335 research outputs found
The Homogeneity of Interstellar Oxygen in the Galactic Disk
We present an analysis of high resolution HST Space Telescope Imaging
Spectrograph (STIS) observations of O I 1356 and H I Lyman-alpha absorption in
36 sight lines that probe a variety of Galactic disk environments and include
paths that range over nearly 4 orders of magnitude in f(H_2), over 2 orders of
magnitude in mean sight line density, and that extend up to 6.5 kpc in length.
Consequently, we have undertaken the study of gas-phase O/H abundance ratio
homogeneity using the current sample and previously published Goddard
High-Resolution Spectrograph (GHRS) results. Two distinct trends are identified
in the 56 sight line sample: an apparent decrease in gas-phase oxygen abundance
with increasing mean sight line density and a gap between the mean O/H ratio
for sight lines shorter and longer than about 800 pc. The first effect is a
smooth transition between two depletion levels associated with large mean
density intervals; it is centered near a density of 1.5 cm^-3 and is similar to
trends evident in gas-phase abundances of other elements. Paths less dense than
the central value exhibit a mean O/H ratio of log_10 (O/H) = -3.41+/-0.01 (or
390+/-10 ppm), which is consistent with averages determined for several long,
low-density paths observed by STIS (Andre et al. 2003) and short low-density
paths observed by FUSE (Moos et al. 2002). Sight lines of higher mean density
exhibit an average O/H value of log_10 (O/H) = -3.55+/-0.02 (284+/-12 ppm). The
datapoints for low-density paths are scattered more widely than those for
denser sight lines, due to O/H ratios for paths shorter than 800 pc that are
generally about 0.10 dex lower than the values for longer ones.Comment: 33 pages, including 8 figures and 4 tables; accepted for publication
in ApJ, tentatively in Oct 200
Observation of interstellar lithium in the low-metallicity Small Magellanic Cloud
The primordial abundances of light elements produced in the standard theory
of Big Bang nucleosynthesis (BBN) depend only on the cosmic ratio of baryons to
photons, a quantity inferred from observations of the microwave background. The
predicted primordial 7Li abundance is four times that measured in the
atmospheres of Galactic halo stars. This discrepancy could be caused by
modification of surface lithium abundances during the stars' lifetimes or by
physics beyond the Standard Model that affects early nucleosynthesis. The
lithium abundance of low-metallicity gas provides an alternative constraint on
the primordial abundance and cosmic evolution of lithium that is not
susceptible to the in situ modifications that may affect stellar atmospheres.
Here we report observations of interstellar 7Li in the low-metallicity gas of
the Small Magellanic Cloud, a nearby galaxy with a quarter the Sun's
metallicity. The present-day 7Li abundance of the Small Magellanic Cloud is
nearly equal to the BBN predictions, severely constraining the amount of
possible subsequent enrichment of the gas by stellar and cosmic-ray
nucleosynthesis. Our measurements can be reconciled with standard BBN with an
extremely fine-tuned depletion of stellar Li with metallicity. They are also
consistent with non-standard BBN.Comment: Published in Nature. Includes main text and Supplementary
Information. Replaced with final title and abstrac
Reddening law and interstellar dust properties along Magellanic sight-lines
This study establishes that SMC, LMC and Milky Way extinction curves obey the
same extinction law which depends on the 2200A bump size and one parameter, and
generalizes the Cardelli, Clayton and Mathis (1989) relationship. This suggests
that extinction in all three galaxies is of the same nature. The role of linear
reddening laws over all the visible/UV wavelength range, particularly important
in the SMC but also present in the LMC and in the Milky Way, is also
highlighted and discussed.Comment: accepted for publication in Astrophysics and Space Science. 16 pages,
12 figures. Some figures are colour plot
Properties and Origin of the High-Velocity Gas Toward the Large Magellanic Cloud
In the spectra of 139 early-type Large Magellanic Cloud (LMC) stars observed
with FUSE and with deep radio Parkes HI 21-cm observations along those stars,
we search for and analyze the absorption and emission from high-velocity gas at
+90<v<+175 km/s. The HI column density of the high-velocity clouds (HVCs) along
these sightlines ranges from <10^18.4 to 10^19.2 cm^-2. The incidence of the
HVC metal absorption is 70%, significantly higher than the HI emission
occurrence of 32%. We find that the mean metallicity of the HVC is [OI/HI] =
-0.51 (+0.12,-0.16). There is no strong evidence for a large variation in the
HVC metallicity, implying that thes e HVCs have a similar origin and are part
of the same complex. The mean and scatter of the HVC metallicities are more
consistent with the present-day LMC oxygen abundance than that of the Small
Magellanic Cloud or the Milky Way. We find that on average [SiII/OI] = +0.48
(+0.15,- 0.25) and [FeII/OI] = +0.33 (+0.14,-0.21), implying that the HVC
complex is dominantly ionized. The HVC complex has a multiphase structure with
a neutral (OI, FeII), weakly ionized (FeII, NII), and highly ionized (OVI)
components, and has evidence of dust but no molecules. All the observed
properties of the HVC can be explained by an energetic outflow from the LMC.
