726 research outputs found
Cloudy_3D, a new pseudo-3D photoionization code
We developed a new quick pseudo-3D photoionization code based on Cloudy (G.
Ferland) and IDL (RSI) tools. The code is running the 1D photoionization code
Cloudy various times, changing at each run the input parameters (e.g. inner
radius, density law) according to an angular law describing the morphology of
the object. Then a cube is generated by interpolating the outputs of Cloudy. In
each cell of the cube, the physical conditions (electron temperature and
density, ionic fractions) and the emissivities of lines are determined.
Associated tools (VISNEB and VELNEB_3D) are used to rotate the nebula and to
compute surface brightness maps and emission line profiles, given a velocity
law and taking into account the effect of the thermal broadening and eventually
the turbulence. Integrated emission line profiles are computed, given aperture
shapes and positions (seeing and instrumental width effects are included). The
main advantage of this tool is the short time needed to compute a model (a few
tens minutes).Comment: To appear in Proc. IAU Symp. 234, Planetary Nebulae in Our Galaxy and
Beyond (3-7 Apr 2006), eds. M.J. Barlow & R.H. Mendez (Cambridge Univ. Press
Excitation properties of galaxies with the highest [OIII]/[OII] ratios: No evidence for massive escape of ionizing photons
The possibility that star-forming galaxies may leak ionizing photons is at
the heart of many present-day studies that investigate the reionization of the
Universe. We test this hypothesis on local blue compact dwarf galaxies of very
high excitation. We assembled a sample of such galaxies by examining the
spectra from Data Releases 7 and 10 of the Sloan Digital Sky Survey. We argue
that reliable conclusions cannot be based on strong lines alone, and adopt a
strategy that includes important weak lines such as [OI] and the
high-excitation HeII and [ArIV] lines. Our analysis is based on purely
observational diagrams and on a comparison of photoionization models with
well-chosen emission-line ratio diagrams. We show that spectral energy
distributions from current stellar population synthesis models cannot account
for all the observational constraints, which led us to mimick several scenarios
that could explain the data. These include the additional presence of hard
X-rays or of shocks. We find that only ionization-bounded models (or models
with an escape fraction of ionizing photons lower than 10%) are able to
simultaneously explain all the observational constraints.Comment: accepted in Astronomy & Astrophysic
O stars effective temperature and HII regions ionization parameter gradients in the Galaxy
Extensive photoionization model grids are computed for single star HII
regions using stellar atmosphere models from the WM-basic code. Mid-IR emission
line intensities are predicted and diagnostic diagrams of [NeIII]/[NeII] and
[SIV]/[SIII] excitation ratio are build, taking into account the metallicities
of both the star and the HII region. The diagrams are used in conjunction with
galactic HII region observations obtained with the ISO Observatory to determine
the effective temperature Teff of the exciting O stars and the mean ionization
parameter U. Teff and U are found to increase and decrease, respectively, with
the metallicity of the HII region represented by the [Ne/Ne_sol] ratio. No
evidence is found for gradients of Teff or U with galactocentric distance Rgal.
The observed excitation sequence with Rgal is mainly due to the effect of the
metallicity gradient on the spectral ionizing shape, upon which the effect of
an increase in Teff with Z is superimposed. We show that not taking properly
into account the effect of metallicity on the ionizing shape of the stellar
atmosphere would lead to an apparent decrease of Teff with Z and an increase of
Teff with Rgal.Comment: Accepted in Ap
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