4,880 research outputs found
Radiative Transfer Effects in He I Emission Lines
We consider the effect of optical depth of the 2 ^{3}S level on the nebular
recombination spectrum of He I for a spherically symmetric nebula with no
systematic velocity gradients. These calculations, using many improvements in
atomic data, can be used in place of the earlier calculations of Robbins. We
give representative Case B line fluxes for UV, optical, and IR emission lines
over a range of physical conditions: T=5000-20000 K, n_{e}=1-10^{8} cm^{-3},
and tau_{3889}=0-100. A FORTRAN program for calculating emissivities for all
lines arising from quantum levels with n < 11 is also available from the
authors.
We present a special set of fitting formulae for the physical conditions
relevant to low metallicity extragalactic H II regions: T=12,000-20,000 K,
n_{e}=1-300 cm^{-3}, and tau_{3889} < 2.0. For this range of physical
conditions, the Case B line fluxes of the bright optical lines 4471 A, 5876 A,
and 6678 A, are changed less than 1%, in agreement with previous studies.
However, the 7065 A corrections are much smaller than those calculated by
Izotov & Thuan based on the earlier calculations by Robbins. This means that
the 7065 A line is a better density diagnostic than previously thought. Two
corrections to the fitting functions calculated in our previous work are also
given.Comment: To be published in 10 April 2002 ApJ; relevant code available at
ftp://wisp.physics.wisc.edu/pub/benjamin/Heliu
Theoretical He I Emissivities in the Case B Approximation
We calculate the He I case B recombination cascade spectrum using improved
radiative and collisional data. We present new emissivities over a range of
electron temperatures and densities. The differences between our results and
the current standard are large enough to have a significant effect not only on
the interpretation of observed spectra of a wide variety of objects but also on
determinations of the primordial helium abundance.Comment: Accepted to ApJ
Contemporary Innovation Policy and Instruments: Challenges and Implications
In this paper we review major theoretical (neoclassical economics, evolutionary, systemic and knowledge-based) insights about innovation and we analyse their implications for the characteristics of contemporary innovation policy and instruments. We show that the perspectives complement each other but altogether reveal the need to redefine the current general philosophy as well as the modes of operationalisation of contemporary innovation policy. We argue that systemic instruments ensuring proper organisation of innovation systems give a promise of increased rates and desired (more sustainable) direction of innovation.systemic instruments, innovation policy, innovation theory, policy mix, innovation system, sustainability
The Primordial Abundance of He4: An Update
We include new data in an updated analysis of helium in low metallicity
extragalactic HII regions with the goal of deriving the primordial abundance of
He4 (Y_P). We show that the new observations of Izotov et al (ITL) are
consistent with previous data. However they should not be taken in isolation to
determine (Y_P) due to the lack of sufficiently low metallicity points. We use
the extant data in a semi-empirical approach to bounding the size of possible
systematic uncertainties in the determination of (Y_P). Our best estimate for
the primordial abundance of He4 assuming a linear relation between He4 and O/H
is Y_P = 0.230 \pm 0.003 (stat) based on the subset of HII regions with the
lowest metallicity; for our full data set we find Y_P = 0.234 \pm 0.002 (stat).
Both values are entirely consistent with our previous results. We discuss the
implications of these values for standard big bang nucleosynthesis (SBBN),
particularly in the context of recent measurements of deuterium in high
redshift, low metallicity QSO absorption-line systems.Comment: 26 pages, latex, 6 ps figure
A Dataset on Comparative Historical National Accounts, ca.1870-1950:A Time-Series Perspective
Wall-bounded turbulent flows at high Reynolds numbers: Recent advances and key issues
Wall-bounded turbulent flows at high Reynolds numbers have become an increasingly active area of
research in recent years. Many challenges remain in theory, scaling, physical understanding,
experimental techniques, and numerical simulations. In this paper we distill the salient advances of
recent origin, particularly those that challenge textbook orthodoxy. Some of the outstanding
questions, such as the extent of the logarithmic overlap layer, the universality or otherwise of the
principal model parameters such as the von Kármán “constant,” the parametrization of roughness
effects, and the scaling of mean flow and Reynolds stresses, are highlighted. Research avenues that
may provide answers to these questions, notably the improvement of measuring techniques and the
construction of new facilities, are identified. We also highlight aspects where differences of opinion
persist, with the expectation that this discussion might mark the beginning of their resolution
Improving Predictions for Helium Emission Lines
We have combined the detailed He I recombination model of Smits with the
collisional transitions of Sawey & Berrington in order to produce new accurate
helium emissivities that include the effects of collisional excitation from
both the 2 (3)S and 2 (1) S levels. We present a grid of emissivities for a
range of temperature and densities along with analytical fits and error
estimates.
Fits accurate to within 1% are given for the emissivities of the brightest
lines over a restricted range for estimates of primordial helium abundance. We
characterize the analysis uncertainties associated with uncertainties in
temperature, density, fitting functions, and input atomic data. We estimate
that atomic data uncertainties alone may limit abundance estimates to an
accuracy of 1.5%; systematic errors may be greater than this. This analysis
uncertainty must be incorporated when attempting to make high accuracy
estimates of the helium abundance. For example, in recent determinations of the
primordial helium abundance, uncertainties in the input atomic data have been
neglected.Comment: ApJ, accepte
New limits on the population of normal and millisecond pulsars in the Large and Small Magellanic Clouds
We model the potentially observable populations of normal and millisecond
radio pulsars in the Large and Small Magellanic Clouds (LMC and SMC) where the
known population currently stands at 19 normal radio pulsars. Taking into
account the detection thresholds of previous surveys, and assuming optimal
period and luminosity distributions based on studies of Galactic pulsars, we
estimate there are (1.79 +/- 0.20) x 10^4 and (1.09 +/- 0.16) x 10^4 normal
pulsars in the LMC and SMC respectively. When we attempt to correct for beaming
effects, and the fraction of high-velocity pulsars which escape the clouds, we
estimate birth rates in both the LMC and SMC to be comparable and in the range
0.5--1 pulsar per century. Although higher than estimates for the rate of
core-collapse supernovae in the clouds, these pulsar birth rates are consistent
with historical supernova observations in the past 300 yr. A substantial
population of active radio pulsars (of order a few hundred thousand) have
escaped the LMC and SMC and populate the local intergalactic medium. For the
millisecond pulsar (MSP) population, the lack of any detections from current
surveys leads to respective upper limits (at the 95% confidence level) of
15,000 for the LMC and 23,000 for the SMC. Several MSPs could be detected by a
currently ongoing survey of the SMC with improved time and frequency resolution
using the Parkes multibeam system. Giant-pulse emitting neutron stars could
also be seen by this survey.Comment: 5 pages, 2 figures, accepted for publication in MNRAS Letter
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