1,439 research outputs found
The formation of the first galaxies and the transition to low-mass star formation
The formation of the first galaxies at redshifts z ~ 10-15 signaled the
transition from the simple initial state of the universe to one of ever
increasing complexity. We here review recent progress in understanding their
assembly process with numerical simulations, starting with cosmological initial
conditions and modelling the detailed physics of star formation. In this
context we emphasize the importance and influence of selecting appropriate
initial conditions for the star formation process. We revisit the notion of a
critical metallicity resulting in the transition from primordial to present-day
initial mass functions and highlight its dependence on additional cooling
mechanisms and the exact initial conditions. We also review recent work on the
ability of dust cooling to provide the transition to present-day low-mass star
formation. In particular, we highlight the extreme conditions under which this
transition mechanism occurs, with violent fragmentation in dense gas resulting
in tightly packed clusters.Comment: 16 pages, 7 figures, appeared in the conference proceedings for IAU
Symposium 255: Low-Metallicity Star Formation: From the First Stars to Dwarf
Galaxies, a high resolution version (highly recommended) can be found at
http://www.ita.uni-heidelberg.de/~tgreif/files/greif08.pd
Viable tax constitutions
Taxation is only sustainable if the general public complies with it. This observation is uncontroversial with tax practitioners but has been ignored by the public finance tradition, which has interpreted tax constitutions as binding contracts by which the power to tax is irretrievably conferred by individuals to government, which can then levy any tax it chooses. However, in the absence of an outside party enforcing contracts between members of a group, no arrangement within groups can be considered to be a binding contract, and therefore the power of tax must be sanctioned by individuals on an ongoing basis. In this paper we offer, for the first time, a theoretical analysis of this fundamental compliance problem associated with taxation, obtaining predictions that in some cases point to a re-interptretation of the theoretical constructions of the public finance tradition while in others call them into question
Open questions in the study of population III star formation
The first stars were key drivers of early cosmic evolution. We review the
main physical elements of the current consensus view, positing that the first
stars were predominantly very massive. We continue with a discussion of
important open questions that confront the standard model. Among them are
uncertainties in the atomic and molecular physics of the hydrogen and helium
gas, the multiplicity of stars that form in minihalos, and the possible
existence of two separate modes of metal-free star formation.Comment: 15 pages, 2 figures. To appear in the conference proceedings for IAU
Symposium 255: Low-Metallicity Star Formation: From the First Stars to Dwarf
Galaxie
Determination of Pericardial Adipose Tissue Increases the Prognostic Accuracy of Coronary Artery Calcification for Future Cardiovascular Events
Objectives: Pericardial adipose tissue (PAT) is associated with coronary artery plaque accumulation and the incidence of coronary heart disease. We evaluated the possible incremental prognostic value of PAT for future cardiovascular events. Methods: 145 patients (94 males, age 60 10 years) with stable coronary artery disease underwent coronary artery calcification (CAC) scanning in a multislice CT scanner, and the volume of pericardial fat was measured. Mean observation time was 5.4 years. Results: 34 patients experienced a severe cardiac event. They had a significantly higher CAC score (1,708 +/- 2,269 vs. 538 +/- 1,150, p 400, 3.5 (1.9-5.4; p = 0.007) for scores > 800 and 5.9 (3.7-7.8; p = 0.005) for scores > 1,600. When additionally a PAT volume > 200 cm(3) was determined, there was a significant increase in the event rate and relative risk. We calculated a relative risk of 2.9 (1.9-4.2; p = 0.01) for scores > 400, 4.0 (2.1-5.0; p = 0.006) for scores > 800 and 7.1 (4.1-10.2; p = 0.005) for scores > 1,600. Conclusions:The additional determination of PAT increases the predictive power of CAC for future cardiovascular events. PAT might therefore be used as a further parameter for risk stratification. Copyright (C) 2012 S. Karger AG, Base
The First Supernova Explosions: Energetics, Feedback, and Chemical Enrichment
We perform three-dimensional smoothed particle hydrodynamics simulations in a
realistic cosmological setting to investigate the expansion, feedback, and
chemical enrichment properties of a 200 M_sun pair-instability supernova in the
high-redshift universe. We find that the SN remnant propagates for a Hubble
time at z = 20 to a final mass-weighted mean shock radius of 2.5 kpc (proper),