This is the first example of a large (>10^6 M_sun) HVC complex that is linked
to stellar feedback occurring in a dwarf spiral galaxy.Comment: Accepted for publication in the Ap
Origin and evolution of the light nuclides
After a short historical (and highly subjective) introduction to the field, I
discuss our current understanding of the origin and evolution of the light
nuclides D, He-3, He-4, Li-6, Li-7, Be-9, B-10 and B-11. Despite considerable
observational and theoretical progress, important uncertainties still persist
for each and every one of those nuclides. The present-day abundance of D in the
local interstellar medium is currently uncertain, making it difficult to infer
the recent chemical evolution of the solar neighborhood. To account for the
observed quasi-constancy of He-3 abundance from the Big Bang to our days, the
stellar production of that nuclide must be negligible; however, the scarce
observations of its abundance in planetary nebulae seem to contradict this
idea. The observed Be and B evolution as primaries suggests that the source
composition of cosmic rays has remained quasi-constant since the early days of
the Galaxy, a suggestion with far reaching implications for the origin of
cosmic rays; however, the main idea proposed to account for that constancy,
namely that superbubbles are at the source of cosmic rays, encounters some
serious difficulties. The best explanation for the mismatch between primordial
Li and the observed "Spite-plateau" in halo stars appears to be depletion of Li
in stellar envelopes, by some yet poorly understood mechanism. But this
explanation impacts on the level of the recently discovered early ``Li-6
plateau'', which (if confirmed), seriously challenges current ideas of cosmic
ray nucleosynthesis.Comment: 18 pages, 9 figs. Invited Review in "Symposium on the Composition of
Matter", honoring Johannes Geiss on the occasion of his 80th birthday
(Grindelwald, Switzerland, Sept. 2006), to be published in Space Science
Series of ISS
A Census of Oxygen in Star-Forming Galaxies: An Empirical Model Linking Metallicities, Star Formation Rates and Outflows
In this contribution we present the first census of oxygen in star-forming
galaxies in the local universe. We examine three samples of galaxies with
metallicities and star formation rates at z = 0.07, 0.8 and 2.26, including the
SDSS and DEEP2 surveys. We infer the total mass of oxygen produced and mass of
oxygen found in the gas-phase from our local SDSS sample. The star formation
history is determined by requiring that galaxies evolve along the relation
between stellar mass and star formation rate observed in our three samples. We
show that the observed relation between stellar mass and star formation rate
for our three samples is consistent with other samples in the literature. The
mass-metallicity relation is well established for our three samples and from
this we empirically determine the chemical evolution of star-forming galaxies.
Thus, we are able to simultaneously constrain the star formation rates and
metallicities of galaxies over cosmic time allowing us to estimate the mass of
oxygen locked up in stars. Combining this work with independent measurements
reported in the literature we conclude that the loss of oxygen from the
interstellar medium of local star-forming galaxies is likely to be a ubiquitous
process with the oxygen mass loss scaling (almost) linearly with stellar mass.
