695 research outputs found
Eliminating Error in the Chemical Abundance Scale for Extragalactic HII Regions
In an attempt to remove the systematic errors which have plagued the
calibration of the HII region abundance sequence, we have theoretically modeled
the extragalactic HII region sequence. We then used the theoretical spectra so
generated in a double blind experiment to recover the chemical abundances using
both the classical electron temperature + ionization correction factor
technique, and the technique which depends on the use of strong emission lines
(SELs) in the nebular spectrum to estimate the abundance of oxygen. We find a
number of systematic trends, and we provide correction formulae which should
remove systematic errors in the electron temperature + ionization correction
factor technique. We also provide a critical evaluation of the various
semi-empirical SEL techniques. Finally, we offer a scheme which should help to
eliminate systematic errors in the SEL-derived chemical abundance scale for
extragalactic HII regions.Comment: 24 pages, 9 Tables, 13 figures, accepted for publication in MNRAS.
Updated considering minor changes during the final edition process and some
few missing reference
Positioning Axes of Sustainable Tourist Destinations: The Case of Aragón
The purpose of this research is to determine the optimal positioning strategies of Aragón as a sustainable tourist destination. An exploratory research is carried out based on the analysis of statistical information sources, the identification of global and multisectoral macro-trends with direct impact on the evolution of the tourist sector, and the use of the qualitative technique of the focus group. The main and secondary axes to provide a basis for the positioning strategy of the autonomous community are presented as a resul
Chemical abundances in the protoplanetary disk LV2 (Orion): clues to the causes of the abundance anomaly in HII regions
Optical integral field spectroscopy of the archetype protoplanetary disk LV2
in the Orion Nebula is presented, taken with the VLT FLAMES/Argus fibre array.
The detection of recombination lines of CII and OII from this class of objects
is reported, and the lines are utilized as abundance diagnostics. The study is
complemented with the analysis of HST Faint Object Spectrograph ultraviolet and
optical spectra of the target contained within the Argus field of view. By
subtracting the local nebula background the intrinsic spectrum of the proplyd
is obtained and its elemental composition is derived for the first time. The
proplyd is found to be overabundant in carbon, oxygen and neon compared to the
Orion Nebula and the sun. The simultaneous coverage over LV2 of the CIII]
1908-A and [OIII] 5007-A collisionally excited lines (CELs) and CII and OII
recombination lines (RLs) has enabled us to measure the abundances of C++ and
O++ for LV2 with both sets of lines. The two methods yield consistent results
for the intrinsic proplyd spectrum, but not for the proplyd spectrum
contaminated by the generic nebula spectrum, thus providing one example where
the long-standing abundance anomaly plaguing metallicity studies of HII regions
has been resolved. These results would indicate that the standard
forbidden-line methods used in the derivation of light metal abundances in HII
regions in our own and other galaxies underestimate the true gas metallicity.Comment: Accepted by MNRAS November 8; 16 pages, 9 figs; typos corrected,
error in FWHMs in table 4 corrected in this versio
Enrichment of the ISM by metal-rich droplets and the abundance bias in HII regions
We critically examine a scenario for the enrichment of the interstellar
medium (ISM) in which supernova ejecta follow a long (10^8 yr) journey before
falling back onto the galactic disk in the form of metal-rich ``droplets'',
These droplets do not become fully mixed with the interstellar medium until
they become photoionized in HII regions. We investigate the hypothesis that the
photoionization of these highly metallic droplets can explain the observed
``abundance discrepancy factors'' (ADFs), which are found when comparing
abundances derived from recombination lines and from collisionally excited
lines, both in Galactic and extragalactic HII regions. We derive bounds of
10^{13}--10^{15} cm on the droplet sizes inside HII regions in order that (1)
they should not have already been detected by direct imaging of nearby nebulae,
and (2) they should not be too swiftly destroyed by diffusion in the ionized
gas. From photoionization modelling we find that, if this inhomogeneous
enrichment scenario holds, then the recombination lines strongly overestimate
the metallicities of the fully mixed HII regions. The abundances derived from
collisionally excited lines also suffer some bias, although to a much lesser
extent. In the absence of any recipe for correcting these biases, we recommend
the discarding of all objects showing large ADFs from studies of galactic
chemical evolution. These biases must also be kept in mind when comparing the
galactic abundance gradients for elements derived from recombination lines with
those derived from collisionally excited lines. Finally, we propose a set of
observations that could be undertaken to test our scenario and improve our
understanding of element mixing in the ISM.Comment: 14 pages, 5 figures, accepted in Astronomy and Astrophysic
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