372 research outputs found
The metal and dust yields of the first massive stars
We quantify the role of Population (Pop) III core-collapse supernovae (SNe)
as the first cosmic dust polluters. Starting from a homogeneous set of stellar
progenitors with masses in the range [13 - 80] Msun, we find that the mass and
composition of newly formed dust depend on the mixing efficiency of the ejecta
and the degree of fallback experienced during the explosion. For standard Pop
III SNe, whose explosions are calibrated to reproduce the average elemental
abundances of Galactic halo stars with [Fe/H] < -2.5, between 0.18 and 3.1 Msun
(0.39 - 1.76 Msun) of dust can form in uniformly mixed (unmixed) ejecta, and
the dominant grain species are silicates. We also investigate dust formation in
the ejecta of faint Pop III SN, where the ejecta experience a strong fallback.
By examining a set of models, tailored to minimize the scatter with the
abundances of carbon-enhanced Galactic halo stars with [Fe/H ] < -4, we find
that amorphous carbon is the only grain species that forms, with masses in the
range 2.7 10^{-3} - 0.27 Msun (7.5 10^{-4} - 0.11 Msun) for uniformly mixed
(unmixed) ejecta models. Finally, for all the models we estimate the amount and
composition of dust that survives the passage of the reverse shock, and find
that, depending on circumstellar medium densities, between 3 and 50% (10 - 80%)
of dust produced by standard (faint) Pop III SNe can contribute to early dust
enrichment.Comment: Accepted by MNRAS, 22 pages, 12 figures, 12 table
School management and moral regulation in Argentina: Case study in a peripheral school
La “Nueva Gestión Pública” (NGP) viene ejerciendo, desde finales de la década del 80, claros
efectos en la vida de las escuelas y en la regulación de las prácticas de los directivos. En esta
ponencia se aborda el trabajo del director escolar y los procesos de regulación de las políticas
educativas orientadas a garantizar la escolaridad secundaria obligatoria. Presentamos un
estudio en caso donde abordamos el trabajo del director de una escuela localizada en una
Ciudad-Barrio de la Ciudad de Córdoba a la que asisten alumnos sumidos en crecientes
procesos de pauperización social. A modo de hipótesis proponemos que el llamado al
compromiso para con el alumnado y sus demandas sociales insatisfechas se transforman
en el motor que regula moralmente al director y lo obliga a trabajar para y a pesar de no
recibirlos los prometidos fondos que los proyectos ministeriales deben proveerSince the 1980s, the "New Public Management" (NGP) has had clear effects on the life of
schools and in the regulation of principal’s practices. This paper addresses the work of the
principal and the processes of regulation of educational policies aimed at ensuring
compulsory secondary schooling. We present a study in case focused in a school located in
a City-Neighborhood of of Cordoba were students are immersed in growing processes of
social impoverishment. As a hypothesis, we propose that the call to commitment to students
and their unmet social demands are transformed into the motor that morally regulates the
principal and forces him to work for and despite not receiving the promised funds that the
ministerial projects must provid
A Follow-Up Study (1970-1972) of the 1969 State Scholarship Program in the State of Rhode Island
Introduction
The philosophy and objectives underlying the Rhode Island State Scholarship Program encompassed severa1 basic assumptions about secondary education. Some of these assumptions have changed substantially over the past fifteen years and deserve close consideration. The new emphasis toward equalizing educational opportunities had been reflected in the changing philosophy that guided the administration of student financial aid. The genesis of most aid programs could be found in a concept that fostered the belief in a system built entirely upon meritocracy. This approach provided aid to students with high achievement and measured potential, irrespective of financial need. Educators seeing the inequity and irrationality of this approach developed a more standardized and need-analysis system. High ability students who needed assistance were given scholarships, but as the research over the past .five years had indicated, this kind of approach benefited only a few of the truly financially impoverished students. The overriding criterion of previous academic success has had the effect of excluding the most needy from participation. The review of the literature for this study was divided into three areas: (l) state scholarship or grant programs, {2) federal scholarship grant or loan programs, and (3) general student aid program models.