roughly half the size of the HII region, and in this process sweeps up a total
gas mass of 2.5*10^5 M_sun. The morphology of the shock becomes highly
anisotropic once it leaves the host halo and encounters filaments and
neighboring minihalos, while the bulk of the shock propagates into the voids of
the intergalactic medium. The SN entirely disrupts the host halo and terminates
further star formation for at least 200 Myr, while in our specific case it
exerts positive mechanical feedback on neighboring minihalos by
shock-compressing their cores. In contrast, we do not observe secondary star
formation in the dense shell via gravitational fragmentation, due to the
previous photoheating by the progenitor star. We find that cooling by metal
lines is unimportant for the entire evolution of the SN remnant, while the
metal-enriched, interior bubble expands adiabatically into the cavities created
by the shock, and ultimately into the voids with a maximum extent similar to
the final mass-weighted mean shock radius. Finally, we conclude that dark
matter halos of at least M_vir > 10^8 M_sun must be assembled to recollect all
components of the swept-up gas.Comment: 16 pages, 14 figures, published in Ap
Direct observation of incommensurate magnetism in Hubbard chains
The interplay between magnetism and doping is at the origin of exotic
strongly correlated electronic phases and can lead to novel forms of magnetic
ordering. One example is the emergence of incommensurate spin-density waves
with a wave vector that does not match the reciprocal lattice. In one dimension
this effect is a hallmark of Luttinger liquid theory, which also describes the
low energy physics of the Hubbard model. Here we use a quantum simulator based
on ultracold fermions in an optical lattice to directly observe such
incommensurate spin correlations in doped and spin-imbalanced Hubbard chains
using fully spin and density resolved quantum gas microscopy. Doping is found
to induce a linear change of the spin-density wave vector in excellent
agreement with Luttinger theory predictions. For non-zero polarization we
observe a decrease of the wave vector with magnetization as expected from the
Heisenberg model in a magnetic field. We trace the microscopic origin of these
incommensurate correlations to holes, doublons and excess spins which act as
delocalized domain walls for the antiferromagnetic order. Finally, when
inducing interchain coupling we observe fundamentally different spin
correlations around doublons indicating the formation of a magnetic polaron
Radiation Hydrodynamical Instabilities in Cosmological and Galactic Ionization Fronts
Ionization fronts, the sharp radiation fronts behind which H/He ionizing
photons from massive stars and galaxies propagate through space, were
ubiquitous in the universe from its earliest times. The cosmic dark ages ended
with the formation of the first primeval stars and galaxies a few hundred Myr
after the Big Bang. Numerical simulations suggest that stars in this era were
very massive, 25 - 500 solar masses, with H II regions of up to 30,000
light-years in diameter. We present three-dimensional radiation hydrodynamical
calculations that reveal that the I-fronts of the first stars and galaxies were
prone to violent instabilities, enhancing the escape of UV photons into the
early intergalactic medium (IGM) and forming clumpy media in which supernovae
later exploded. The enrichment of such clumps with metals by the first
supernovae may have led to the prompt formation of a second generation of
low-mass stars, profoundly transforming the nature of the first protogalaxies.
Cosmological radiation hydrodynamics is unique because ionizing photons coupled
strongly to both gas flows and primordial chemistry at early epochs,
introducing a hierarchy of disparate characteristic timescales whose relative
magnitudes can vary greatly throughout a given calculation. We describe the
adaptive multistep integration scheme we have developed for the self-consistent
transport of both cosmological and galactic ionization fronts.Comment: 6 pages, 4 figures, accepted for proceedings of HEDLA2010, Caltech,
March 15 - 18, 201
The Interspersed Spin Boson Lattice Model
We describe a family of lattice models that support a new class of quantum
magnetism characterized by correlated spin and bosonic ordering [Phys. Rev.
Lett. 112, 180405 (2014)]. We explore the full phase diagram of the model using
Matrix-Product-State methods. Guided by these numerical results, we describe a
modified variational ansatz to improve our analytic description of the
groundstate at low boson frequencies. Additionally, we introduce an
experimental protocol capable of inferring the low-energy excitations of the
system by means of Fano scattering spectroscopy. Finally, we discuss the
implementation and characterization of this model with current circuit-QED
technology.Comment: Submitted to EPJ ST issue on "Novel Quantum Phases and Mesoscopic
Physics in Quantum Gases
- …