95 research outputs found
The effect of collisional enhancement of Balmer lines on the determination of the primordial helium abundance
This paper describes a new determination of the primordial helium abundance
(Y_P), based on the abundance analysis of five metal-poor extragalactic HII
regions. For three regions of the sample (SBS 0335-052, I Zw 18, and H29) we
present tailored photoionization models based on improved calculations with
respect to previous models. In particular, we use the photoionization models to
study quantitatively the effect of collisional excitation of Balmer lines on
the determination of the helium abundance (Y) in the individual regions. This
effect is twofold: first, the intensities of the Balmer lines are enhanced with
respect to the pure recombination value, mimicking a higher hydrogen abundance;
second, the observed reddening is larger than the true extinction, due to the
differential effect of collisions on different Balmer lines. In addition to
these effects, our analysis takes into account the following features of HII
regions: (i) the temperature structure, (ii) the density structure, (iii) the
presence of neutral helium, (iv) the collisional excitation of the HeI lines,
(v) the underlying absorption of the HeI lines, and (vi) the optical thickness
of the HeI lines. The object that shows the highest increase in Y after the
inclusion of collisional effects in the analysis is SBS 0335-052, whose helium
abundance has been revised by Delta Y = +0.0107. The revised Y values for the
five objects in our sample yield an increase of +0.0035 in Y_P, giving Y_P =
0.2391 +/- 0.0020.Comment: 59 pages, 8 figures. AAS Latex. Accepted for publication in the
Astrophysical Journa
Fluorescent excitation of Balmer lines in gaseous nebulae: case D
(abridged) Non-ionizing stellar continua are a source of photons for
continuum pumping in the hydrogen Lyman transitions. In the environments where
these transitions are optically thick, deexcitation occurs through higher
series lines, so that the flux in these lines has a fluorescent contribution in
addition to recombination; in particular, Balmer emissivities are
systematically enhanced above case B. The effectiveness of such mechanism in
HII regions and the adequacy of photoionization models as a tool to study it
are the two main focuses of this work. We find that photoionization models of H
II regions illuminated by low-resolution population synthesis models
significantly overpredict the fluorescent contribution to the Balmer lines.
Conversely, photoionization models in which the non-ionizing part of the
continuum is omitted or is not transferred underpredict the fluorescent
contribution to the Balmer lines, producing a bias of similar amplitude in the
opposite direction. In this paper, we carry out realistic estimations of the
fluorescent Balmer intensity and discuss the variations to be expected as the
simulated observational setup and the stellar population's parameters are
varied. In all the cases explored, we find that fluorescent excitation provides
a significant contribution. We also show that differential fluorescent
enhancement may produce line-of-sight differences in the Balmer decrement,
mimicking interstellar extinction. Fluorescent excitation emerges from our
study as a small but important mechanism for the enhancement of Balmer lines,
which should be taken into account in the abundance analysis of photoionized
regions, particularly in the case of high-precision applications such as the
determination of primordial helium.Comment: 49 pages, 14 figures. AAS Latex. Accepted for publication in the
Astrophysical Journa
Relationship between different livestock managements and stress response in dairy ewes
Abstract. The gradual diffusion of intensive and semi-intensive production systems, especially in dairy sheep breeds, has led to the growing concern of consumers about the life conditions of farmed animals. Space allowance and structures of sheep houses are described as the main potential sources of discomfort for housed flocks, together with inappropriate milking procedures and human–animal interactions. The aim of the present study was to evaluate whether the structure relative to milking room could represent a stressor in Sarda dairy ewes. Animals were divided into two groups according to their farm of origin. Group A (n=40) was from a farm whose milking room was an old warehouse with a waiting area limited and located outdoors and at a different level with respect to the milking room. The passageway of the entrance in the milking room was narrow and perpendicular to the milking positioning so the animal must bend 90∘ to enter in the room. Group B (n=40) was from a farm whose milking room was wide and modern with a large waiting area located at the same level. From all animals blood samples were collected at T0 (2h before milking procedure at 06:00), at T1 (immediately after the animals entered the milking room, about 08:00) and at T2 (after milking procedure). In addition plasma cortisol and glucose values were evaluated. Statistical analysis showed significant effect of milking room (P&lt;0.001) and of sampling time (P&lt;0.05) on cortisol and glucose levels. The results obtained in the present study suggest that, in addition to milking, the characteristics of the room where this procedure occurs represent stressful stimuli that could influence negatively the productivity and welfare of dairy ewes.</p
Relationship between different livestock managements and stress response in dairy ewes
The gradual diffusion of intensive and semi-intensive production systems,
especially in dairy sheep breeds, has led to the growing concern of consumers
about the life conditions of farmed animals. Space allowance and structures
of sheep houses are described as the main potential sources of discomfort
for housed flocks, together with inappropriate milking procedures and
human–animal interactions. The aim of the present study was to evaluate
whether the structure relative to milking room could represent a stressor in
Sarda dairy ewes. Animals were divided into two groups according to their
farm of origin. Group A (n = 40) was from a farm whose milking room
was an old warehouse with a waiting area limited and located outdoors and at
a different level with respect to the milking room. The passageway of the entrance in
the milking room was narrow and perpendicular to the milking positioning so
the animal must bend 90° to enter in the room. Group B