Procedures
The procedures followed in the conduct of the study were typical of those used in descriptive research design. A series of questions was identified bearing on important educational concerns; the related literature was reviewed; a survey instrument was designed and tested; follow ups were made; and the collected data were analyzed for significant findings and implications. As a means of assessing the effectiveness ·of the Rhode Island scholarship programs, the investigators conducted a survey of Rhode Island high school seniors in November 1970 and 1971. Four major variables relating to tests for higher education, academic and economic readiness for higher education, and specific demographic characteristics were investigated through a questionnaire administered in the high schools. The pooled responses to the questionnaire totaled 19,313 returns over the two-year period. This return represented 74 per cent of the 25,920 seniors in Rhode Island high schools during 1970 and 1971. Because of this relatively high return, the findings of the study were assumed to be indicative of the larger population and adequately described the characteristics of this total population. The significant finding indicated that a major discrepancy existed between the needs of the students and the delivery system of the Rhode Islan
Halo dust detection around NGC 891
Observations of edge-on galaxies allow us to investigate the vertical extent
and properties of dust, gas and stellar distributions. NGC 891 has been studied
for decades and represents one of the best studied cases of an edge-on galaxy.
We use deep PACS data together with IRAC, MIPS and SPIRE data to study the
vertical extent of dust emission around NGC 891. We also test the presence of a
more extended, thick dust component. By performing a convolution of an
intrinsic vertical profile emission with each instrument PSF and comparing it
with observations we derived the scaleheight of a thin and thick dust disc
component. For all wavelengths considered the emission is best fit with the sum
of a thin and a thick dust component. The scaleheight of both dust components
shows a gradient passing from 70 m to 250 m. This could be due to a
drop in dust heating (and thus dust temperature) with the distance from the
plane, or to a sizable contribution () of an unresolved thin
disc of hotter dust to the observed surface brightness at shorter wavelengths.
The scaleheight of the thick dust component, using observations from 70 m
to 250 m has been estimated to be kpc, consistent with
previous estimates (extinction and scattering in optical bands and MIR
emission). The amount of dust mass at distances larger than kpc from
the midplane represents % of the total galactic dust mass and the
relative abundance of small grains with respect to large grains is almost
halved comparing to that in the midplane. The paucity of small grains high
above the midplane might indicate that dust is hit by interstellar shocks or
galactic fountains and entrained together with gas. The halo dust component is
likely to be embedded in an atomic / molecular gas and heated by a thick
stellar disc.Comment: 15 pages, 13 figures, A&A accepte
Hippocampal Theta Input to the Amygdala Shapes Feedforward Inhibition to Gate Heterosynaptic Plasticity
The dynamic interactions between hippocampus and amygdala are critical for emotional memory. Theta synchrony between these structures occurs during fear memory retrieval and may facilitate synaptic plasticity, but the cellular mechanisms are unknown. We report that interneurons of the mouse basal amygdala are activated during theta network activity or optogenetic stimulation of ventral CA1 pyramidal cell axons, whereas principal neurons are inhibited. Interneurons provide feedforward inhibition that transiently hyperpolarizes principal neurons. However, synaptic inhibition attenuates during theta frequency stimulation of ventral CA1 fibers, and this broadens excitatory postsynaptic potentials. These effects are mediated by GABAB receptors and change in the Cl- driving force. Pairing theta frequency stimulation of ventral CA1 fibers with coincident stimuli of the lateral amygdala induces long-term potentiation of lateral-basal amygdala excitatory synapses. Hence, feedforward inhibition, known to enforce temporal fidelity of excitatory inputs, dominates hippocampus-amygdala interactions to gate heterosynaptic plasticity. Video Abstract: Hippocampal-amygdala interactions are critical for emotional memory, but the cellular mechanisms are unknown. In this paper, Bazelot, Bocchio et al. functionally demonstrate that GABAergic neurons of the basal amygdala gate principal neuron firing and heterosynaptic plasticity in the mouse amygdala
