499 research outputs found
Recalibration of Pagel's method for HII regions considering the thermal structure, the ionization structure, and the depletion of O into dust grains
Using a sample of 28 HII regions from the literature with measured
temperature inhomogeneity parameter, t^2, we present a statistical correction
to the chemical abundances determined with the Te(4363/5007) method. We used
the t^2 values to correct the oxygen gaseous abundances and consider the oxygen
depletion into dust to calculate the total abundances for these objects. This
correction is used to obtain a new calibration of Pagel's strong-line method,
R_{23}, to determine oxygen abundances in HII regions. Our new calibration
simultaneously considers the temperature structure, the ionization structure,
and the fraction of oxygen depleted into dust grains. Previous calibrations in
the literature have included one or two of these factors; this is the first
time all three are taken into account. This recalibration conciliates the
systematic differences among the temperatures found from different methods. We
find that the total correction due to thermal inhomogeneities and dust
depletion amounts to an increase in the O/H ratio of HII regions by factors of
1.7 to 2.2 (or 0.22 to 0.35 dex). This result has important implications in
various areas of astrophysics such as the study of the higher end of the
initial mass function, the star formation rate, and the mass-metallicity
relation of galaxies, among others.Comment: 16 pages (preprint), 4 figures, 1 Table, accepted in ApJ
Analysis of two SMC HII Regions Considering Thermal Inhomogeneities: Implications for the Determinations of Extragalactic Chemical Abundances
We present long slit spectrophotometry considering the presence of thermal
inhomogeneities (t^2) of two HII regions in the Small Magellanic Cloud (SMC):
NGC 456 and NGC 460. Physical conditions and chemical abundances were
determined for three positions in NGC 456 and one position in NGC 460, first
under the assumption of uniform temperature and then allowing for the
possibility of thermal inhomogeneities. We determined t^2 values based on three
different methods: i) by comparing the temperature derived using oxygen
forbidden lines with the temperature derived using helium recombination lines,
ii) by comparing the abundances derived from oxygen forbidden lines with those
derived from oxygen recombination lines, and iii) by comparing the abundances
derived from ultraviolet carbon forbidden lines with those derived from optical
carbon recombination lines. The first two methods averaged t^2=0.067+-0.013 for
NGC 456 and t^2=0.036+-0.027 for NGC 460. These values of t^2 imply that when
gaseous abundances are determined with collisionally excited lines they are
underestimated by a factor of nearly 2. From these objects and others in the
literature, we find that in order to account for thermal inhomogeneities and
dust depletion, the O/H ratio in low metallicity HII regions should be
corrected by 0.25-0.45 dex depending on the thermal structure of the nebula, or
by 0.35 dex if such information is not available.Comment: Accepted for publication in The Astrophysical Journal. 41 pages in
pre-print format. 3 figure
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
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
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