349 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
The bends in the slopes of radial abundance gradients in the disks of spiral galaxies -- do they exist?
Spiral galaxies with a reported bend in the slope of gradient in the oxygen
abundances (O/H)_R23, derived with traditionally used R23 - method, were
examined. It is shown that the artificial origin of the reported bends can be
naturally explained. Two reasons that result in a false bend in the slope of
(O/H)_R23 gradient are indicated. It is concluded that at the present time
there is no example of a galaxy with an undisputable established bend in the
slope of the oxygen abundance gradient.Comment: 6 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
On the electron temperature determination in high-metallicity HII regions
The problem of determination of the electron temperature t_2 in the OII zone
of high-metallicity HII region was examined. It was shown that the ratio of
nebular to auroral nitrogen line intensities, which is an indicator of the
electron temperature t_2, can be expressed in terms of the nebular line
intensities of oxygen. This solves the problem of the determination of the
electron temperature t_2, since the oxygen nebular lines are strong and,
consequently, are readily observable. A relation between electron temperatures
in the OII and OIII zones in high-metallicity HII regions was studied. It was
found that there is no one-to-one correspondance between t_2 and t_3
temperatures. Instead the t_2 - t_3 relation is dependent on excitation
parameter.Comment: 7 pages with 6 figures, accepted for publication in MNRA
The oxygen abundance deficiency in irregular galaxies
The observed oxygen abundances in a number of irregular galaxies have been
compared with predictions of the closed-box model of chemical and photometric
evolution of galaxies. Oxygen is found to be deficient with respect to the
predicted abundances. This is an indicator in favor of loss of heavy elements
via galactic winds or/and of infall of low--abundance gas onto the galaxy.
The oxygen abundance deficiency observed within the optical edge of a galaxy
cannot be explained by mixing with the gas envelope observed outside the
optical limit. We confirm the widespread idea that a significant part of the
heavy elements is ejected by irregular galaxies in the intergalactic medium.Comment: 8 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
New calibrations for abundance determinations in HII regions
Simple relations for deriving the oxygen abundance in HII regions with
intensities of the three strong emission lines R_2, R_3, and N_2 (R
calibration) or S_2, R_3, and N_2 (S calibration) in their spectra are
suggested. A sample of 313 reference HII regions of the counterpart method is
used as calibrating data points. Relations for the determination of nitrogen
abundances, the R calibration, are also constructed. We find that the oxygen
and nitrogen abundances in high-metallicity HII regions can be estimated using
the intensities of the two strong lines R_2 and N_2 (or S_2 and N_2 for oxygen)
only. The corresponding two-dimensional relations are provided. There are
considerable advantages of the suggested calibration relations as compared to
the existing ones. First, the oxygen and nitrogen abundances estimated through
the suggested calibrations agree with the Te-based abundances within ~0.1 dex
over the whole metallicity range, i.e., the relative accuracy of the
calibration-based abundances is 0.1 dex. Although we constructed distinct
relations for high- and low-metallicity objects, the separation between these
two can be simply obtained from the intensity of the N_2 line. Moreover, the
applicability ranges of the high- and low-metallicity relations overlap for
adjacent metallicities, i.e., the transition zone disappears. Second, the
oxygen abundances produced by the two suggested calibrations are in remarkable
agreement with each other. In fact, the R-based and S-based oxygen abundances
agree within ~0.05 dex in the majority of cases for more than three thousand
HII region spectra.Comment: 16 pages, 9 figues, accepted to MNRA
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
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