5,957 research outputs found
Multi-Element Abundance Measurements from Medium-Resolution Spectra. III. Metallicity Distributions of Milky Way Dwarf Satellite Galaxies
We present metallicity distribution functions (MDFs) for the central regions
of eight dwarf satellite galaxies of the Milky Way: Fornax, Leo I and II,
Sculptor, Sextans, Draco, Canes Venatici I, and Ursa Minor. We use the
published catalog of abundance measurements from the previous paper in this
series. The measurements are based on spectral synthesis of iron absorption
lines. For each MDF, we determine maximum likelihood fits for Leaky Box,
Pre-Enriched, and Extra Gas (wherein the gas supply available for star
formation increases before it decreases to zero) analytic models of chemical
evolution. Although the models are too simplistic to describe any MDF in
detail, a Leaky Box starting from zero metallicity gas fits none of the
galaxies except Canes Venatici I well. The MDFs of some galaxies, particularly
the more luminous ones, strongly prefer the Extra Gas Model to the other
models. Only for Canes Venatici I does the Pre-Enriched Model fit significantly
better than the Extra Gas Model. The best-fit effective yields of the less
luminous half of our galaxy sample do not exceed 0.02 Z_sun, indicating that
gas outflow is important in the chemical evolution of the less luminous
galaxies. We surmise that the ratio of the importance of gas infall to gas
outflow increases with galaxy luminosity. Strong correlations of average [Fe/H]
and metallicity spread with luminosity support this hypothesis.Comment: 17 pages, 5 figures; accepted for publication in ApJ; minor
corrections in v3; corrected typographical errors in Tables 1 and 3 in v
Domain specific interactions of S-RNase binding protein with stylar 120 kDA glycoprotein in Nicotiana [abstract]
Abstract only availableFaculty Mentor: Bruce McClure, BiochemistryThrough the process of evolution many flowering plants have developed a biochemical mechanism to prevent self-pollination and pollination by closely related plants. Gametophytic self-incompatibility (SI) is one such system that prevents inbreeding, a well-characterized disadvantage for organisms. A recently discovered protein S-RNase Binding Protein (SBP1) may be involved in SI in several species of flowering plants. SBP1 has been isolated in Nicotiana and has been shown to interact with the c-terminal of 120 kDa protein (120K), a key protein player in SI. It is unclear which domain or domains of SBP1 interact with 120K. In this experiment NaSBP1 has been cloned into pMAL-C2x an N-terminal fusion Maltose Binding Protein (MBP) expression vector. Using nucleotide primers it has been possible to amplify the desired NaSBP1 sequences for cloning into pMAL-C2x and expression in E. Coli. Transformants have been screened for expression using SDS-PAGE and western blotting. Clones that express the fusion proteins were sequenced to verify that no mutations in the DNA have occurred during PCR amplification. Binding experiments (pull-down assays) with domain specific MB
Episodic starbursts in dwarf spheroidal galaxies: a simple model
Dwarf galaxies in the Local Group appear to be stripped of their gas within
270 kpc of the host galaxy. Color-magnitude diagrams of these dwarfs, however,
show clear evidence of episodic star formation (\Delta{}t ~ a few Gyr) over
cosmic time. We present a simple model to account for this behaviour. Residual
gas within the weak gravity field of the dwarf experiences dramatic variations
in the gas cooling time around the eccentric orbit. This variation is due to
two main effects. The azimuthal compression along the orbit leads to an
increase in the gas cooling rate of ~([1+\epsilon]/[1-\epsilon])^2. The
Galaxy's ionizing field declines as 1/R^2 for R>R_disk although this reaches a
floor at R~150 kpc due to the extragalactic UV field ionizing intensity. We
predict that episodic SF is mostly characteristic of dwarfs on moderately
