6,602 research outputs found
Quantifying stellar radial migration in an N-body simulation: blurring, churning, and the outer regions of galaxy discs
Radial stellar migration in galactic discs has received much attention in
studies of galactic dynamics and chemical evolution, but remains a dynamical
phenomenon that needs to be fully quantified. In this work, using a Tree-SPH
simulation of an Sb-type disc galaxy, we quantify the effects of blurring
(epicyclic excursions) and churning (change of guiding radius). We quantify
migration (either blurring or churning) both in terms of flux (the number of
migrators passing at a given radius), and by estimating the population of
migrators at a given radius at the end of the simulation compared to
non-migrators, but also by giving the distance over which the migration is
effective at all radii. We confirm that the corotation of the bar is the main
source of migrators by churning in a bar-dominated galaxy, its intensity being
directly linked to the episode of a strong bar, in the first 1-3 Gyr of the
simulation. We show that within the outer Lindblad resonance (OLR), migration
is strongly dominated by churning, while blurring gains progressively more
importance towards the outer disc and at later times. Most importantly, we show
that the OLR limits the exchange of angular momentum, separating the disc in
two distinct parts with minimal or null exchange, except in the transition
zone, which is delimited by the position of the OLR at the epoch of the
formation of the bar, and at the final epoch. We discuss the consequences of
these findings for our understanding of the structure of the Milky Way disc.
Because the Sun is situated slightly outside the OLR, we suggest that the solar
vicinity may have experienced very limited churning from the inner disc.Comment: Accepted for publication in Astronomy and Astrophysics (acceptance
date: 27/04/15), 24 pages, 24 figure
Hiding its age: the case for a younger bulge
The determination of the age of the bulge has led to two contradictory
results. On the one side, the color-magnitude diagrams in different bulge
fields seem to indicate a uniformly old (10 Gyr) population. On the other
side, individual ages derived from dwarfs observed through microlensing events
seem to indicate a large spread, from 2 to 13 Gyr. Because the
bulge is now recognised as being mainly a boxy peanut-shaped bar, it is
suggested that disk stars are one of its main constituents, and therefore also
stars with ages significantly younger than 10 Gyr. Other arguments as well
point to the fact that the bulge cannot be exclusively old, and in particular
cannot be a burst population, as it is usually expected if the bulge was the
fossil remnant of a merger phase in the early Galaxy. In the present study, we
show that given the range of metallicities observed in the bulge, a uniformly
old population would be reflected into a significant spread in color at the
turn-off which is not observed. Inversely, we demonstrate that the correlation
between age and metallicity expected to hold for the inner disk would conspire
to form a color-magnitude diagram with a remarkably small spread in color, thus
mimicking the color-magnitude diagram of a uniformly old population. If stars
younger than 10 Gyr are part of the bulge, as must be the case if the bulge has
been mainly formed through dynamical instabilities in the disk, then a very
small spread at the turn-off is expected, as seen in the observations.Comment: 11 pages, 11 figures. Accepted for publication in A&
The age structure of stellar populations in the solar vicinity. Clues of a two-phase formation history of the Milky Way disk
We analyze high quality abundances data of solar neighborhood stars and show
that there are two distinct regimes of [alpha/Fe] versus age which we identify
as the epochs of the thick and thin disk formation. A tight correlation between
metallicity and [alpha/Fe] versus age is clearly identifiable on thick disk
stars, implying that this population formed from a well mixed ISM, over a time
scale of 4-5 Gyr. Thick disk stars vertical velocity dispersion correlate with
age, with the youngest objects having as small scale heights as those of thin
disk stars. A natural consequence of these two results is that a vertical
metallicity gradient is expected in this population. We suggest that the thick
disk set the initial conditions for the formation of the inner thin disk. This
provides also an explanation of the apparent coincidence between the step in
metallicity at 7-10 kpc in the thin disk and the confinment of the thick disk
at about R<10 kpc. We suggest that the outer thin disk developped outside the
influence of the thick disk, but also that the high alpha-enrichment of the
outer regions may originate from a primordial pollution by the gas expelled
from the thick disk. Local metal-poor thin disk stars, whose properties are
best explained by an origin in the outer disk, are shown to be as old as the
youngest thick disk (9-10 Gyr), implying that the outer thin disk started to
form while the thick disk formation was still on-going in the inner Galaxy. We
point out that, given the tight age-abundance relations in the thick disk, an
inside-out process would give rise to a radial gradient in abundances in this
population which is not observed. Finally, we argue that the data discussed
here leave little room for radial migration, either to have contaminated the
solar vicinity, or, to have redistributed stars in significant proportion
