278 research outputs found
The effects of spatially distributed ionisation sources on the temperature structure of HII region
Spatially resolved studies of star forming regions show that the assumption
of spherical geometry is not realistic in most cases, with a major complication
posed by the gas being ionised by multiple non-centrally located stars or star
clusters. We try to isolate the effects of multiple non-centrally located stars
on the temperature and ionisation structure of HII regions, via the
construction of 3D photoionisation models using the 3D Monte Carlo
photoionisation code MOCASSIN. We find that the true temperature fluctuations
due to the stellar distribution (as opposed to the large-scale temperature
gradients due to other gas properties) are small in all cases and not a
significant cause of error in metallicity studies. Strong emission lines from
HII regions are often used to study the metallicity of star-forming regions. We
compare integrated emission line spectra from our models and quantify any
systematic errors caused by the simplifying assumption of a single, central
location for all ionising sources. We find that the dependence of the
metallicity indicators on the ionisation parameter causes a clear bias, due to
the fact that models with a fully distributed configuration of stars always
display lower ionisation parameters than their fully concentrated counterparts.
The errors found imply that the geometrical distribution of ionisation sources
may partly account for the large scatter in metallicities derived using
model-calibrated empirical methods.Comment: 13 pages, 6 figures, Accepted by MNRA
Primordial 4He abundance: a determination based on the largest sample of HII regions with a methodology tested on model HII regions
We verified the validity of the empirical method to derive the 4He abundance
used in our previous papers by applying it to CLOUDY (v13.01) models. Using
newly published HeI emissivities, for which we present convenient fits as well
as the output CLOUDY case B hydrogen and HeI line intensities, we found that
the empirical method is able to reproduce the input CLOUDY 4He abundance with
an accuracy of better than 1%. The CLOUDY output data also allowed us to derive
the non-recombination contribution to the intensities of the strongest Balmer
hydrogen Halpha, Hbeta, Hgamma, and Hdelta emission lines and the ionisation
correction factors for He. With these improvements we used our updated
empirical method to derive the 4He abundances and to test corrections for
several systematic effects in a sample of 1610 spectra of low-metallicity
extragalactic HII regions, the largest sample used so far. From this sample we
extracted a subsample of 111 HII regions with Hbeta equivalent width EW(Hbeta)
> 150A, with excitation parameter x = O^{2+}/O > 0.8, and with helium mass
fraction Y derived with an accuracy better than 3%. With this subsample we
derived the primordial 4He mass fraction Yp = 0.254+/-0.003 from linear
regression Y-O/H. The derived value of Yp is higher at the 68% confidence level
(CL) than that predicted by the standard big bang nucleosynthesis (SBBN) model,
possibly implying the existence of different types of neutrino species in
addition to the three known types of active neutrinos. Using the most recently
derived primordial abundances D/H = (2.60+/-0.12)x10^{-5} and Yp =
0.254+/-0.003 and the chi^2 technique, we found that the best agreement between
abundances of these light elements is achieved in a cosmological model with
baryon mass density Omegab h^2 = 0.0234+/-0.0019 (68% CL) and an effective
number of the neutrino species Neff = 3.51+/-0.35 (68% CL).Comment: 23 pages, 14 figures, accepted for publication in Astronomy and
Astrophysic
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
Abundances in the Galactic bulge: results from planetary nebulae and giant stars
Our understanding of the chemical evolution of the Galactic bulge requires
the determination of abundances in large samples of giant stars and planetary
nebulae (PNe). We discuss PNe abundances in the Galactic bulge and compare
these results with those presented in the literature for giant stars. We
present the largest, high-quality data-set available for PNe in the direction
of the Galactic bulge (inner-disk/bulge). For comparison purposes, we also
consider a sample of PNe in the Large Magellanic Cloud (LMC). We derive the
element abundances in a consistent way for all the PNe studied. By comparing
the abundances for the bulge, inner-disk, and LMC, we identify elements that
have not been modified during the evolution of the PN progenitor and can be
used to trace the bulge chemical enrichment history. We then compare the PN
abundances with abundances of bulge field giant. At the metallicity of the
bulge, we find that the abundances of O and Ne are close to the values for the
interstellar medium at the time of the PN progenitor formation, and hence these
elements can be used as tracers of the bulge chemical evolution, in the same
way as S and Ar, which are not expected to be affected by nucleosynthetic
processes during the evolution of the PN progenitors. The PN oxygen abundance
distribution is shifted to lower values by 0.3 dex with respect to the
distribution given by giants. A similar shift appears to occur for Ne and S. We
discuss possible reasons for this PNe-giant discrepancy and conclude that this
is probably due to systematic errors in the abundance derivations in either
giants or PNe (or both). We issue an important warning concerning the use of
absolute abundances in chemical evolution studies.Comment: 23 pages, 15 figures, 16 pages of online material, A&A in pres
Planetary nebulae in M32 and the bulge of M31: Line intensities and oxygen abundances
We present spectroscopy of planetary nebulae in M32 and in the bulge of M31
that we obtained with the MOS spectrograph at the Canada-France-Hawaii
Telescope. Our sample includes 30 planetary nebulae in M31 and 9 planetary
nebulae in M32. We also observed one H II region in the disk of M31. We
detected [O III]4363 in 18 of the planetary nebulae, 4 in M32 and 14
in the bulge of M31. We use our line intensities to derive electron
temperatures and oxygen abundances for the planetary nebulae.Comment: 17 pages, 12 figures, accepted by Astronomy & Astrophysics Supplement
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