3,695 research outputs found
O/Fe in metal-poor main sequence and subgiant stars
A study of the O/Fe ratio in metal-poor main sequence and subgiant stars is
presented using the [OI] 6300A line, the OI 7774A triplet, and a selection of
weak FeII lines observed on high-resolution spectra acquired with the VLT UVES
spectrograph. The [OI] line is detected in the spectra of 18 stars with -0.5 <
[Fe/H] < -2.4, and the triplet is observed for 15 stars with [Fe/H] ranging
from -1.0 to -2.7. The abundance analysis was made first using standard model
atmospheres taking into account non-LTE effects on the triplet: the [OI] line
and the triplet give consistent results with [O/Fe] increasing quasi-linearly
with decreasing [Fe/H] reaching [O/Fe] ~ +0.7 at [Fe/H] = -2.5. When
hydrodynamical model atmospheres representing stellar granulation in dwarf and
subgiant stars replace standard models, the [O/Fe] from the [OI] and FeII lines
is decreased by an amount which increases with decreasing [Fe/H]. The [O/Fe] vs
[Fe/H] relation remains quasi-linear extending to [O/Fe] ~ +0.5 at [Fe/H] =
-2.5, but with a tendency of a plateau with [O/Fe] ~ +0.3 for -2.0 < [Fe/H] <
-1.0, and a hint of cosmic scatter in [O/Fe] at [Fe/H] ~ -1.0. Use of the
hydrodynamical models disturbs the broad agreement between the oxygen
abundances from the [OI], OI, and OH lines, but 3D non-LTE effects may serve to
erase these differences.Comment: ps file, 18 pages (including 10 figures) - Accepted for publication
in A&
Observations of spatial and velocity structure in the Orion Molecular Cloud
Observations are reported of H2 IR emission in the S(1) v=1-0 line at 2.121
microns in the Orion Molecular Cloud, OMC1, using the GriF instrument on the
Canada-France-Hawaii Telescope. GriF is a combination of adaptive optics and
Fabry-Perot interferometry, yielding a spatial resolution of 0.15" to 0.18" and
a velocity discrimination as high as 1 km/s. Thanks to the high spatial and
velocity resolution of the GriF data, 193 bright H2 emission regions can be
identified in OMC1. The general characteristics of these features are described
in terms of radial velocities, brightness and spatial displacement of maxima of
velocity and brightness, the latter to yield the orientation of flows in the
plane of the sky. Strong spatial correlation between velocity and bright H2
emission is found and serves to identify many features as shocks. Important
results are: (i) velocities of the excited gas illustrate the presence of a
zone to the south of BN-IRc2 and Peak 1, and the west of Peak 2, where there is
a powerful blue-shifted outflow with an average velocity of -18 km/s. This is
shown to be the NIR counterpart of an outflow identified in the radio from
source I, a very young O-star. (ii) There is a band of weak velocity features
(<5 km/s) in Peak 1 which may share a common origin through an explosive event,
in the BN-IRc2 region, with the fast-moving fingers (or bullets) to the NW of
OMC1. (iii) A proportion of the flows are likely to represent sites of low mass
star formation and several regions show multiple outflows, probably indicative
of multiple star formation within OMC1. The high spatial and velocity
resolution of the GriF data show these and other features in more detail than
has previously been possible.Comment: 27 pages, 19 figures, submitted to A&A Version 2: Several additions,
including a section on protostellar candidates in OMC1, have been made based
on the referee's suggestions v3: corrected typograph
The First Galaxies: Clues from Element Abundances
It has recently become possible to measure directly the abundances of several
chemical elements in a variety of environments at redshifts up to z = 5. In
this review I summarise the latest observations of Lyman break galaxies, damped
Lyman alpha systems and the Lyman alpha forest with a view to uncovering any
clues which these data may offer to the first episodes of star formation. The
picture which is emerging is one where the universe at z = 3 already included
many of the components of today's galaxies--even at these early times we see
evidence for Populations I and II stars, while the `smoking gun' for Population
III objects may be hidden in the chemical composition of the lowest density
regions of the IGM, yet to be deciphered.Comment: 15 pages, LaTex, 8 Postscript Figures. To appear in the Philosophical
Transactions of The Royal Society, Series
Si and Mn Abundances in Damped Lya Systems with Low Dust Content
We have measured the abundances of Zn, Si, Mn, Cr, Fe, and Ni in three damped
Lyman alpha systems at redshifts z < 1 from high resolution echelle spectra of
QSOs recorded with the Keck I telescope. In all three cases the abundances of
Cr, Fe, and Ni relative to Zn indicate low levels of dust depletions. We
propose that when the proportion of refractory elements locked up in dust
grains is less than about 50 percent, it is plausible to assume an
approximately uniform level of depletion for all grain constituents and, by
applying a small dust correction, recover the intrisic abundances of Si and Mn.
