90,101 research outputs found
Are Galaxies Optically Thin to Their Own Lyman Continuum Radiation? II. NGC 6822
Halpha and UBV photometry of NGC 6822 are used to study the distribution of
OB stars and HII regions in the galaxy and to determine whether individual
regions of the galaxy are in a state of ionization balance. Four distinct
components of the Halpha emission (bright, halo, diffuse and field)
differentiated by their surface brightnesses are identified. We find that
approximately 1/2 of all OB stars in NGC 6822 are located in the field while
only 1/4 are found in the combined bright and halo regions, suggesting that OB
stars spend roughly 3/4 of their lifetimes outside ``classical'' H II regions.
Comparing the observed Halpha emission with that predicted from stellar
ionizing flux models, we find that although the bright, halo and diffuse
regions are probably in ionization balance, the field region is producing at
least 6 times as much ionizing flux as is observed. The ionization balance
results in NGC 6822 suggest that star formation rates obtained from Halpha
luminosities must underestimate the true star formation rate in this galaxy by
about 50%. Comparing our results for NGC 6822 with previous results for the
spiral galaxy M33, we find that the inner kiloparsec of M33 is in a more
serious state of ionization imbalance, perhaps due to its higher surface
density of blue stars.Comment: Replaced version should now compile with standard aastex style files.
28 pages, aastex preprint format. Accepted in ApJ. Hardcopies of figures
available on request to [email protected]
Thermodynamic property measurements in reflected shock air plasmas at 12,000 - 16,000 K
Reflected shock air plasma thermodynamic properties at 12,000 to 16,000 deg
Renormalization Group Treatment of Nonrenormalizable Interactions
The structure of the UV divergencies in higher dimensional nonrenormalizable
theories is analysed. Based on renormalization operation and renormalization
group theory it is shown that even in this case the leading divergencies
(asymptotics) are governed by the one-loop diagrams the number of which,
however, is infinite. Explicit expression for the one-loop counter term in an
arbitrary D-dimensional quantum field theory without derivatives is suggested.
This allows one to sum up the leading asymptotics which are independent of the
arbitrariness in subtraction of higher order operators. Diagrammatic
calculations in a number of scalar models in higher loops are performed to be
in agreement with the above statements. These results do not support the idea
of the na\"ive power-law running of couplings in nonrenormalizable theories and
fail (with one exception) to reveal any simple closed formula for the leading
terms.Comment: LaTex, 11 page
The Stokes boundary layer for a thixotropic or antithixotropic fluid
We present a mathematical investigation of the oscillatory boundary layer (‘Stokes layer’) in a semi-infinite fluid bounded by an oscillating wall (the socalled ‘Stokes problem’), when the fluid has a thixotropic or antithixotropic rheology. We obtain asymptotic solutions in the limit of small-amplitude oscillations, and we use numerical integration to validate the asymptotic solutions and to explore the behaviour of the system for larger-amplitude oscillations. The solutions that we obtain differ significantly from the classical solution for a Newtonian fluid. In particular, for antithixotropic fluids the velocity reaches zero at a finite distance from the wall, in contrast to the exponential decay for a thixotropic or a Newtonian fluid. For small amplitudes of oscillation, three regimes of behaviour are possible: the structure parameter may take values defined instantaneously by the shear rate, or by a long-term average; or it may behave hysteretically. The regime boundaries depend on the precise specification of structure build-up and breakdown rates in the rheological model, illustrating the subtleties of complex fluid models in non-rheometric settings. For larger amplitudes of oscillation the dominant behaviour is hysteretic. We discuss in particular the relationship between the shear stress and the shear rate at the oscillating wall
Conceptual design for Mobile Geological Laboratory position and heading fix system
Conceptual design of position fixing system for Mobile Geological Laboratory in Lunar Mobile Laboratory simulatio
Strings and Branes in Nonabelian Gauge Theory
It is an old speculation that SU(N) gauge theory can alternatively be
formulated as a string theory. Recently this subject has been revived, in the
wake of the discovery of D-branes. In particular, it has been argued that at
least some conformally invariant cousins of the theory have such a string
representation. This is a pedagogical introduction to these developments for
non-string theorists. Some of the existing arguments are simplified.Comment: Reference adde
Quantum Phase Transitions beyond the Landau's Paradigm in Sp(4) Spin System
We propose quantum phase transitions beyond the Landau's paradigm of Sp(4)
spin Heisenberg models on the triangular and square lattices, motivated by the
exact Sp(4) SO(5) symmetry of spin-3/2 fermionic cold atomic system
with only wave scattering. On the triangular lattice, we study a phase
transition between the spin ordered phase and a
spin liquid phase, this phase transition is described by an O(8) sigma model in
terms of fractionalized spinon fields, with significant anomalous scaling
dimensions of spin order parameters. On the square lattice, we propose a
deconfined critical point between the Neel order and the VBS order, which is
described by the CP(3) model, and the monopole effect of the compact U(1) gauge
field is expected to be suppressed at the critical point.Comment: 6 pages, 3 figure
Exact renormalization group equations and the field theoretical approach to critical phenomena
After a brief presentation of the exact renormalization group equation, we
illustrate how the field theoretical (perturbative) approach to critical
phenomena takes place in the more general Wilson (nonperturbative) approach.
Notions such as the continuum limit and the renormalizability and the presence
of singularities in the perturbative series are discussed.Comment: 15 pages, 7 figures, to appear in the Proceedings of the 2nd
Conference on the Exact Renormalization Group, Rome 200
- …