eccentric orbits (\epsilon>0.2) that do not come too close to the centre
(R>R_disk) and do not spend their entire orbit far away from the centre (R>200
kpc). Up to 40% of early infall dwarf spheroidals can be expected to have
already had at least one burst since the initial epoch of star formation, and
10% of these dwarf spheriodals experiencing a second burst. Such a model can
explain the timing of bursts in the Carina dwarf spheroidal and restrict the
orbit of the Fornax dwarf spheroidal. However, this model fails to explain why
some dwarfs, such as Ursa Minor, experience no burst post-infall.Comment: 8 pages, 8 figures. ApJ accepte
Factors Associated with Findings of Published Trials of Drug–Drug Comparisons: Why Some Statins Appear More Efficacious than Others
Lisa Bero and colleagues found published trials comparing one statin with another were more likely to report results and conclusions favoring the sponsor's product than the comparison drug
Safety in maritime oil sector: Content analysis of machinery space fire hazards
An in-depth study of the practice within the maritime oil industry was undertaken to ascertain safety issues in seafaring vessels. It was more concentrated on the type of accidents that occur in machine spaces of seafaring vessels in this industry. The main focus of the research was streamlined to fire in machinery spaces. The literature review later concentrated on two of such incidences, they are oil spill and fire events. An investigation was done to assess those factors which actually contribute or are in association to fire outbreak. A content analysis methodology was used to investigate the associative relationships to fire outbreak with the aid of NVivo 9.0 software. The investigation focused on 15 key in-depth reports on machinery space incidences which were uploaded into the software. The results indicate that leakages on hot surfaces were the major causes of fire hazards in seafaring vessels. The results from using this methodology also highlighted two more fire hazards that were not so apparent in previous studies. They are generator fire and compressors fire. The results supported other studies about leakages on hot surfaces as a major contributor, but also clearly show that there are other hazardous factors of fire in machinery spaces that require further investigation
Carrier-mediated antiferromagnetic interlayer exchange coupling in diluted magnetic semiconductor multilayers GaMnAs/GaAs:Be
We use neutron reflectometry to investigate the interlayer exchange coupling
between GaMnAs ferromagnetic semiconductor layers separated
by non-magnetic Be-doped GaAs spacers. Polarized neutron reflectivity measured
below the Curie temperature of GaMnAs reveals a
characteristic splitting at the wave vector corresponding to twice the
multilayer period, indicating that the coupling between the ferromagnetic
layers are antiferromagnetic (AFM). When the applied field is increased to
above the saturation field, this AFM coupling is suppressed. This behavior is
not observed when the spacers are undoped, suggesting that the observed AFM
coupling is mediated by charge carriers introduced via Be doping. The behavior
of magnetization of the multilayers measured by DC magnetometry is consistent
with the neutron reflectometry results.Comment: 4 pages, 4 figure
Carbon in Red Giants in Globular Clusters and Dwarf Spheroidal Galaxies
We present carbon abundances of red giants in Milky Way globular clusters and
dwarf spheroidal galaxies (dSphs). Our sample includes measurements of carbon
abundances for 154 giants in the clusters NGC 2419, M68, and M15 and 398 giants
in the dSphs Sculptor, Fornax, Ursa Minor, and Draco. This sample doubles the
number of dSph stars with measurements of [C/Fe]. The [C/Fe] ratio in the
clusters decreases with increasing luminosity above log(L/L_sun) ~= 1.6, which
can be explained by deep mixing in evolved giants. The same decrease is
observed in dSphs, but the initial [C/Fe] of the dSph giants is not uniform.
