2,186 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
The Curious Conundrum Regarding Sulfur Abundances In Planetary Nebulae
Sulfur abundances derived from optical emission line measurements and
ionization correction factors in planetary nebulae are systematically lower
than expected for the objects' metallicities. We have carefully considered a
large range of explanations for this "sulfur anomaly", including: (1)
correlations between the size of the sulfur deficit and numerous nebular and
central star properties; (2) ionization correction factors which under-correct
for unobserved ions; (3) effects of dielectronic recombination on the sulfur
ionization balance; (4) sequestering of S into dust and/or molecules; and (5)
excessive destruction of S or production of O by AGB stars. It appears that all
but the second scenario can be ruled out. However, we find evidence that the
sulfur deficit is generally reduced but not eliminated when S^+3 abundances
determined directly from IR measurements are used in place of the customary
sulfur ionization correction factor. We tentatively conclude that the sulfur
anomaly is caused by the inability of commonly used ICFs to properly correct
for populations of ionization stages higher than S^+2.Comment: 40 pages, 14 figures, 3 tables. Accepted for publication in the
Astrophysical Journa
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