6,139 research outputs found
Clumpy Ultracompact HII Regions I: Fully Supersonic Wind-blown Models
We propose that a significant fraction of the ultracompact HII regions found
in massive star-forming clouds are the result of the interaction of the wind
and ionizing radiation from a young massive star with the clumpy molecular
cloud gas in its neighbourhood. Distributed mass loading in the flow allows the
compact nebulae to be long-lived. In this paper, we discuss a particularly
simple case, in which the flow in the HII region is everywhere supersonic. The
line profiles predicted for this model are highly characteristic, for the case
of uniform mass loading. We discuss briefly other observational diagnostics of
these models.Comment: To appear in Monthly Notices of the Royal Astronomical Society. 5
pages LaTeX (uses mn.sty and epsf.sty macros) + 4 PS figures. Also available
via http://axp2.ast.man.ac.uk:8000/Preprints.htm
Does Inflation Provide Natural Initial Conditions for the Universe?
If our universe underwent inflation, its entropy during the inflationary
phase was substantially lower than it is today. Because a low-entropy state is
less likely to be chosen randomly than a high-entropy one, inflation is
unlikely to arise through randomly-chosen initial conditions. To resolve this
puzzle, we examine the notion of a natural state for the universe, and argue
that it is a nearly-empty spacetime. If empty space has a small vacuum energy,
however, inflation can begin spontaneously in this background. This scenario
explains why a universe like ours is likely to have begun via a period of
inflation, and also provides an origin for the cosmological arrow of time.Comment: Submitted to Gravity Research Foundation Essay Competition; based on
hep-th/041027
Poincare Recurrences and Topological Diversity
Finite entropy thermal systems undergo Poincare recurrences. In the context
of field theory, this implies that at finite temperature, timelike two-point
functions will be quasi-periodic. In this note we attempt to reproduce this
behavior using the AdS/CFT correspondence by studying the correlator of a
massive scalar field in the bulk. We evaluate the correlator by summing over
all the SL(2,Z) images of the BTZ spacetime. We show that all the terms in this
sum receive large corrections after at certain critical time, and that the
result, even if convergent, is not quasi-periodic. We present several arguments
indicating that the periodicity will be very difficult to recover without an
exact re-summation, and discuss several toy models which illustrate this.
Finally, we consider the consequences for the information paradox.Comment: 18 + 8 pages, 5 figures. v2: reference adde
ACTIVATION AND CONTRIBUTION OF TRUNK AND LEG MUSCULATURE TO FORCE PRODUCTION DURING ON-WATER SPRINT KAYAK PERFORMANCE
Velocity production during sprint kayaking has been shown to be dependent on the magnitude of forces produced during the stroke cycle. However, while the importance of the upper body in force production has been promoted by previous research, the importance of the trunk and lower body are yet to be established. Eight international level paddlers completed 5 on-water sprint trials during which paddle force and trunk and leg muscle activations were recorded. Significant correlations (
Kepler and the Kuiper Belt
The proposed field-of-view of the Kepler mission is at an ecliptic latitude
of ~55 degrees, where the surface density of scattered Kuiper Belt Objects
(KBOs) is a few percent that in the ecliptic plane. The rate of occultations of
Kepler target stars by scattered KBOs with radii r>10km is ~10^-6 to 10^-4 per
star per year, where the uncertainty reflects the current ignorance of the
thickness of the scattered KBO disk and the faint-end slope of their magnitude
distribution. These occultation events will last only ~0.1% of the planned
t_exp=15 minute integration time, and thus will appear as single data points
that deviate by tiny amounts. However, given the target photometric accuracy of
Kepler, these deviations will nevertheless be highly significant, with typical
signal-to-noise ratios of ~10. I estimate that 1-20 of the 10^5 main-sequence
stars in Kepler's field-of-view will exhibit detectable occultations during its
four-year mission. For unresolved events, the signal-to-noise of individual
occultations scales as t_exp^{-1/2}, and the minimum detectable radius could be
decreased by an order of magnitude to ~1 km by searching the individual
3-second readouts for occultations. I propose a number of methods by which
occultation events may be differentiated from systematic effects. Kepler should
measure or significantly constrain the frequency of highly-inclined, ~10
km-sized KBOs.Comment: 5 pages, 1 figure. No changes. Accepted to ApJ, to appear in the
August 1, 2004 issue (v610
Black Holes Surrounded by Uniformly Rotating Rings
Highly accurate numerical solutions to the problem of Black Holes surrounded
by uniformly rotating rings in axially symmetric, stationary spacetimes are
presented. The numerical methods developed to handle the problem are discussed
in some detail. Related Newtonian problems are described and numerical results
provided, which show that configurations can reach an inner mass-shedding limit
as the mass of the central object increases. Exemplary results for the full
relativistic problem for rings of constant density are given and the
deformation of the event horizon due to the presence of the ring is
demonstrated. Finally, we provide an example of a system for which the angular
momentum of the central Black Hole divided by the square of its mass exceeds
one.Comment: 12 pages, 14 figures, revtex, v4: minor changes, Eq. (17) corrected,
corresponds to version in PR
Tachyons in de Sitter space and analytical continuation from dS/CFT to AdS/CFT
We discuss analytic continuation from d-dimensional Lorentzian de Sitter
(dS) to d-dimensional Lorentzian anti-de Sitter (AdS) spacetime. We
show that AdS, with opposite signature of the metric, can be obtained as
analytic continuation of a portion of dS. This implies that the dynamics of
(positive square-mass) scalar particles in AdS can be obtained from the
dynamics of tachyons in dS. We discuss this correspondence both at the
level of the solution of the field equations and of the Green functions. The
AdS/CFT duality is obtained as analytic continuation of the dS/CFT duality.Comment: 17 pages, 1 figure, JHEP styl
Spectral correlations : understanding oscillatory contributions
We give a different derivation of a relation obtained using a supersymmetric nonlinear sigma model by Andreev and Altshuler [Phys. Rev. Lett. 72, 902 (1995)], which connects smooth and oscillatory components of spectral correlation functions. We show that their result is not specific to the random matrix theory. Also, we show that despite an apparent contradiction, the results obtained using their formula are consistent with earlier perspectives on random matrix models
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