Stars in dSphs at lower metallicities have larger [C/Fe] ratios. We hypothesize
that [C/Fe] (corrected to the initial carbon abundance) declines with
increasing [Fe/H] due to the metallicity dependence of the carbon yield of
asymptotic giant branch stars and due to the increasing importance of Type Ia
supernovae at higher metallicities. We also identified 11 very carbon-rich
giants (8 previously known) in three dSphs. However, our selection biases
preclude a detailed comparison to the carbon-enhanced fraction of the Milky Way
stellar halo. Nonetheless, the stars with [C/Fe] < +1 in dSphs follow a
different [C/Fe] track with [Fe/H] than the halo stars. Specifically, [C/Fe] in
dSphs begins to decline at lower [Fe/H] than in the halo. The difference in the
metallicity of the [C/Fe] "knee" adds to the evidence from [alpha/Fe]
distributions that the progenitors of the halo had a shorter timescale for
chemical enrichment than the surviving dSphs.Comment: accepted to ApJ; 20 pages, 11 figures, 2 machine-readable table
A finite element approach for the acoustic modelling of perforated dissipative mufflers with non-homogeneous properties
[EN] In this work, a finite element approach is presented for modeling sound propagation in
perforated dissipative mufflers with non-homogeneous properties. The spatial variations
of the acoustic properties can arise, for example, from uneven filling processes during
manufacture and degradation associated with the flow of soot particles within the
absorbent material. First, the finite element method is applied to the wave equation for
a propagation medium with variable properties (outer chamber with absorbent material)
and a homogeneous medium (central passage). For the case of a dissipative muffler, the
characterization of the absorbent material is carried out by means of its equivalent complex
density and speed of sound. To account for the spatial variations of these properties, a
coordinate-dependent function is proposed for the filling density of the absorbent material.
The coupling between the outer chamber and the central passage is achieved by using
the acoustic impedance of the perforated central pipe, that relates the acoustic pressure
jump and the normal velocity through the perforations. The acoustic impedance of the
perforated central duct includes the influence of the absorbent material and therefore
a spatial variation of the impedance is also taken into account. A detailed study is then
presented to assess the influence of the heterogeneous properties and the perforated duct
porosity on the acoustic attenuation performance of the muffler.The authors gratefully acknowledge the financial support of Ministerio de Ciencia e Innovacion and the European Regional Development Fund by means of the projects DPI2007-62635 and DPI2010-15412.Antebas, A.; Denia Guzmán, FD.; Pedrosa Sanchez, AM.; Fuenmayor Fernández, FJ. (2013). A finite element approach for the acoustic modelling of perforated dissipative mufflers with non-homogeneous properties. Mathematical and Computer Modelling. 57(7):1970-1978. https://doi.org/10.1016/j.mcm.2012.01.021S1970197857
The SEGUE Stellar Parameter Pipeline. V. Estimation of Alpha-Element Abundance Ratios From Low-Resolution SDSS/SEGUE Stellar Spectra
We present a method for the determination of [alpha/Fe] ratios from
low-resolution (R = 2000) SDSS/SEGUE stellar spectra. By means of a
star-by-star comparison with degraded spectra from the ELODIE spectral library
and with a set of moderately high-resolution (R = 15,000) and medium-resolution
(R = 6000) spectra of SDSS/SEGUE stars, we demonstrate that we are able to
measure [alpha/Fe] from SDSS/SEGUE spectra (with S/N > 20/1) to a precision of
better than 0.1 dex, for stars with atmospheric parameters in the range Teff =
[4500, 7000] K, log g = [1.5, 5.0], and [Fe/H] = [-1.4, +0.3], over the range
[alpha/Fe] = [-0.1, +0.6]. For stars with [Fe/H] < -1.4, our method requires
spectra with slightly higher signal-to-noise to achieve this precision (S/N >
25/1). Over the full temperature range considered, the lowest metallicity star
for which a confident estimate of [alpha/Fe] can be obtained from our approach
is [Fe/H] ~ -2.5; preliminary tests indicate that a metallicity limit as low as
[Fe/H] ~ -3.0 may apply to cooler stars. As a further validation of this
approach, weighted averages of [alpha/Fe] obtained for SEGUE spectra of likely
member stars of Galactic globular clusters (M15, M13, and M71) and open
clusters (NGC 2420, M67, and NGC 6791) exhibit good agreement with the values
of [alpha/Fe] from previous studies. The results of the comparison with NGC
6791 imply that the metallicity range for the method may extend to ~ +0.5.Comment: 47 pages, 11 figures, 5 tables, to appear in A
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