring the relative growth velocity of domains during ripening, we estimate that the driving force for alignment is on the order of 0.1 meV per C atom and increases with rotation angle. A simple model of the orientation-dependent energy associated with the moiré corrugation of the graphene sheet due to local variations in the graphene-substrate interaction reproduces the results. This work suggests new strategies for improving the van der Waals epitaxy of 2D materials
The Birth of a Galaxy - III. Propelling reionisation with the faintest galaxies
Starlight from galaxies plays a pivotal role throughout the process of cosmic
reionisation. We present the statistics of dwarf galaxy properties at z > 7 in
haloes with masses up to 10^9 solar masses, using a cosmological radiation
hydrodynamics simulation that follows their buildup starting with their
Population III progenitors. We find that metal-enriched star formation is not
restricted to atomic cooling ( K) haloes, but can occur
in haloes down to masses ~10^6 solar masses, especially in neutral regions.
Even though these smallest galaxies only host up to 10^4 solar masses of stars,
they provide nearly 30 per cent of the ionising photon budget. We find that the
galaxy luminosity function flattens above M_UV ~ -12 with a number density that
is unchanged at z < 10. The fraction of ionising radiation escaping into the
intergalactic medium is inversely dependent on halo mass, decreasing from 50 to
5 per cent in the mass range . Using our galaxy
statistics in a semi-analytic reionisation model, we find a Thomson scattering
optical depth consistent with the latest Planck results, while still being
consistent with the UV emissivity constraints provided by Ly forest
observations at z = 4-6.Comment: 21 pages, 15 figures, 4 tables. Accepted in MNRA
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