293 research outputs found
Oxygen abundances in dwarf irregular galaxies and the metallicity - luminosity relationship
The low-luminosity dwarf irregular galaxies are considered. The oxygen
abundances in HII regions of dwarf irregular galaxies were recalculated from
published spectra through the recently suggested P - method. It has been found
that the metallicity of low-luminosity dwarf irregular galaxies, with a few
exceptions, correlates well with galaxy luminosity. The dispersion of oxygen
abundances around the metallicity - luminosity relationship increases with
decreasing of galaxy luminosity, as was found by Richer and McCall (1995). No
relationship between the oxygen abundance and the absolute magnitude in the
blue band for irregular galaxies obtained by Hidalgo-Gamez and Olofsson (1998)
can be explained by the large uncertainties in the oxygen abundances derived
through the Te - method, that in turn can be explained by the large
uncertainties in the measurements of the strengths of the weak oxygen line
[OIII]4363 used in the Te - method.Comment: 9 pages, 5 figures, accepted for publication in Astronomy and
Astrophysic
Abundance determinations in HII regions: model fitting versus Te-method
The discrepancy between the oxygen abundances in high-metallicity HII regions
determined through the Te-method (and/or through the corresponding "strong
lines - oxygen abundance" calibration) and that determined through the model
fitting (and/or through the corresponding "strong lines - oxygen abundance"
calibration) is discussed. It is suggested to use the interstellar oxygen
abundance in the solar vicinity, derived with very high precision from the
high-resolution observations of the weak interstellar absorption lines towards
the stars, as a "Rosetta stone" to verify the validity of the oxygen abundances
derived in HII regions with the Te-method at high abundances. The agreement
between the value of the oxygen abundance at the solar galactocentric distance
traced by the abundances derived in HII regions through the Te-method and that
derived from the interstellar absorption lines towards the stars is strong
evidence in favor of that i) the two-zone model for Te seems to be a realistic
interpretation of the temperature structure within HII regions, and ii) the
classic Te-method provides accurate oxygen abundances in HII regions. It has
been concluded that the "strong lines - oxygen abundance" calibrations must be
based on the HII regions with the oxygen abundances derived with the Te-method
but not on the existing grids of the models for HII regions.Comment: 5 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
Abundance determination in HII regions from spectra without the [OII]3727+3729 line
We suggest an empirical calibration for determination of oxygen and nitrogen
abundances and electron temperature in HII regions where the [OII]3727+3729
line (R_2) is not available. The calibration is based on the strong emission
lines of OIII, NII, and SII (NS calibration) and derived using the spectra of
HII regions with measured electron temperatures as calibration datapoints. The
NS calibration makes it possible to derive abundances for HII regions in nearby
galaxies from the SDSS spectra where R_2 line is out of the measured wavelength
range, but can also be used for the oxygen and nitrogen abundances
determinations in any HII region independently whether the nebular oxygen line
[OII]3727+3729 is available or not. The NS calibration provides reliable oxygen
and nitrogen abundances for HII regions over the whole range of metallicities.Comment: 6 pages, 4 figures, accepted for publication in the MNRA
Chemical abundances in spiral and irregular galaxies. O and N abundances derived from global emission--line spectra
The validity of oxygen and nitrogen abundances derived from the global
emission-line spectra of galaxies via the P-method has been investigated using
a collection of published spectra of individual HII regions in irregular and
spiral galaxies. The conclusions of Kobulnicky, Kennicutt & Pizagno (1999) that
global emission-line spectra can reliably indicate the chemical properties of
galaxies has been confirmed. It has been shown that the comparison of the
global spectrum of a galaxy with a collection of spectra of individual HII
regions can be used to distinguish high and low metallicity objects and to
estimate accurate chemical abundances in a galaxy. The oxygen and nitrogen
abundances in samples of UV-selected and normal nearby galaxies have been
determined. It has been found that the UV-selected galaxies occupy the same
area in the N/O -- O/H diagram as individual HII regions in nearby galaxies.
Finally, we show that intermediate-redshift galaxies systematically deviate
from the metallicity -- luminosity trend of local galaxies.Comment: 15 pages, 17 figures, accepted for publication in Astronomy and
Astrophysic
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