The evolution of amorphous hydrocarbons in the ISM: dust modelling from a new vantage point
Context. The evolution of amorphous hydrocarbon materials, a-C(:H),
principally resulting from ultraviolet (UV) photon absorption- induced
processing, are likely at the heart of the variations in the observed
properties of dust in the interstellar medium. Aims. The consequences of the
size-dependent and compositional variations in a-C(:H), from aliphatic-rich
a-C:H to aromatic-rich a-C, are studied within the context of the interstellar
dust extinction and emission. Methods. Newly-derived optical property data for
a-C(:H) materials, combined with that for an amorphous forsterite-type silicate
with iron nano-particle inclusions, a-SilFe, are used to explore dust evolution
in the interstellar medium. Results. We present a new dust model that consists
of a power-law distribution of small a-C grains and log-normal distributions of
large a-SilFe and a-C(:H) grains. The model, which is firmly anchored by
laboratory-data, is shown to quite naturally explain the variations in the
infrared (IR) to far-ultraviolet (FUV) extinction, the 217 nm UV bump, the IR
absorption and emission bands and the IR-mm dust emission. Conclusions. The
major strengths of the new model are its inherent simplicity and built-in
capacity to follow dust evolution in interstellar media. We show that mantle
accretion in molecular clouds and UV photo-processing in photo-dominated
regions are likely the major drivers of dust evolution.Comment: 22 pages, 20 figure
Photonic mode density effects on single-molecule fluorescence blinking
We investigated the influence of the photonic mode density (PMD) on the
triplet dynamics of individual chromophores on a dielectric interface by
comparing their response in the presence and absence of a nearby gold film.
Lifetimes of the excited singlet state were evaluated in ordet to measure
directly the PMD at the molecules position. Triplet state lifetimes were
simultaneously determined by statistical analysis of the detection time of the
fluorescence photons. The observed singlet decay rates are in agreement with
the predicted PMD for molecules with different orientations. The triplet decay
rate is modified in a fashion correlated to the singlet decay rate. These
results show that PMD engineering can lead to an important suppression of the
fluorescence, introducing a novel aspect of the physical mechanism to enhance
fluorescence intensity in PMD-enhancing systems such as plasmonic devices
Psychological features in men with erectile dysfunction with or without preclinical atherosclerosis
n/
Gas-to-Dust mass ratios in local galaxies over a 2 dex metallicity range
This paper analyses the behaviour of the gas-to-dust mass ratio (G/D) of
local Universe galaxies over a large metallicity range. We combine three
samples: the Dwarf Galaxy Survey, the KINGFISH survey and a subsample from
Galametz et al. (2011) totalling 126 galaxies, covering a 2 dex metallicity
range, with 30% of the sample with 12+log(O/H) < 8.0. The dust masses are
homogeneously determined with a semi-empirical dust model, including submm
constraints. The atomic and molecular gas masses are compiled from the
literature. Two XCO are used to estimate molecular gas masses: the Galactic
XCO, and a XCO depending on the metallicity (as Z^{-2}). Correlations with
morphological types, stellar masses, star formation rates and specific star
formation rates are discussed. The trend between G/D and metallicity is
empirically modelled using power-laws (slope of -1 and free) and a broken
power-law. We compare the evolution of the G/D with predictions from chemical
evolution models. We find that out of the five tested galactic parameters,
metallicity is the galactic property driving the observed G/D. The G/D versus
metallicity relation cannot be represented by a power-law with a slope of -1
over the whole metallicity range. The observed trend is steeper for
metallicities lower than ~ 8.0. A large scatter is observed in the G/D for a
given metallicity, with a dispersion of 0.37 dex in metallicity bins of ~0.1
dex. The broken power-law reproduces best the observed G/D and provides
estimates of the G/D that are accurate to a factor of 1.6. The good agreement
of the G/D and its scatter with the three tested chemical evolution models
shows that the scatter is intrinsic to galactic properties, reflecting the
different star formation histories, dust destruction efficiencies, dust grain
size distributions and chemical compositions across the sample. (abriged)Comment: 23 pages, 12 figures, accepted in Astronomy & Astrophysic
- …