114 research outputs found
The chemical composition of metal-poor emission-line galaxies in the Data Release 3 of the Sloan Digital Sky Survey
We have re-evaluated empirical expressions for the abundance determination of
N, O, Ne, S, Cl, Ar and Fe taking into account the latest atomic data and
constructing an appropriate grid of photoionization models with state-of-the
art model atmospheres. Using these expressions we have derived heavy element
abundances in the 310 emission-line galaxies from the Data Release 3 of
the Sloan Digital Sky Survey (SDSS)with an observed Hbeta flux F(Hbeta)> 1E-14
erg s^{-1} cm^{-2} and for which the [O III] 4363 emission line was detected at
least at a 2sigma level, allowing abundance determination by direct methods.
The oxygen abundance 12 + log O/H of the SDSS galaxies lies in the range from ~
7.1 (Zsun/30) to 8.5 (0.7 Zsun). The SDSS sample is merged with a sample of 109
blue compact dwarf (BCD) galaxies with high quality spectra, which contains
extremely low-metallicity objects. We use the merged sample to study the
abundance patterns of low-metallicity emission-line galaxies. We find that
extremely metal-poor galaxies (12 + log O/H < 7.6, i.e. Z < Zsun/12) are rare
in the SDSS sample. The alpha element-to-oxygen abundance ratios do not show
any significant trends with oxygen abundance, in agreement with previous
studies, except for a slight increase of Ne/O with increasing metallicity,
which we interpret as due to a moderate depletion of O onto grains in the most
metal-rich galaxies. The Fe/O abundance ratio is smaller than the solar value,
by up to 1 dex at the high metallicity end. (abridged)Comment: 17 pages, 12 figures. Accepted for publication in the Astronomy and
Astrophysic
Abundance patterns in the low-metallicity emission-line galaxies from the Early Data Release of the Sloan Digital Sky Survey
We have derived element abundances in 310 emission-line galaxies from the
Early Data Release of the Sloan Digital Sky Survey (SDSS) for which the [O {\sc
iii}] 4363 emission line was detected, allowing abundance determination by
direct methods. We found no extremely metal-deficient galaxy (Z<Zsun/12),
probably as a consequence of selection effects in the SDSS sample. The oxygen
abundance 12 + log O/H of the SDSS galaxies sample lies in the range from ~7.6
(Zsun/12) to ~8.4 (Zsun/2). This sample is merged with a sample of ~100 blue
compact dwarf galaxies with high quality spectra containing some very
low-metallicity objects to study the abundance patterns of low-metallicity
emission-line galaxies. We find that the element-to-oxygen abundance
ratios do not show any significant trends with the oxygen abundance, in
agreement with previous studies. The Fe/O abundance ratio is smaller than the
solar value, which we interpret as an indication that type Ia supernovae have
not yet appeared in these galaxies, implying an age of less than 1-2 Gyr.
However, a slight decrease of the Fe/O abundance ratio with increasing
metallicity suggests some depletion of iron onto dust in the galaxies with
higher metallicities. The N/O abundance ratio ranges from log N/O= -1.6 to
-0.8. The fact that no galaxy with log N/O < -1.6 was discovered implies that
local low-metallicity emission-line galaxies are of a different nature than
high-redshift damped Lyalpha systems with log N/O of ~-2.3 and that their ages
are probably larger than 100-300 Myr. Our data indicate the existence of a
gradual nitrogen enrichment on a time-scale of a few Myr.Comment: 8 pages, 7 figures, accepted for publication in A&
Evolution of Planetary Nebulae I. An improved synthetic model
We present a new synthetic model to follow the evolution of a planetary
nebula (PN) and its central star, starting from the onset of AGB phase up to
the white dwarf cooling sequence. The model suitably combines various
analytical prescriptions to account for different (but inter-related) aspects
of planetary nebulae, such as: the dynamical evolution of the primary shell and
surrounding ejecta, the photoionisation of H and He by the central star, the
nebular emission of a few relevant optical lines (e.g. Hbeta; HeII4686;
[OIII]5007). Predictions of the synthetic model are tested by comparison with
both findings of hydrodynamical calculations, and observations of Galactic PNe.
The sensitiveness of the results to the models parameters (e.g. transition
time, mass of the central star, H-/He-burning tracks, etc.) is also discussed.