across the solar annulus.Comment: Accepted in A&A, Revised version with new figures and extended
discussio
Revisiting two local constraints of the Galactic chemical evolution
I review the uncertainties in two observational constraints of the Galactic
disc chemical evolution: the metallicity distribution (MD) of long-lived dwarfs
and the age-metallicity relation (AMR). It is shown first that the observed MD,
designed with standard methods, is more fit to a closed-box model than to the
infall MD. We argue that this is due to the specific contribution of the thick
disc population, which has been overlooked both in the derivation of the
observed MD and in the standard chemical evolution models. Although this
agreement disqualifies the MD as the best supportive evidence for infall, we
argue that the evolution must be more complex than described by either the
closed-box or the standard infall models. A new age-metallicity distribution is
obtained, where particularities of the previous recent determinations are
phased out. The new AMR shows a mean increase limited to about a factor of 2 in
Z over the disc age. It is shown that below 3 Gyrs, the dispersion in
metallicity is about 0.1 dex, which, (given the observational uncertainties),
is compatible with the small cosmic dispersion measured on the interstellar
medium and meteoritic pre-solar dust grains. A population that is progressively
older and more metal rich arises at a metallicity greater than that of the
Hyades, to reach [Fe/H] ~ +0.5 dex at ages > 5 Gyr. We suggest that this is
best explained by radial migration. A symmetrical widening towards lower
metallicities is seen at about the same age. Finally, the new derived ages are
sufficiently consistent that an AMR within the thick disc is confirmed. These
new features altogether draw a picture of the chemical evolution where
dynamical effects and complexity in the AMR dominate, rather than a generalized
high dispersion at all ages. (abridged)Comment: 19 pages, 13 figures. Accepted for publication in MNRA
A Seven Year Reaction to the Single Audit Act of 1984
The Single Audit Act of 1984 attempted to provide a sense of uniformity amongst governmental audits. It provided strict regulations and procedures the auditor must follow when conducting the audit. Having only one audit provides efficient use of resources, rather than having many audits instituted by each relevant federal agency. The auditors must follow generally accepted governmental auditing standards when performing the audit. Controls and compliance with proper regulations have to be examined to ensure they are being utilized effectively. Reactions from all of the relevant parties have been positive and supportive, so the Single Audit Act should be around for a long time guiding the auditors in their audits of governmental units.B.A. (Bachelor of Arts
Juan de Torres's Poetics of Vision : "Oiosqueyanovesque"
In this article, I examine Juan de Torres's poetics of the visual through a reading of his Cancionero de Palacio dealing with the visual sphere. I argue that his poetry demonstrates familiarity with medieval scholastic psychology, particularly in relation to sight and memory. The visual sphere can be understood as both the external, phenomenological and somatic world of experience and intersubject interaction, and as the interior world of the psyche and the affect. Lacanian and Jamesonian reading strategies are deployed to approach latent psychological and socio-political content in Torres's representation of the psychic disarray of the lover. The visual sphere is the medium by and through which desire is apprehended and the subject inscribes itself in the symbolic order, seeks the desire of the Other and is subject to the surveying gaze of power hierarchies. Torres's work shows great skill and wit, and stands as a particularly good example of the way in which a highly abstract poetic corpus deals directly with the visual understood as power and hierarchy.El presente artículo examina una gama de obras de Juan de Torres del Cancionero de Palacio en que el vate trata un temario que engloba el campo visual. En las obras bajo consideración Torres recurre a la teoría de las facultades o potencias del alma según la psicología escolástica, sobre todo en su relación con el sentido de la vista y la memoria. El campo visual abarca tanto el mundo externo, fenomenológico y somático de interacciones intersubjetivas como el interno del ánimo y los sentimientos. Uso las ópticas lacaniana y jamesoniana para explicar el contenido psicológico latente y socio-político de la representación de la confusión anímica del yo-lírico. El campo visual es el medio por el cual se percibe el deseo y el sujeto se inscribe en el orden simbólico, busca el deseo del Otro y es sujeto a la vigilancia del poder. La poética de Torres manifiesta con gran destreza y cierto tono humorístico la manera en que una poesía con alto grado de abstracción trata directamente con el campo visual como registro de poder y jerarquía
When the Milky Way turned off the lights: APOGEE provides evidence of star formation quenching in our Galaxy
Quenching, the cessation of star formation, is one of the most significant
events in the life cycle of galaxies. We show here the first evidence that the
Milky Way experienced a generalised quenching of its star formation at the end
of its thick disk formation 9 Gyr ago. Elemental abundances of stars
studied as part of the APOGEE survey reveal indeed that in less than 2
Gyr the star formation rate in our Galaxy dropped by an order-of-magnitude.
