23 research outputs found
Ionization Correction Factors for Planetary Nebulae: I- Using optical spectra
We compute a large grid of photoionization models that covers a wide range of
physical parameters and is representative of most of the observed PNe. Using
this grid, we derive new formulae for the ionization correction factors (ICFs)
of He, O, N, Ne, S, Ar, Cl, and C. Analytical expressions to estimate the
uncertainties arising from our ICFs are also provided. This should be useful
since these uncertainties are usually not considered when estimating the error
bars in element abundances. Our ICFs are valid over a variety of assumptions
such as the input metallicities, the spectral energy distribution of the
ionizing source, the gas distribution, or the presence of dust grains. Besides,
the ICFs are adequate both for large aperture observations and for pencil-beam
observations in the central zones of the nebulae. We test our ICFs on a large
sample of observed PNe that extends as far as possible in ionization, central
star temperature, and metallicity, by checking that the Ne/O, S/O, Ar/O, and
Cl/O ratios show no trend with the degree of ionization. Our ICFs lead to
significant differences in the derived abundance ratios as compared with
previous determinations, especially for N/O, Ne/O, and Ar/O.Comment: 19 pages, 22 figures. Accepted for publication in MNRA
Emission line taxonomy and the nature of AGN-looking galaxies in the SDSS
Massive spectroscopic surveys like the SDSS have revolutionized the way we
study AGN and their relations to the galaxies they live in. A first step in any
such study is to define samples of different types of AGN on the basis of
emission line ratios. This deceivingly simple step involves decisions on which
classification scheme to use and data quality censorship. Galaxies with weak
emission lines are often left aside or dealt with separately because one cannot
fully classify them onto the standard Star-Forming, Seyfert of LINER
categories. This contribution summarizes alternative classification schemes
which include this very numerous population. We then study how star-formation
histories and physical properties of the hosts vary from class to class, and
present compelling evidence that the emission lines in the majority of
LINER-like systems in the SDSS are not powered by black-hole accretion. The
data are fully consistent with them being galaxies whose old stars provide all
the ionizing power needed to explain their line ratios and luminosities. Such
retired galaxies deserve a place in the emission line taxonomy.Comment: 8 pages, 4 figures, Proceedings of IAU Symposium 267, Co-Evolution of
Central Black Holes and Galaxie
Oxygen enrichment in carbon-rich planetary nebulae
We study the relation between the chemical composition and the type of dust
present in a group of 20 Galactic planetary nebulae (PNe) that have high
quality optical and infrared spectra. The optical spectra are used, together
with the best available ionization correction factors, to calculate the
abundances of Ar, C, Cl, He, N, Ne, and O relative to H. The infrared spectra
are used to classify the PNe in two groups depending on whether the observed
dust features are representative of oxygen-rich or carbon-rich environments.
The sample contains one object from the halo, eight from the bulge, and eleven
from the local disc. We compare their chemical abundances with nucleosynthesis
model predictions and with the ones obtained in seven Galactic H II regions of
the solar neighbourhood.
We find evidence of O enrichment (by 0.3 dex) in all but one of the
PNe with carbon-rich dust (CRD). Our analysis shows that Ar, and especially Cl,
are the best metallicity indicators of the progenitors of PNe. There is a tight
correlation between the abundances of Ar and Cl in all the objects, in
agreement with a lockstep evolution of both elements. The range of
metallicities implied by the Cl abundances covers one order of magnitude and we
find significant differences in the initial masses and metallicities of the PNe
with CRD and oxygen-rich dust (ORD). The PNe with CRD tend to have intermediate
masses and low metallicities, whereas most of the PNe with ORD show higher
enrichments in N and He, suggesting that they had high-mass progenitors.Comment: Accepted for publication in MNRAS. 14 pages, 8 figures, 5 table
Constraining jet production scenarios by studies of Narrow-Line-Radio-Galaxies
We study a large sample of narrow-line radio galaxies (NLRGs) with extended
radio structures. Using 1.4 GHz radio luminosities, , narrow optical
emission line luminosities, L_{\oiii} and , as well as black
hole masses derived from stellar velocity dispersions measured from
the optical spectra obtained with the Sloan Digital Sky Survey, we find that:
(i) NLRGs cover about 4 decades of the Eddington ratio, ; (ii) strongly
correlates with ; (iii) radio-loudness, , strongly anti-correlates with . A very broad range
of the Eddington ratio indicates that the parent population of NLRGs includes
both radio-loud quasars (RLQs) and broad-line radio galaxies (BLRGs). The
correlations they obey and their high jet production efficiencies favor a jet
production model which involves the so-called 'magnetically choked' accretion
scenario. In this model, production of the jet is dominated by the
Blandford-Znajek mechanism, and the magnetic fields in the vicinity of the
central black hole are confined by the ram pressure of the accretion flow.