(n = 40) was from a farm whose milking room was wide and modern
with a large waiting area located at the same level. From all animals blood
samples were collected at T0
(2 h before milking procedure at 06:00), at T1 (immediately after the animals entered the milking room,
about 08:00) and at T2 (after milking procedure). In addition plasma cortisol and
glucose values were evaluated. Statistical analysis showed significant
effect of milking room (P < 0.001) and of sampling time (P < 0.05) on cortisol and glucose levels. The results obtained in the present
study suggest that, in addition to milking, the characteristics of the room
where this procedure occurs represent stressful stimuli that could influence
negatively the productivity and welfare of dairy ewes
Effect of body condition score, treatment period and month of the previous lambing on the reproductive resumption of melatonin-treated sarda breed sheep during spring
Stakeholders place great emphasis upon rationalizing the management and rearing techniques which are utilized within sheep farms. The present study aimed to investigate factors which may improve the reproductive performance of melatonin-treated Sardinian sheep via a series of three trials. The first trial (n = 100) investigated the effect of melatonin treatment alongside body condition score (BCS), the second trial (n = 150) investigated the effect of treatment alongside the date of treatment (treatment period) and the third trial (n = 150) investigated the effect of treatment alongside the previous lambing of the ewes. The findings indicated that melatonin is an effective tool for anticipating and improving the reproductive activity of in Sarda breed sheep during the springtime. Furthermore, to obtain optional results, melatonin implantation should be conducted in April, in ewes that have a BCS of >2.5 and that have passed their third month of lactation. © 2021 by the authors. Licensee MDPI, Basel, Switzerland
The Oxygen Abundance of Nearby Galaxies from Sloan Digital Sky Survey Spectra
We have derived the oxygen abundance for a sample of nearby galaxies in the
Data Release 5 of the Sloan Digital Sky Survey (SDSS) which possess at least
two independent spectra of one or several HII regions with a detected
[OIII]4363 auroral line. Since, for nearby galaxies, the [OII]3727 nebular line
is out of the observed wavelength range, we propose a method to derive (O/H)_ff
abundances using the classic Te method coupled with the ff relation. (O/H)_7325
abundances have also been determined, based on the [OII]7320,7330 line
intensities, and using a small modification of the standard Te method. The
(O/H)_ff and (O/H)_7325 abundances have been derived with both the one- and
two-dimensional t_2 - t_3 relations. It was found that the (O/H)_ff abundances
derived with the parametric two-dimensional t_2 - t_3 relation are most
reliable. Oxygen abundances have been determined in 29 nearby galaxies, based
on 84 individual abundance determinations in HII regions. Because of our
selection methods, the metallicity of our galaxies lies in the narrow range 8.2
< 12 + log (O/H) < 8.4. The radial distribution of oxygen abundances in the
disk of the spiral galaxy NGC 4490 is determined for the first time.Comment: 39 pages, 10 figures, 4 tables, accepted for publication in the
Astrophysical Journa
A New Approach to Systematic Uncertainties and Self-Consistency in Helium Abundance Determinations
Tests of big bang nucleosynthesis and early universe cosmology require
precision measurements for helium abundance determinations. However, efforts to
determine the primordial helium abundance via observations of metal poor H II
regions have been limited by significant uncertainties. This work builds upon
previous work by providing an updated and extended program in evaluating these
uncertainties. Procedural consistency is achieved by integrating the hydrogen
based reddening correction with the helium based abundance calculation, i.e.,
all physical parameters are solved for simultaneously. We include new atomic
data for helium recombination and collisional emission based upon recent work
by Porter et al. and wavelength dependent corrections to underlying absorption
are investigated. The set of physical parameters has been expanded here to
include the effects of neutral hydrogen collisional emission. Because of a
degeneracy between the solutions for density and temperature, the precision of
the helium abundance determinations is limited. Also, at lower temperatures (T
\lesssim 13,000 K) the neutral hydrogen fraction is poorly constrained
resulting in a larger uncertainty in the helium abundances. Thus the derived
errors on the helium abundances for individual objects are larger than those
typical of previous studies. The updated emissivities and neutral hydrogen
correction generally raise the abundance. From a regression to zero
metallicity, we find Y_p as 0.2561 \pm 0.0108, in broad agreement with the WMAP
result. Tests with synthetic data show a potential for distinct improvement,
via removal of underlying absorption, using higher resolution spectra. A small
bias in the abundance determination can be reduced significantly and the
calculated helium abundance error can be reduced by \sim 25%.Comment: 51 pages, 13 figure
The Structure and Star-Formation History of NGC 5461
We compute photoionization models for the giant extragalactic H II region NGC
5461, and compare their predictions to several observational constraints. Since
we aim at reproducing not only the global properties of the region, but its
local structure also, the models are constrained to reproduce the observed
density profile, and our analysis takes into consideration the bias introduced
by the shapes and sizes of the slits used by different observers. We find that
an asymmetric nebula with a gaussian density distribution, powered by a young
burst of 3.1 Myr, satisfactorily reproduces most of the constraints, and that
the star-formation efficiency inferred from the model agrees with current
estimates. Our results strongly depend on the assumed density law, since
constant density models overestimate the hardness of the ionizing field,
affecting the deduced properties of the central stellar cluster. We illustrate
the features of our best model, and discuss the possible sources of errors and
uncertainties affecting the outcome of this type of studies.Comment: 33 pages (LaTeX), 3 .eps figures. to be published in ApJ, May 200
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