378 research outputs found
The Cocoon Nebula and its ionizing star: do stellar and nebular abundances agree?
(Abridged) Main sequence massive stars embedded in an HII region should have
the same chemical abundances as the surrounding nebular gas+dust. The Cocoon
nebula, a close-by Galactic HII region ionized by a narrow line B0.5 V single
star (BD+46 3474), is an ideal target to perform a detailed comparison of
nebular and stellar abundances in the same Galactic HII region. We investigate
the chemical content of O, N and S in the Cocoon nebula from two different
points of view: an empirical analysis of the nebular spectrum and a detailed
spectroscopic analysis of its ionizing B-type star using state-of-the-art
stellar atmosphere modeling. By comparing the stellar and nebular abundances,
we aim to indirectly address the long-standing problem of the discrepancy found
between abundances obtained from collisionally excited lines (CELs) and optical
recombination lines in photoionized nebulae. We collect spatially resolved
spectroscopy of the Cocoon nebula and a high resolution optical spectrum of its
ionizing star. Standard nebular techniques are used to compute the physical
conditions and gaseous abundances of O, N and S. We perform a self-consistent
spectroscopic abundance analysis of BD+46 3474 based on the atmosphere code
FASTWIND to determine the stellar parameters and Si, O, and N abundances. The
Cocoon nebula and its ionizing star, located at a distance of 800+-80 pc, have
a very similar chemical composition as the Orion nebula and other B-type stars
in the solar vicinity. This result agrees with the high degree of homogeneity
of the present-day composition of the solar neighbourhood as derived from the
study of the local cold-gas ISM. The comparison of stellar and nebular CELs
abundances in the Cocoon nebula indicates that O and N gas+dust nebular values
are in better agreement with stellar ones assuming small temperature
fluctuations, of the order of those found in the Orion nebula.Comment: Accepted for publication in A&A. 13 pages, 7 tables and 6 figure
Comparison of sustainable architecture from two European countries trough indicators (The Netherlands and Spain)
The Netherlands was pioneer in incorporating ecological concern to its aims and politics. By 1972 the Dutch government spread the message of responsible and ecologic production and consumption, wheras Spain was still far from considering sustainability as a main concern. The way that both countries have followed is worth studying and comparing. In this research, the status of the construction sector in Spain and the Netherlands will be analysed, by using indices. Both countries enjoy a similar HDI (Human Development Index), and differences, among others, in their constructive development, as the results show. Considering sustainability as a compound result of environmental, social and economical matters, diverse indices related to these aspects have been searched for in order to locate construction in its real context. This way, proper comparisons can be established, which will help to learn and improve, both in the general aims and in the process of construction. The aims will be analysed by comparing existing and possible situations. The processes will be analysed by indicating the areas whose indices should be established and studied in more depth, and detecting as well the weaknesses of knowledge that should be improved. As for the temporal development, the indices selected were those related to sustainability in the different stages of the life cycle of a building; from the pre-design until the demolition of it. Where necessary, trends have been defined by presenting temporary cycles. The indices have been obtained from several official organisations considering the availability of data about both countries as for concept, time and reliability. This is the case of institutes, universities, congresses and research centres. In some cases, the sources of the information are common to both countries: EU statistics. All conclusions establish forcefully, how (in a comparable framework, where differences are not essential) there is a space of improvement and learning that can be shared. It also identifies which areas should be considered as those requiring priority attentions and the trend towards a likely common factor
The chemical composition of the Orion star-forming region: II. Stars, gas, and dust: the abundance discrepancy conundrum
We re-examine the recombination/collisional emission line (RL/CEL) nebular
abundance discrepancy problem in the light of recent high-quality abundance
determinations in young stars in the Orion star-forming region.
We re-evaluate the CEL and RL abundances of several elements in the Orion
nebula and estimate the associated uncertainties, taking into account the
uncertainties in the ionization correction factors for unseen ions. We estimate
the amount of oxygen trapped in dust grains for several scenarios of dust
formation. We compare the resulting gas+dust nebular abundances with the
stellar abundances of a sample of 13 B-type stars from the Orion star-forming
region (Ori\,OB1), analyzed in Papers I and III of this series.