We briefly illustrate the systematic differences that are expected in the
luminosities and lifetimes of PNe with either H- or He-burning central stars,
which result in different ``detection probabilities'' across the H-R diagram,
in both Hbeta and [OIII]5007 lines. Adopting reasonable values of the model
parameters, we are able to reproduce, in a satisfactory way, many general
properties of PNe, like the ionised mass--nebular radius relationship, the
trends of a few main nebular line ratios, and the observed ranges of nebular
shell thicknesses, electron densities, and expansion velocities. The models
naturally predict also the possible transitions from optically-thick to
optically-thin configurations (and vice versa). In this context, the origin of
the Zanstra discrepancy is also analysed. (abridged)Comment: 28 pages, 23 postscript figures, to appear in A&
Evolution of planetary nebulae II. Population effects on the bright cut-off of the PNLF
We investigate the bright cut-off of the [OIII]l5007 planetary nebula
luminosity function (PNLF), that has been suggested as a powerful extragalactic
distance indicator. Theoretical PNLFs are constructed via Monte-Carlo
simulations of populations of PNe, whose individual properties are described
with the aid of recent PN synthetic models (Marigo et al. 2001), coupled to a
detailed photoionisation code (CLOUDY). The basic dependences of the cut-off
magnitude M* are then discussed. We find that: (i) In galaxies with recent or
ongoing star formation, the modelled PNLF present M* values between -4 and -5,
very close to the observationally-calibrated value for the LMC. (ii) In these
galaxies, the PNLF cut-off is produced by PNe with progenitor masses of about
2.5 Msun, while less massive stars give origin to fainter PNe. As a consequence
M* is expected to depend strongly on the age of the last burst of star
formation, dimming by as much as 5 mag as we go from young to 10-Gyr old
populations. (iii) Rather than on the initial metallicity of a stellar
population, M* depends on the actual [O/H] of the observed PNe, a quantity that
may differ significantly from the initial value (due to dredge-up episodes),
especially in young and intermediate-age PN populations. (iv) Also the
transition time from the end of AGB to the PN phase, and the nuclear-burning
properties (i.e. H- or He-burning) of the central stars introduce
non-negligible effects on M*. The strongest indication derived from the present
calculations is a serious difficulty to explain the age-invariance of the
cut-off brightness over an extended interval, say from 1 to 13 Gyr, that
observations of PNLFs in galaxies of late-to-early type seem to suggest.Comment: 22 pages, to appear in Astronomy & Astrophysic
ISO SWS Observations of H II Regions in NGC 6822 and I ZW 36: Sulfur Abundances and Temperature Fluctuations
We report ISO SWS infrared spectroscopy of the H II region Hubble V in NGC
6822 and the blue compact dwarf galaxy I Zw 36. Observations of Br alpha, [S
III] at 18.7 and 33.5 microns, and [S IV] at 10.5 microns are used to determine
ionic sulfur abundances in these H II regions. There is relatively good
agreement between our observations and predictions of S^+3 abundances based on
photoionization calculations, although there is an offset in the sense that the
models overpredict the S^+3 abundances. We emphasize a need for more
observations of this type in order to place nebular sulfur abundance
determinations on firmer ground. The S/O ratios derived using the ISO
observations in combination with optical data are consistent with values of
S/O, derived from optical measurements of other metal-poor galaxies.
We present a new formalism for the simultaneous determination of the
temperature, temperature fluctuations, and abundances in a nebula, given a mix
of optical and infrared observed line ratios. The uncertainties in our ISO
measurements and the lack of observations of [S III] lambda 9532 or lambda 9069
do not allow an accurate determination of the amplitude of temperature
fluctuations for Hubble V and I Zw 36. Finally, using synthetic data, we
illustrate the diagnostic power and limitations of our new method.Comment: 32 Pages total, including 6 encapsulated postscript figures (one with
two parts). Accepted for Publication in the 20 Dec 2002 Ap
Photoionization Models of NGC 346
We present spherically symmetric and plane parallel photoionization models of
NGC 346, an HII region in the Small Magellanic Cloud. The models are based on
CLOUDY and on the observations of Peimbert, Peimbert, & Ruiz (2000). We find
that approximately 45% of the H ionization photons escape from the HII region
providing an important ionizing source for the low density interstellar medium
of the SMC. The predicted I(4363)/I(5007) value is smaller than that observed,
probably implying that there is an additional source of energy not taken into
account by the models. From the ionization structure of the best model and the
observed line intensities we determine the abundances of N, Ne, S, Ar, and Fe
relative to O.Comment: 23 pages, 2 figures, Accepted for publication in Ap
On the most metal-poor PN and its binary central star
PN G135.9+55.9 is the most metal-poor PN known in our Galaxy. The central
star resides in a short-period binary system with a compact component, probably
a white dwarf. We describe new observations, which allowed us to determine the
orbital period. The lower limit for the combined mass of both stars is close to
the Chandrasekhar limit for white dwarfs, making this binary a possible
progenitor of a supernova type Ia. The binary system must have recently emerged
from a common envelope phase.Comment: Proceedings of the conference "Planetary Nebulae as Astronomical
Tools", Gdansk, June/July 2005. To appear in AIP conference serie
Spectroscopic and photometric studies of low-metallicity star-forming dwarf galaxies. I. SBS 1129+576
Spectroscopy and V,I CCD photometry of the dwarf irregular galaxy SBS
1129+576 are presented for the first time. The CCD images reveal a chain of
compact H II regions within the elongated low-surface-brightness (LSB)
component of the galaxy. Star formation takes place mainly in two
high-surface-brightness H II regions. The mean (V-I) colour of the LSB
component in the surface brightness interval between 23 and 26 mag/sq.arcsec is
relatively blue ~0.56+/-0.03 mag, as compared to the (V-I)~0.9-1.0 for the
majority of known dwarf irregular and blue compact dwarf (BCD) galaxies.
