59 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
On the O/H, Mg/H, Si/H and Fe/H Gas and Dust Abundance Ratios in Galactic and Extragalactic H II Regions
We derive the Mg/H ratio in the Orion nebula and in 30 Doradus. We also
derive the O/H and the Fe/O ratios in the extremely metal poor galaxy SBS
0335-052. We estimate the dust depletions of Mg, Si, and Fe in Galactic and
extragalactic H II regions. Based on these depletions we estimate the fraction
of O atoms embedded in dust as a function of the O/H ratio. We find an
increasing depletion of O with increasing O/H. The O depletion increases from
about 0.08 dex, for the metal poorest H II regions known, to about 0.12 dex,
for metal rich H II regions. This depletion has to be considered when comparing
nebular with stellar abundances.Comment: 22 pages, 6 figures, submitted to The Astrophysical Journal.
Rewritten for clarity (following the recomendations by the ApJ referee
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
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