We find that the oxygen nebular abundance based on recombination lines agrees
much better with the stellar abundances than the one derived from the
collisionally excited lines. This result calls for further investigation. If
the CEL/RL abundance discrepancy were caused by temperature fluctuations in the
nebula, as argued by some authors, the same kind of discrepancy should be seen
for the other elements, such as C, N and Ne, which is not what we find in the
present study. Another problem is that with the RL abundances, the energy
balance of the Orion nebula is not well understood. We make some suggestions
concerning the next steps to undertake to solve this problem.Comment: 11 pages, 8 tables, 5 figures (To be published in A&A
Carbon and oxygen in HII regions of the Magellanic Clouds: abundance discrepancy and chemical evolution
We present C and O abundances in the Magellanic Clouds derived from deep
spectra of HII regions. The data have been taken with the Ultraviolet-Visual
Echelle Spectrograph at the 8.2-m VLT. The sample comprises 5 HII regions in
the Large Magellanic Cloud (LMC) and 4 in the Small Magellanic Cloud (SMC). We
measure pure recombination lines (RLs) of CII and OII in all the objects,
permitting to derive the abundance discrepancy factors (ADFs) for O^2+, as well
as their O/H, C/H and C/O ratios. We compare the ADFs with those of other HII
regions in different galaxies. The results suggest a possible metallicity
dependence of the ADF for the low-metallicity objects, but more uncertain for
high-metallicity objects. We compare nebular and B-type stellar abundances and
we find that the stellar abundances agree better with the nebular ones derived
from collisionally excited lines (CELs). Comparing these results with other
galaxies we observe that stellar abundances seem to agree better with the
nebular ones derived from CELs in low-metallicity environments and from RLs in
high-metallicity environments. The C/H, O/H and C/O ratios show almost flat
radial gradients, in contrast with the spiral galaxies where such gradients are
negative. We explore the chemical evolution analysing C/O vs. O/H and comparing
with the results of HII regions in other galaxies. The LMC seems to show a
similar chemical evolution to the external zones of small spiral galaxies and
the SMC behaves as a typical star-forming dwarf galaxy.Comment: Accepted for publication in MNRAS, 17 pages, 11 figures, 8 table
Dielectric Strength of Nanofluid-Impregnated Transformer Solid Insulation
The interest in developing new fluids that can be used as dielectric liquids for transformers
has driven the research on dielectric nanofluids in the last years. A number of authors have reported
promising results on the electrical and thermal properties of dielectric nanofluids. Less attention
has been paid to the interaction of these fluids with the cellulose materials that constitute the solid
insulation of the transformers. In the present study, the dielectric strength of cellulose insulation is
investigated, comparing its behavior when it is impregnated with transformer mineral oil and when
it is impregnated with a dielectric nanofluid. The study includes the analysis of the AC breakdown
voltage and the impulse breakdown voltage of the samples. Large improvements were observed on
the AC breakdown voltages of the specimens impregnated with nanofluids, while the enhancements
were lower in the case of the impulse tests. The reasons for the increase in AC breakdown voltage
were investigated, considering the dielectric properties of the nanofluids used to impregnate the
samples of cellulose. The analysis was completed with a finite element study that revealed the
effect of the nanoparticles on the electric field distribution within the test cell, and its role in the
observed enhancement.This work was supported by the Spanish State Research Agency under grant PID2019-
107126RB-C21/ AEI/10.13039/501100011033 and by the Spanish Ministry of Economy and Competitiveness under grant DPI2015-71219-C2-2-
Integral field spectroscopy of selected areas of the Bright Bar and Orion-S cloud in the Orion Nebula
We present integral field spectroscopy of two selected zones in the Orion
Nebula obtained with the Potsdam Multi-Aperture Spectrophotometer (PMAS),
covering the optical spectral range from 3500 to 7200 A and with a spatial
resolution of 1". The observed zones are located on the prominent Bright Bar
and on the brightest area at the northeast of the Orion South cloud, both
containing remarkable ionization fronts. We obtain maps of emission line fluxes
and ratios, electron density and temperatures, and chemical abundances. We
study the ionization structure and morphology of both fields, which ionization
fronts show different inclination angles with respect to the plane of the sky.
We find that the maps of electron density, O+/H+ and O/H ratios show a rather
similar structure. We interpret this as produced by the strong dependence on
density of the [OII] lines used to derive the O+ abundance, and that our
nominal values of electron density-derived from the [SII] line ratio-may be
slightly higher than the appropriate value for the O+ zone. We measure the
faint recombination lines of OII in the field at the northeast of the Orion
South cloud allowing us to explore the so-called abundance discrepancy problem.
We find a rather constant abundance discrepancy across the field and a mean
value similar to that determined in other areas of the Orion Nebula, indicating
that the particular physical conditions of this ionization front do not
contribute to this discrepancy.Comment: 15 pages, 10 figures. Accepted for publication in MNRA
Physical Conditions in Barnard's Loop, Components of the Orion-Eridanus Bubble, and Implications for the WIM Component of the ISM
We have supplemented existing spectra of Barnard's Loop with high accuracy
spectrophotometry of one new position. Cloudy photoionization models were
calculated for a variety of ionization parameters and stellar temperatures and
compared with the observations. After testing the procedure with recent
observations of M43, we establish that Barnard's Loop is photoionized by four
candidate ionizing stars, but agreement between the models and observations is
only possible if Barnard's Loop is enhanced in heavy elements by about a factor
of 1.4. Barnard's Loop is very similar in properties to the brightest
components of the Orion-Eridanus Bubble and the Warm Ionized Medium (WIM). We
are able to establish models that bound the range populated in low-ionization
color-color diagrams (I([SII])/I(H{\alpha}) versus I([NII])/I(H{\alpha})) using
only a limited range of ionization parameters and stellar temperatures.
Previously established variations in the relative abundance of heavy elements
render uncertain the most common method of determining electron temperatures
for components of the Orion-Eridanus Bubble and the WIM based on only the
I([NII])/I(H{\alpha}) ratio, although we confirm that the lowest surface
brightness components of the WIM are on average of higher electron temperature.
The electron temperatures for a few high surface brightness WIM components
determined by direct methods are comparable to those of classical bright H II
regions. In contrast, the low surface brightness HII regions studied by the
Wisconsin H{\alpha} Mapper are of lower temperatures than the classical bright
HII regions
- …