Because of the tight correlation between age and alpha abundance, this event
reflects in the dearth of stars along the inner disk sequence in the
[Fe/H]-[/Fe] plane. Before this phase, which lasted about 1.5 Gyr, the
Milky Way was actively forming stars. Afterwards, the star formation resumed at
a much lower level to form the thin disk. These events are very well matched by
the latest observation of MW-type progenitors at high redshifts. In late type
galaxies, quenching is believed to be related to a long and secular exhaustion
of gas. In our Galaxy, it occurred on a much shorter time scale, while the
chemical continuity before and after the quenching indicates that it was not
due to the exhaustion of the gas. While quenching is generally associated with
spheroids, our results show that it also occurs in galaxies like the Milky Way,
possibly when they are undergoing a morphological transition from thick to thin
disks. Given the demographics of late type galaxies in the local universe, in
which classical bulges are rare, we suggest further that this may hold true
generally in galaxies with mass lower than or approximately , where
quenching could be directly a consequence of thick disk formation. We emphasize
that the quenching phase in the Milky Way could be contemporaneous with, and
related to, the formation of the bar. We sketch a scenario on how a strong bar
may inhibit star formation.Comment: 17 pages, 8 figures. Published versio
Signatures of radial migration in barred galaxies: Azimuthal variations in the metallicity distribution of old stars
By means of N-body simulations, we show that radial migration in galaxy
disks, induced by bar and spiral arms, leads to significant azimuthal
variations in the metallicity distribution of old stars at a given distance
from the galaxy center. Metals do not show an axisymmetric distribution during
phases of strong migration. Azimuthal variations are visible during the whole
phase of strong bar phase, and tend to disappear as the effect of radial
migration diminishes, together with a reduction in the bar strength. These
results suggest that the presence of inhomogeneities in the metallicity
distribution of old stars in a galaxy disk can be a probe of ongoing strong
migration. Such signatures may be detected in the Milky Way by Gaia (and
complementary spectroscopic data), as well as in external galaxies, by IFU
surveys like CALIFA and ATLAS3D. Mixing - defined as the tendency toward a
homogeneous, azimuthally symmetric, stellar distribution in the disk - and
migration turns out to be two distinct processes, the effects of mixing
starting to be visible when strong migration is over.Comment: 8 pages, 10 figures, accepted for publication on Astronomy and
Astrophysic
Composting paper and grass clippings with anaerobically treated palm oil mill effluent
Purpose The purpose of this study is to investigate the composting performance of anaerobically treated palm oil mill effluent (AnPOME) mixed with paper and grass clippings. Methods Composting was conducted using a laboratory scale system for 40 days. Several parameters were determined: temperature, mass reduction, pH, electrical conductivity, colour, zeta potential, phytotoxicity and final compost nutrients. Results The moisture content and compost mass were reduced by 24 and 18 %, respectively. Both final compost pH value and electrical conductivity were found to increase in value. Colour (measured as PtCo) was not suitable as a maturity indicator. The negative zeta potential values decreased from −12.25 to −21.80 mV. The phytotoxicity of the compost mixture was found to decrease in value during the process and the final nutrient value of the compost indicates its suitability as a soil conditioner. Conclusions From this study, we conclude that the addition of paper and grass clippings can be a potential substrate to be composted with anaerobically treated palm oil mill effluent (AnPOME). The final compost produced is suitable for soil conditioner
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