We estimate the total baryonic mass loss and argue that only a small fraction
of the baryons inferred from cosmological observations accrete onto galaxies.Comment: 24 pages, 18 figures. Accepted for publication in Ap
Shifting Characterizations of the ‘Common People’ in Modern English Retranslations of Thucydides’ History of the Peloponnesian War: A corpus-based analysis
Little research has yet explored the impact of (re)translation on narrative characterization, that is, on the process through which the various actors depicted in a narrative are attributed particular traits and qualities. Moreover, the few studies that have been published on this topic are either rather more anecdotal than systematic, or their focus is primarily on the losses in character information that inevitably occur when a narrative is retold for a new audience in a new linguistic context. They do not explore how the translator’s own background knowledge and ideological beliefs might affect the characterization process for readers of their target-language text. Consequently, this paper seeks to make two contributions to the field: first, it presents a corpus-based methodology developed as part of the Genealogies of Knowledge project for the comparative analysis of characterization patterns in multiple retranslations of a single source text. Such an approach is valuable, it is argued, because it can enhance our ability to engage in a more systematic manner with the accumulation of characterization cues spread throughout a narrative. Second, the paper seeks to move discussions of the effects of translation on narrative characterization away from a paradigm of loss, deficiency and failure, promoting instead a perspective which embraces the productive role translators often play in reconfiguring the countless narratives through which we come to know, imagine and make sense of the past, our present and futures. The potential of this methodology and theoretical standpoint is illustrated through a case study exploring changes in the characterization of ‘the common people’ in two English-language versions of classical Greek historian Thucydides’ History of the Peloponnesian War, the first produced by Samuel Bloomfield in 1829 and the second by Steven Lattimore in 1998. Particular attention is paid to the referring expressions used by each translator—such as the multitude vs. the common people—as well as the specific attributes assigned to this narrative actor. In this way, the study attempts to gain deeper insight into the ways in which these translations reflect important shifts in attitudes within key political debates concerning the benefits and dangers of democracy
Concentration or representation : the struggle for popular sovereignty
There is a tension in the notion of popular sovereignty, and the notion of democracy associated with it, that is both older than our terms for these notions themselves and more fundamental than the apparently consensual way we tend to use them today. After a review of the competing conceptions of 'the people' that underlie two very different understandings of democracy, this article will defend what might be called a 'neo-Jacobin' commitment to popular sovereignty, understood as the formulation and imposition of a shared political will. A people's egalitarian capacity to concentrate both its collective intelligence and force, from this perspective, takes priority over concerns about how best to represent the full variety of positions and interests that differentiate and divide a community
The rationale and design of the antihypertensives and vascular, endothelial, and cognitive function (AVEC) trial in elderly hypertensives with early cognitive impairment: Role of the renin angiotensin system inhibition
<p>Abstract</p> <p>Background</p> <p>Prior evidence suggests that the renin angiotensin system and antihypertensives that inhibit this system play a role in cognitive, central vascular, and endothelial function. Our objective is to conduct a double-blind randomized controlled clinical trial, the antihypertensives and vascular, endothelial, and cognitive function (AVEC), to compare 1 year treatment of 3 antihypertensives (lisinopril, candesartan, or hydrochlorothiazide) in their effect on memory and executive function, cerebral blood flow, and central endothelial function of seniors with hypertension and early objective evidence of executive or memory impairments.</p> <p>Methods/Design</p> <p>The overall experimental design of the AVEC trial is a 3-arm double blind randomized controlled clinical trial. A total of 100 community eligible individuals (60 years or older) with hypertension and early cognitive impairment are being recruited from the greater Boston area and randomized to lisinopril, candesartan, or hydrochlorothiazide ("active control") for 12 months. The goal of the intervention is to achieve blood pressure control defined as SBP < 140 mm Hg and DBP < 90 mm Hg. Additional antihypertensives are added to achieve this goal if needed. Eligible participants are those with hypertension, defined as a blood pressure 140/90 mm Hg or greater, early cognitive impairment without dementia defined (10 or less out of 15 on the executive clock draw test or 1 standard deviation below the mean on the immediate memory subtest of the repeatable battery for the assessment of neuropsychological status and Mini-Mental-Status-exam >20 and without clinical diagnosis of dementia or Alzheimer's disease). Individuals who are currently receiving antihypertensives are eligible to participate if the participants and the primary care providers are willing to taper their antihypertensives. Participants undergo cognitive assessment, measurements of cerebral blood flow using Transcranial Doppler, and central endothelial function by measuring changes in cerebral blood flow in response to changes in end tidal carbon dioxide at baseline (off antihypertensives), 6, and 12 months. Our outcomes are change in cognitive function score (executive and memory), cerebral blood flow, and carbon dioxide cerebral vasoreactivity.</p> <p>Discussion</p> <p>The AVEC trial is the first study to explore impact of antihypertensives in those who are showing early evidence of cognitive difficulties that did not reach the threshold of dementia. Success of this trial will offer new therapeutic application of antihypertensives that inhibit the renin angiotensin system and new insights in the role of this system in aging.</p> <p>Trial Registration</p> <p>Clinicaltrials.gov NCT00605072</p
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