559 research outputs found
On the abundance discrepancy problem in HII regions
The origin of the abundance discrepancy is one of the key problems in the
physics of photoionized nebula. In this work, we analize and discuss data for a
sample of Galactic and extragalactic HII regions where this abundance
discrepancy has been determined. We find that the abundance discrepancy factor
(ADF) is fairly constant and of the order of 2 in all the available sample of
HII regions. This is a rather different behaviour than that observed in
planetary nebulae, where the ADF shows a much wider range of values. We do not
find correlations between the ADF and the O/H, O++/H+ ratios, the ionization
degree, Te(High), Te(Low)/ Te(High), FWHM, and the effective temperature of the
main ionizing stars within the observational uncertainties. These results
indicate that whatever mechanism is producing the abundance discrepancy in HII
regions it does not substantially depend on those nebular parameters. On the
contrary, the ADF seems to be slightly dependent on the excitation energy, a
fact that is consistent with the predictions of the classical temperature
fluctuations paradigm. Finally, we obtain that Te values obtained from OII
recombination lines in HII regions are in agreement with those obtained from
collisionally excited line ratios, a behaviour that is again different from
that observed in planetary nebulae. These similar temperature determinations
are in contradiction with the predictions of the model based on the presence of
chemically inhomogeneous clumps but are consistent with the temperature
fluctuations paradigm. We conclude that all the indications suggest that the
physical mechanism responsible of the abundance discrepancy in HII regions and
planetary nebulae are different.Comment: 14 pages, 8 figures, 9 tables. Accepted for publication in the Ap
New insights into the physical state of gaseous nebulae
The impact of knowledge of H II regions, planetary nebulae and supernova remnants due to International Ultraviolet Explorer is examined. The more relevant aspects related to the physical conditions of gaseous nebulae are reviewed. The following properties of gaseous nebulae are discussed: (1) density and temperature distribution; (2) ionization structure; (3) chemical composition; (4) internal dust; and (5) shock velocity for supernova remnants. The CNO abundances of planetary nebulae are compared with stellar evolution models
Ionization Corrections For Low-Metallicity H II Regions and the Primordial Helium Abundance
Helium and hydrogen recombination lines observed in low-metallicity,
extragalactic H II regions provide the data used to infer the primordial helium
mass fraction, Y_P. The ionization corrections for unseen neutral helium (or
hydrogen) are usually assumed to be absent; i.e., the ionization correction
factor is taken to be unity (icf = 1). In this paper we revisit the question of
the icf for H II regions ionized by clusters of young, hot, metal-poor stars.
Our key result is that for the H II regions used in the determination of Y_P,
there is a ``reverse'' ionization correction: icf < 1. We explore the effect on
the icf of more realistic inhomogeneous H II region models and find that for
those regions ionized by young stars, with ``hard'' radiation spectra, the icf
is reduced further below unity. In Monte Carlos using H II region data from the
literature (Izotov and Thuan 1998) we estimate a reduction in the published
value of Y_P of order 0.003, which is roughly twice as large as the quoted
statistical error in the Y_P determination.Comment: 23 pages, 2 postscript figures; ApJ accepted; minor change
Faint emission lines in the Galactic H II regions M16, M20 and NGC 3603
We present deep echelle spectrophotometry of the Galactic {\hii} regions M16,
M20 and NGC 3603. The data have been taken with the Very Large Telescope
Ultraviolet-Visual Echelle Spectrograph in the 3100 to 10400 \AA range. We have
detected more than 200 emission lines in each region. Physical conditions have
been derived using different continuum and line intensity ratios. We have
derived He, C and O abundances from pure recombination
lines as well as abundances from collisionally excited lines for a large number
of ions of different elements. We have obtained consistent estimations of the
temperature fluctuation parameter, {\ts}, using different methods. We also
report the detection of deuterium Balmer lines up to D (M16) and to
D (M20) in the blue wings of the hydrogen lines, which excitation
mechanism seems to be continuum fluorescence. The temperature fluctuations
paradigm agree with the results obtained from optical CELs and the more
uncertain ones from far IR fine structure CELs in NGC 3603, although, more
observations covering the same volume of the nebula are necessary to obtain
solid conclusions.Comment: 22 pages, 13 Tables, 7 Figures. Accepted for publication by MNRA
The Chemical Composition of the Small Magellanic Cloud H II Region NGC 346 and the Primordial Helium Abundance
Spectrophotometry in the 3400-7400 range is presented for 13 areas of the
brightest H II region in the SMC: NGC 346. The observations were obtained at
CTIO with the 4-m telescope. Based on these observations its chemical
composition is derived. The helium and oxygen abundances by mass are given by:
Y(SMC)=0.2405+-0.0018 and O(SMC)=0.00171+-0.00025. From models and observations
of irregular and blue compact galaxies it is found that dY/dO=3.5+-0.9 and
consequently that the primordial helium abundance by mass is given by:
Yp=0.2345+-0.0026 (1-sigma). This result is compared with values derived from
Big Bang nucleosynthesis, and with other determinations of Yp.Comment: 32 pages + 5 figures Referee Revised Versio
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