2,142 research outputs found
Hydrogen Two-Photon Continuum Emission from the Horseshoe Filament in NGC 1275
Far ultraviolet emission has been detected from a knot of Halpha emission in
the Horseshoe filament, far out in the NGC 1275 nebula. The flux detected
relative to the brightness of the Halpha line in the same spatial region is
very close to that expected from Hydrogen two-photon continuum emission in the
particle heating model of Ferland et al. (2009) if reddening internal to the
filaments is taken into account. We find no need to invoke other sources of far
ultraviolet emission such as hot stars or emission lines from CIV in
intermediate temperature gas to explain these data.Comment: 9 pages, 8 figures. Accepted for publication in MNRA
Atomic Data for Permitted Resonance Lines of Atoms and Ions from H to Si, and S, Ar, Ca and Fe
We list vacuum wavelengths, energy levels, statistical weights, transition
probabilities and oscillator strengths for permitted resonance spectral lines
of all ions of 18 astrophysically important elements (H through Si, S, Ar, Ca,
Fe). Using a compilation of experimental energy levels, we derived accurate
wavelengths for 5599 lines of 1828 ground-term multiplets which have gf-values
calculated in the Opacity Project. We recalculated the Opacity Project
multiplet gf-values to oscillator strengths and transition probabilities of
individual lines. For completeness, we added 372 resonance lines of NeI, ArI,
FeI and FeII ions which are not covered by the Opacity Project.
Intercombination and forbidden lines are not included in the present
compilation.Comment: 6 pages of text, latex, 1 figure, 4 tables; tables in ASCII format
available at ftp://asta.pa.uky.edu/dima/lines/ or at
http://www.pa.uky.edu/~verner/atom.html Accepted by Atomic Data Nucl. Data
Table
Photoionized gas in hydrostatic equilibrium: the role of gravity
We present a method to include the effects of gravity in the plasma physics
code Cloudy. More precisely, a term is added to the desired gas pressure in
order to enforce hydrostatic equilibrium, accounting for both the self-gravity
of the gas and the presence of an optional external potential. As a test case,
a plane-parallel model of the vertical structure of the Milky Way disk near the
solar neighbourhood is considered. It is shown that the gravitational force
determines the scale height of the disk, and it plays a critical role in
setting its overall chemical composition. However, other variables, such as the
shape of incident continuum and the intensity of the Galactic magnetic field,
strongly affect the predicted structure.Comment: 9 pages, 8 figures, MNRAS in pres
Herschel dust emission as a probe of starless cores mass: MCLD 123.5+24.9 of the Polaris Flare
We present newly processed archival Herschel images of molecular cloud MCLD
123.5+24.9 in the Polaris Flare. This cloud contains five starless cores. Using
the spectral synthesis code Cloudy, we explore uncertainties in the derivation
of column densities, hence, masses of molecular cores from Herschel data. We
first consider several detailed grain models that predict far-IR grain
opacities. Opacities predicted by the models differ by more than a factor of
two, leading to uncertainties in derived column densities by the same factor.
Then we consider uncertainties associated with the modified blackbody fitting
process used by observers to estimate column densities. For high column density
clouds (N(H) 10 cm), this fitting technique can
underestimate column densities by about a factor of three. Finally, we consider
the virial stability of the five starless cores in MCLD 123.5+24.9. All of
these cores appear to have strongly sub-virial masses, assuming, as we argue,
that CO line data provide reliable estimates of velocity dispersions.
Evidently, they are not self-gravitating, so it is no surprise that they are
starless.Comment: ApJ, Accepted. Minor typographical errors corrected and figures 6 & 7
updated in v
Locally Optimally-emitting Clouds and the Narrow Emission Lines in Seyfert Galaxies
The narrow emission line spectra of active galactic nuclei are not accurately
described by simple photoionization models of single clouds. Recent Hubble
Space Telescope images of Seyfert 2 galaxies show that these objects are rich
with ionization cones, knots, filaments, and strands of ionized gas. Here we
extend to the narrow line region the ``locally optimally emitting cloud'' (LOC)
model, in which the observed spectra are predominantly determined by powerful
selection effects. We present a large grid of photoionization models covering a
wide range of physical conditions and show the optimal conditions for producing
many of the strongest emission lines. We show that the integrated narrow line
spectrum can be predicted by an integration of an ensemble of clouds, and we
present these results in the form of diagnostic line ratio diagrams making
comparisons with observations. We also predict key diagnostic line ratios as a
function of distance from the ionizing source, and compare these to
observations. The predicted radial dependence of the [O III]/[O II] ratio may
be matched to the observed one in NGC4151, if the narrow line clouds see a more
intense continuum than we see. The LOC scenario when coupled with a simple
Keplerian gravitational velocity field will quite naturally predict the
observed line width versus critical density relationship. The influence of dust
within the ionized portion of the clouds is discussed and we show that the more
neutral gas is likely to be dusty, although a high ionization dust-free region
is most likely present too. This argues for a variety of NLR cloud origins.Comment: 29 pages plus 16 figures, accepted for publication in Ap
Atomic Data for Astrophysics. II. New Analytic Fits for Photoionization Cross Sections of Atoms and Ions
We present a complete set of analytic fits to the non-relativistic
photoionization cross sections for the ground states of atoms and ions of
elements from H through Si, and S, Ar, Ca, and Fe. Near the ionization
thresholds, the fits are based on the Opacity Project theoretical cross
sections interpolated and smoothed over resonances. At higher energies, the
fits reproduce calculated Hartree-Dirac-Slater photoionization cross sections.Comment: 24 pages including Postscript figures and tables, uses aaspp4.sty,
accepted for publication in Astrophysical Journal. Misprint in Eq.(1) is
correcte
Intermediate-line Emission in AGNs: The Effect of Prescription of the Gas Density
The requirement of intermediate line component in the recently observed
spectra of several AGNs points to possibility of the existence of a physically
separate region between broad line region (BLR) and narrow line region (NLR).
In this paper we explore the emission from intermediate line region (ILR) by
using the photoionization simulations of the gas clouds distributed radially
from the AGN center. The gas clouds span distances typical for BLR, ILR and
NLR, and the appearance of dust at the sublimation radius is fully taken into
account in our model. Single cloud structure is calculated under the assumption
of the constant pressure. We show that the slope of the power law cloud density
radial profile does not affect the existence of ILR in major types of AGN. We
found that the low ionization iron line, Fe~II, appears to be highly sensitive
for the presence of dust and therefore becomes potential tracer of dust content
in line emitting regions. We show that the use of disk-like cloud density
profile computed at the upper part of the accretion disc atmosphere reproduces
the observed properties of the line emissivities. In particular, the distance
of H line inferred from our model agrees with that obtained from the
reverberation mapping studies in Sy1 galaxy NGC 5548.Comment: 15 pages, 13 figure
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