Spectroscopy shows that the galaxy is among the most metal-deficient galaxies
with an oxygen abundance 12+log(O/H)= 7.36+/-0.10 in the brightest H II region
and 7.48+/-0.12 in the second brightest H II region, or 1/36 and 1/28 of the
solar value, respectively. Hbeta and Halpha emission lines and Hdelta and
Hgamma absorption lines are detected in a large part of the LSB component. We
use two extinction-insensitive methods based on the equivalent widths of (1)
emission and (2) absorption Balmer lines to put constraints on the age of the
stellar populations in the galaxy. In addition, we use two extinction-dependent
methods based on (3) the spectral energy distribution (SED) and (4) the (V-I)
colour. The observed properties of the LSB component can be reproduced by a
stellar population forming continuously since 10 Gyr ago, provided that the
star formation rate has increased during the last 100 Myr by a factor of 6 to
50 and no extinction is present. However, the observational properties of the
LSB component can be reproduced equally well by continuous star formation which
started not earlier than 100 Myr ago and stopped at 5 Myr, if some extinction
is assumed.(Abridged)Comment: 17 pages, 12 figures, Accepted for publication in A&
The warm interstellar medium around the Cygnus Loop
Observations of the oxygen lines [OII]3729 and [OIII]5007 in the medium
immediately beyond the Cygnus Loop supernova remnant were carried out with the
scanning Fabry-P\'erot spectrophotometer ESOP. Both lines were detected in
three different directions - east, northeast and southwest - and up to a
distance of 15 pc from the shock front. The ionized medium is in the immediate
vicinity of the remnant, as evinced by the smooth brightening of both lines as
the adiabatic shock transition (defined by the X-ray perimeter) is crossed.
These lines are usually brighter around the Cygnus Loop than in the general
background in directions where the galactic latitude is above 5 degrees. There
is also marginal (but significant) evidence that the degree of ionization is
somewhat larger around the Cygnus Loop. We conclude that the energy necessary
to ionize this large bubble of gas could have been supplied by an O8 or O9 type
progenitor or the particles heated by the expanding shock front. The second
possibility, though highly atractive, would have to be assessed by extensive
modelling.Comment: 18 pages, 8 figures, ApJ 512 in pres
Calibration of Nebular Emission-line Diagnostics: II. Abundances
(Abridged) We examine standard methods of measuring nebular chemical
abundances, including estimates based on direct T_e measurements, and also
bright-line diagnostics. We use observations of 4 LMC HII regions whose
ionizing stars have classifications ranging from O7 to WN3. We assume a 2-zone
T_e structure to compute ionic abundances. We compare with photoionization
models tailored to the properties of the individual objects, and emphasize the
importance of correctly relating T_e in the two zones, which can otherwise
cause errors of ~0.2 dex in abundance estimates. There are no spatial
variations to within 0.1 - 0.15 dex in any of the objects, even one hosting 3
WR stars.
Our data agree with the modeled R23 and S23 diagnostics of O and S. We
present the first theoretical tracks for S23, which are in excellent agreement
with a larger dataset. However, contrary to earlier suggestions, S23 is much
more sensitive to the ionization parameter than is R23, because S23 does not
sample S IV. We therefore introduce S234 = ([SII]+[SIII]+[SIV])/H-beta.
Predicted and observed spatial variations in S234 are dramatically reduced in
contrast to S23. The intensity of [SIV]10.5 microns is easily estimated from a
simple relation between [SIV]/[SIII] and [OIII]/[OII]. This method of
estimating S234 yields excellent agreement with our models, hence we give a
theoretical calibration for S234. The double-valued structure of S23 and S234
remains an important problem as for R23, and presently we consider the S
diagnostics reliable only at Z < 0.5 Z_sol. However, the slightly larger
dynamic range and excellent compatibility with theoretical predictions suggest
the S diagnostics to be more effective abundance indicators than R23.Comment: Accepted to ApJ. 24 pages, 11 figures, uses emulateapj.st
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