440 research outputs found
Citizen assessment of the political system is fostered by rational considerations rather than virtuousness
Scholars have often assumed that citizens value fairness with respect to the inclusion and representation of different groups in the electoral process, and therefore are likely to favour proportional over majoritarian systems. However, in a recent study Benjamin Ferland found that citizens actually prefer their party to be advantaged at the expense of others, indicating citizen satisfaction with the political system is shaped more by rational considerations than by virtuousness
Ion-by-ion Cooling efficiencies
We present ion-by-ion cooling efficiencies for low-density gas. We use Cloudy
(ver. 08.00) to estimate the cooling efficiencies for each ion of the first 30
elements (H-Zn) individually. We present results for gas temperatures between
1e4 and 1e8K, assuming low densities and optically thin conditions. When
nonequilibrium ionization plays a significant role the ionization states
deviate from those that obtain in collisional ionization equilibrium (CIE), and
the local cooling efficiency at any given temperature depends on specific
non-equilibrium ion fractions. The results presented here allow for an
efficient estimate of the total cooling efficiency for any ionic composition.
We also list the elemental cooling efficiencies assuming CIE conditions. These
can be used to construct CIE cooling efficiencies for non-solar abundance
ratios, or to estimate the cooling due to elements not explicitly included in
any nonequilibrium computation. All the computational results are listed in
convenient online tables.Comment: Submitted to ApJS. Electronic data available at
http://wise-obs.tau.ac.il/~orlyg/ion_by_ion
A 3D Monte Carlo Photoionization Code for Modeling Diffuse Ionized Gas
We have developed a three dimensional Monte Carlo photoionization code
tailored for the study of Galactic H II regions and the percolation of ionizing
photons in diffuse ionized gas. We describe the code, our calculation of
photoionization, heating & cooling, and the approximations we have employed for
the low density H II regions we wish to study. Our code gives results in
agreement with the Lexington H II region benchmarks. We show an example of a 2D
shadowed region and point out the very significant effect that diffuse
radiation produced by recombinations of helium has on the temperature within
the shadow.Comment: MNRAS accepte
PPAK Integral Field Spectroscopy survey of the Orion Nebula: Data Release
We present a low-resolution spectroscopic survey of the Orion nebula which
data we release for public use. In this article, we intend to show the possible
applications of this dataset analyzing some of the main properties of the
nebula. We perform an integral field spectroscopy mosaic of an area of ~5' X 6'
centered on the Trapezium region of the nebula, including the ionization front
to the south-east. The analysis of the line fluxes and line ratios of both the
individual and integrated spectra allowed us to determine the main
characteristics of the ionization throughtout the nebula.The final dataset
comprises 8182 individual spectra, which sample each one a circular area of
\~2.7" diameter. The data can be downloaded as a single row-stacked spectra
fits file plus a position table or as an interpolated datacube with a final
sampling of 1.5"/pixel. The integrated spectrum across the field-of-view was
used to obtain the main integrated properties of the nebula, including the
electron density and temperature, the dust extinction, the Halpha integrated
flux (after correcting for dust reddening), and the main diagnostic line
ratios. The individual spectra were used to obtain line intensity maps of the
different detected lines. These maps were used to study the distribution of the
ionized hydrogen, the dust extinction, the electron density and temperature,
and the helium and oxygen abundance...Comment: 13 pages, 8 figures, accepted for publishing in Astronomy &
Astrophysic
Uncertainties in Theoretical HeI Emissivities: HII Regions, Primordial Abundance, and Cosmological Recombination
A number of recent works in astronomy and cosmology have relied upon
theoretical He I emissivities, but we know of no effort to quantify the
uncertainties in the atomic data. We analyze and assign uncertainties to all
relevant atomic data, perform Monte Carlo analyses, and report standard
deviations in the line emissivities. We consider two sets of errors, which we
call "optimistic" and "pessimistic." We also consider three different
conditions, corresponding to prototypical Galactic and extragalactic H II
regions and the epoch of cosmological recombination. In the extragalactic H II
case, the errors we obtain are comparable to or larger than the errors in some
recent calculations, including those derived from CMB observations. We
demonstrate a systematic effect on primordial abundance calculations; this
effect cannot be reduced by observing a large number of objects. In the
cosmological recombination case, the errors are comparable to many of the
effects considered in recent calculations.Comment: 5 pages, 3 figures, accepted to MNRAS Letter
A study of the neglected Galactic HII region NGC 2579 and its companion ESO 370-9
The Galactic HII region NGC 2579 has stayed undeservedly unexplored due to
identification problems which persisted until recently. Both NGC 2579 and its
companion ESO 370-9 have been misclassified as planetary or reflection nebula,
confused with each other and with other objects. Due to its high surface
brightness, high excitation, angular size of few arcminutes and relatively low