Since large net magnetic flux accumulated in central regions of the accretion
flow required by the model can take place only via geometrically thick
accretion, we speculate that the massive, 'cold' accretion events associated
with luminous emission-line AGN can be accompanied by an efficient jet
production only if preceded by a hot, very sub-Eddington accretion phase.Comment: 24 pages, 6 figures, published in ApJ, moderate revisions to match
the published versio
Planetary Nebulae and the Ionization of the Interstellar Medium in Galaxies
We discuss the role of planetary nebulae and their progeny in galaxy context in terms of ionization of the galaxy interstellar medium. This regards ionized gas outside the disk of spiral galaxies, the diffuse ionized medium in spiral galaxies, and the weak line emission of elliptical galaxies
Can solid body destruction explain abundance discrepancies in planetary nebulae?
In planetary nebulae, abundances of oxygen and other heavy elements derived
from optical recombination lines are systematically higher than those derived
from collisionally excited lines. We investigate the hypothesis that the
destruction of solid bodies may produce pockets of cool, high-metallicity gas
that could explain these abundance discrepancies. Under the assumption of
maximally efficient radiative ablation, we derive two fundamental constraints
that the solid bodies must satisfy in order that their evaporation during the
planetary nebula phase should generate a high enough gas phase metallicity. A
local constraint implies that the bodies must be larger than tens of meters,
while a global constraint implies that the total mass of the solid body
reservoir must exceed a few hundredths of a solar mass. This mass greatly
exceeds the mass of any population of comets or large debris particles expected
to be found orbiting evolved low- to intermediate-mass stars. We therefore
conclude that contemporaneous solid body destruction cannot explain the
observed abundance discrepancies in planetary nebulae. However, similar
arguments applied to the sublimation of solid bodies during the preceding
asymptotic giant branch (AGB) phase do not lead to such a clear-cut conclusion.
In this case, the required reservoir of volatile solids is only one
ten-thousandth of a solar mass, which is comparable to the most massive debris
disks observed around solar-type stars, implying that this mechanism may
contribute to abundance discrepancies in at least some planetary nebulae, so
long as mixing of the high metallicity gas is inefficient.Comment: 8 pages, no figures, ApJ in pres
The Massive Stellar Content in the Starburst NGC3049: A Test for Hot-Star Mode
We have obtained high-spatial resolution ultraviolet and optical STIS
spectroscopy and imaging of the metal-rich nuclear starburst in NGC3049. The
stellar continuum and the absorption line spectrum in the ultraviolet are used
to constrain the massive stellar population. The strong, blueshifted stellar
lines of CIV and SiIV detected in the UV spectra indicate a metal-rich,
compact, massive (1E6 Msol) cluster of age 3--4 Myr emitting the UV-optical
continuum. We find strong evidence against a depletion of massive stars in this
metal-rich cluster. The derived age and the upper mass-limit cut-off of the
initial mass function are also consistent with the detection of Wolf-Rayet (WR)
features at optical wavelengths. As a second independentconstraint on the
massive stellar content, the nebular emission-line spectrum is modeled with
photoionization codes using stellar spectra from evolutionary synthesis models.
However, the nebular lines are badly reproduced by 3--4 Myr instantaneous
bursts, as required by the UV line spectrum, when unblanketed WR and/or Kurucz
stellar atmospheres are used. The corresponding number of photons above 24 and
54 eV in the synthetic models is too high in comparison with values suggested
by the observed line ratios. Since the ionizing spectrum in this regime is
dominated by emission from WR stars, this discrepancy between observations and
models is most likely the result of incorrect assumptions about the WR stars.
Thus we conclude that the nebular spectrum of high-metallicity starbursts is
poorly reproduced by models for WR dominated populations. However, the new
model set of Smith et al. (2002) with blanketed WR and O atmospheres and
adjusted WR temperatures predicts a softer far-UV radiation field, providing a
better match to the data.Comment: To be published in ApJ, Dec. issue 17 figures, 3 in gif forma
The primordial abundance of 4He: a self-consistent empirical analysis of systematic effects in a large sample of low-metallicity HII regions
We determine the primordial helium mass fraction Yp using 93 spectra of 86
low-metallicity extragalactic HII regions. This sample constitutes the largest
and most homogeneous high-quality data sets in existence for the determination
of Yp. For comparison and to improve the statistics in our investigation of
systematic effects affecting the Yp determination, we have also considered a
sample of 271 low-metallicity HII regions selected from the DR5 of the SDSS.
Although this larger sample shows more scatter, it gives results that are
consistent at the 2sigma level with our original sample. We have considered
known systematic effects which may affect the 4He abundance determination. They
include different sets of HeI line emissivities and reddening laws, collisional
and fluorescent enhancements of HeI recombination lines, underlying HeI stellar
absorption lines, collisional excitation of hydrogen lines, temperature and
ionization structure of the HII region, and deviation of HeI and H emission
line intensities from case B. However, the most likely value of Yp depends on
the adopted set of HeI line emissivities. Using Monte Carlo methods to solve
simultaneously the above systematic effects we find a primordial helium mass
fraction Yp = 0.2472+/-0.0012 when using the HeI emissivities from Benjamin et
al. (1999, 2002) and 0.2516+/-0.0011 when using those from Porter et al.
(2005). The first value agrees well with the value given by SBBN theory, while
the value obtained with likely more accurate emissivities of Porter et al.
(2005) is higher at the 2sigma level. This latter value, if confirmed, would
imply slight deviations from SBBN.Comment: 56 pages, 15 figures, accepted for publication in the Astrophysical
Journa