4,911 research outputs found
The Formation of Population III Binaries from Cosmological Initial Conditions
Previous high resolution cosmological simulations predict the first stars to
appear in the early universe to be very massive and to form in isolation. Here
we discuss a cosmological simulation in which the central 50 solar mass clump
breaks up into two cores, having a mass ratio of two to one, with one fragment
collapsing to densities of 10^{-8} g/cc. The second fragment, at a distance of
800 astronomical units, is also optically thick to its own cooling radiation
from molecular hydrogen lines, but is still able to cool via collision-induced
emission. The two dense peaks will continue to accrete from the surrounding
cold gas reservoir over a period of 10^5 years and will likely form a binary
star system.Comment: Accepted by Science, first published online on July 9, 2009 in
Science Express. 16 pages, 4 figures, includes supporting online materia
Determination of chromium in titanium dioxide pigments by atomic spectrometry
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Operator bases, -matrices, and their partition functions
Relativistic quantum systems that admit scattering experiments are
quantitatively described by effective field theories, where -matrix
kinematics and symmetry considerations are encoded in the operator spectrum of
the EFT. In this paper we use the -matrix to derive the structure of the EFT
operator basis, providing complementary descriptions in (i) position space
utilizing the conformal algebra and cohomology and (ii) momentum space via an
algebraic formulation in terms of a ring of momenta with kinematics implemented
as an ideal. These frameworks systematically handle redundancies associated
with equations of motion (on-shell) and integration by parts (momentum
conservation).
We introduce a partition function, termed the Hilbert series, to enumerate
the operator basis--correspondingly, the -matrix--and derive a matrix
integral expression to compute the Hilbert series. The expression is general,
easily applied in any spacetime dimension, with arbitrary field content and
(linearly realized) symmetries.
In addition to counting, we discuss construction of the basis. Simple
algorithms follow from the algebraic formulation in momentum space. We
explicitly compute the basis for operators involving up to scalar fields.
This construction universally applies to fields with spin, since the operator
basis for scalars encodes the momentum dependence of -point amplitudes.
We discuss in detail the operator basis for non-linearly realized symmetries.
In the presence of massless particles, there is freedom to impose additional
structure on the -matrix in the form of soft limits. The most na\"ive
implementation for massless scalars leads to the operator basis for pions,
which we confirm using the standard CCWZ formulation for non-linear
realizations.Comment: 75 pages plus appendice
First Stars III Conference Summary
The understanding of the formation, life, and death of Population III stars,
as well as the impact that these objects had on later generations of structure
formation, is one of the foremost issues in modern cosmological research and
has been an active area of research during the past several years. We summarize
the results presented at "First Stars III," a conference sponsored by Los
Alamos National Laboratory, the Kavli Institute for Particle Astrophysics and
Cosmology, and the Joint Institute for Nuclear Astrophysics. This conference,
the third in a series, took place in July 2007 at the La Fonda Hotel in Santa
Fe, New Mexico, U.S.A.Comment: 11 pages, no figures; Conference summary for First Stars III, which
was held in Santa Fe, NM on July 15-20, 2007. To appear in "Proceedings of
First Stars III," Eds. Brian W. O'Shea, Alexander Heger & Tom Abe
Maximum likelihood and pseudo score approaches for parametric time-to-event analysis with informative entry times
We develop a maximum likelihood estimating approach for time-to-event Weibull
regression models with outcome-dependent sampling, where sampling of subjects
is dependent on the residual fraction of the time left to developing the event
of interest. Additionally, we propose a two-stage approach which proceeds by
iteratively estimating, through a pseudo score, the Weibull parameters of
interest (i.e., the regression parameters) conditional on the inverse
probability of sampling weights; and then re-estimating these weights (given
the updated Weibull parameter estimates) through the profiled full likelihood.
With these two new methods, both the estimated sampling mechanism parameters
and the Weibull parameters are consistently estimated under correct
specification of the conditional referral distribution. Standard errors for the
regression parameters are obtained directly from inverting the observed
information matrix in the full likelihood specification and by either
calculating bootstrap or robust standard errors for the hybrid pseudo
score/profiled likelihood approach. Loss of efficiency with the latter approach
is considered. Robustness of the proposed methods to misspecification of the
referral mechanism and the time-to-event distribution is also briefly examined.
Further, we show how to extend our methods to the family of parametric
time-to-event distributions characterized by the generalized gamma
distribution. The motivation for these two approaches came from data on time to
cirrhosis from hepatitis C viral infection in patients referred to the
Edinburgh liver clinic. We analyze these data here.Comment: Published in at http://dx.doi.org/10.1214/14-AOAS725 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
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