interstellar extinction, NGC 2579 is an ideal object for investigations in the
optical range. Located in the outer Galaxy, NGC 2579 is an excellent object for
studying the Galactic chemical abundance gradients. In this paper we present
the first comprehensive observational study on the nebular and stellar
properties of NGC 2579 and ESO 370-9, including the determination of electron
temperature, density structure, chemical composition, kinematics, distance, and
the identification and spectral classification of the ionizing stars, and
discuss the nature of ESO 370-9. Long slit spectrophotometric data in the
optical range were used to derive the nebular electron temperature, density and
chemical abundances and for the spectral classification of the ionizing star
candidates. Halpha and UBV CCD photometry was carried out to derive stellar
distances from spectroscopic parallax and to measure the ionizing photon flux.Comment: To be published in Astronomy & Astrophysic
A comprehensive study of reported high metallicity giant HII regions. I. Detailed abundance analysis
We present long-slit observations in the optical and near infrared of
fourteen HII regions in the spiral galaxies: NGC 628, NGC 925, NGC 1232 and NGC
1637, all of them reported to have solar or oversolar abundances according to
empirical calibrations. For seven of the observed regions, ion-weighted
temperatures from optical forbidden auroral to nebular line ratios have been
obtained and for six of them, the oxygen abundances derived by standard methods
turn out to be significantly lower than solar. The other one, named CDT1 in NGC
1232, shows an oxygen abundance of 12+log(O/H) = 8.95+-0.20 and constitutes, to
the best of our knowledge, the first high metallicity HII region for which
accurate line temperatures, and hence elemental abundances, have been derived.
For the rest of the regions no line temperature measurements could be made
and the metallicity has been determined by means of both detailed
photoionisation modelling and the sulphur abundance parameter S_23. Only one of
these regions shows values of O_23 and S_23 implying a solar or oversolar
metallicity.
According to our analysis, only two of the observed regions can therefore be
considered as of high metallicity. The two of them fit the trends previously
found in other high metallicity HII regions, i.e. N/O and S/O abundance ratios
seem to be higher and lower than solar respectively.Comment: Accepted for publication by MNRA
Precision abundance analysis of bright HII galaxies
We present high signal-to-noise spectrophotometric observations of seven
luminous HII galaxies. The observations have been made with the use of a
double-arm spectrograph which provides spectra with a wide wavelength coverage,
from 3400 to 10400\AA free of second order effects, of exactly the same region
of a given galaxy. These observations are analysed applying a methodology
designed to obtain accurate elemental abundances of oxygen, sulphur, nitrogen,
neon, argon and iron in the ionized gas. Four electron temperatures and one
electron density are derived from the observed forbidden line ratios using the
five-level atom approximation. For our best objects errors of 1% in
t_e([OIII]), 3% in t_e([OII]) and 5% in t_e([SIII]) are achieved with a
resulting accuracy of 7% in total oxygen abundances, O/H.
The ionisation structure of the nebulae can be mapped by the theoretical
oxygen and sulphur ionic ratios, on the one side, and the corresponding
observed emission line ratios, on the other -- the \eta and \eta' plots --. The
combination of both is shown to provide a means to test photo-ionisation model
sequences currently applied to derive elemental abundances in HII galaxies.Comment: 24 pages, 8 figures, accepted by MNRA
The effect of photo-ionization on the cooling rates of enriched, astrophysical plasmas
Radiative cooling is central to a wide range of astrophysical problems.
Despite its importance, cooling rates are generally computed using very
restrictive assumptions, such as collisional ionization equilibrium and solar
relative abundances. We simultaneously relax both assumptions and investigate
the effects of photo-ionization of heavy elements by the meta-galactic UV/X-ray
background and of variations in relative abundances on the cooling rates of
optically thin gas in ionization equilibrium. We find that photo-ionization by
the meta-galactic background radiation reduces the net cooling rates by up to
an order of magnitude for gas densities and temperatures typical of the
shock-heated intergalactic medium and proto-galaxies. In addition,
photo-ionization changes the relative contributions of different elements to
the cooling rates. We conclude that photo-ionization by the ionizing background
and heavy elements both need to be taken into account in order for the cooling
rates to be correct to order of magnitude. Moreover, if the rates need to be
known to better than a factor of a few, then departures of the relative
abundances from solar need to be taken into account. We propose a method to
compute cooling rates on an element-by-element basis by interpolating
pre-computed tables that take photo-ionization into account. We provide such
tables for a popular model of the evolving UV/X-ray background radiation,
computed using the photo-ionization package CLOUDY.Comment: 10 pages, 6 figures, Accepted for publication in MNRAS. One figure
added and minor textual changes made to first version. Downloadable tables
and videos available at http://www.strw.leidenuniv.nl/WSS08
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
- âŠ