We use this approach on a small sample of damped systems for which it is
appropriate, with the aim of comparing the metallicity dependence of the ratios
[Si/Fe] and [Mn/Fe] with analogous measurements in Milky Way stars. The main
conclusion is that the relative abundances of both elements in distant galaxies
are broadly in line with expectations based on Galactic data. Si displays a
mild enhancement at low metallicities, as expected for an alpha-capture
element, but there are also examples of near-solar [Si/Fe] at [Fe/H] < -1. The
underabundance of Mn at low metallicities is possibly even more pronounced than
that in metal-poor stars, and no absorption system has yet been found where
[Mn/Fe] is solar. The heterogeneous chemical properties of damped Lyman alpha
systems, evident even from this limited set of measurements, provide further
support for the conclusion from imaging studies that a varied population of
galaxies gives rise to this class of QSO absorbers.Comment: 29 pages, LaTex, 7 Postscript Figures. Accepted for Publication in
the Astrophysical Journa
Cosmic Ray Production of Lithium-6 by Structure Formation Shocks in the Early Milky Way: A Fossil Record of Dissipative Processes during Galaxy Formation
While the abundances of Be and B observed in metal-poor halo stars are well
explained as resulting from spallation of CNO-enriched cosmic rays (CRs)
accelerated by supernova shocks, accounting for the observed Li in such
stars with supernova CRs is more problematic. Here we propose that
gravitational shocks induced by infalling and merging sub-Galactic clumps
during hierarchical structure formation of the Galaxy should dissipate enough
energy at early epochs, and CRs accelerated by such shocks can provide a
natural explanation of the observed Li. In clear constrast to supernovae,
structure formation shocks do not eject freshly synthesized CNO nor Fe, so that
the only effective production channel at low metallicity is
fusion, capable of generating sufficient Li with no accompanying Be or B
and no direct correspondence with Fe. Correlations between the Li abundance
and the kinematic properties of the halo stars may also be expected in this
scenario. Further, more extensive observations of Li in metal-poor halo
stars, e.g. by the Subaru HDS or VLT/UVES, may offer us an invaluable fossil
record of dissipative dynamical processes which occurred during the formation
of our Galaxy.Comment: Ap.J. in press; 6 pages, 1 figur
Correlation of the Drag Characteristics of a Typical Pursuit Airplane Obtained from High-Speed Wind-Tunnel and Flight Tests
In order to obtain a correlation of drag data from wind-tunnel and flight tests at high Mach numbers, a typical pursuit airplane, with the propeller removed, was tested in flight at Mach numbers up to 0.755, and the results were compared with wind-tunnel tests of a 1/3-scale model of the airplane. The tests results show that the drag characteristics of the test airplane can be predicted with satisfactory accuracy from tests in the Ames 16-foot high-speed wind tunnel of the Ames Aeronautical Laboratory at both high and low Mach numbers. It is considered that this result is not unique with the airplane
Big Bang Nucleosynthesis with Bound States of Long-lived Charged Particles
Charged particles (X) decaying after primordial nucleosynthesis are
constrained by the requirement that their decay products should not change the
light element abundances drastically. If the decaying particle is negatively
charged (X-) then it will bind to the nuclei. We consider the effects of the
decay of X when bound to Helium-4 and show that this will modify the Lithium
abundances.Comment: 5 pages; v2: references added, figure caption corrected; v3:
references adde
Haemophilus influenzae type b disease in the era of conjugate vaccines: critical factors for successful eradication
Introduction: Prior to implementation of Haemophilus influenzae type b (Hib)-conjugate vaccination programs in the 1990s, Hib was the commonest cause of bacterial meningitis in children aged <5 years. While the burden of all Hib disease has significantly decreased in the post-vaccination era, Hib still accounted for >29,000 deaths worldwide in children aged <5 years in 2015. Areas covered: We reviewed literature data on the most widely used Hib vaccines and vaccination strategies which led to the global prevention and control of Hib disease and aim to highlight important factors for continued disease control and elimination in the future. Expert commentary: More than 90% of countries worldwide have implemented Hib-conjugate vaccination in their national immunization programs. Vaccines containing Hib polyribosylribitol phosphate (PRP) conjugated with tetanus toxoid (Hib-TT) are the most commonly used. Neisseria meningitidis outer membrane protein complex of PRP (Hib-OMP) is also used. Although the kinetics of the immune response varies with Hib vaccine and schedule used, high control of Hib disease was observed in all settings/scenarios. Further improving global Hib vaccination coverage may result in disease elimination. Plain language summary: What is the context? Haemophilus influenzae is causing a variety of diseases, from otitis media and sinusitis to invasive disease (e.g. meningitis and pneumonia). H. influenzae type b (Hib) was the most common cause of bacterial meningitis in children <5 years of age, and especially among <2-year-olds. Even with appropriate treatment, up to 40% of children with bacterial meningitis can suffer permanent disabilities and up to 5% will die. The development of vaccines to protect against Hib disease has started in the late 1970s and has culminated with the licensure of 4 Hib conjugate vaccines, of which 2 are currently widely used. What is new? In this review, we gathered evidence on the different Hib vaccines and vaccination strategies that have contributed to the global prevention and control of Hib disease. The review indicates: the incidence of Hib disease has decreased considerably due to the introduction of Hib vaccines in national immunization programs worldwide. However, Hib disease is not yet completely eradicated. the vaccines currently used offer protection against Hib over long periods of time. carriage of the pathogen by healthy individuals seem to be less frequent, but data are still needed to fully evaluate the impact of vaccination. other H. influenzae types are now more frequent. Why is this important? Despite the huge success of Hib vaccination, continuous surveillance is needed to anticipate potential re-emergences and devise the best strategies for prevention and control of disease. Hib vaccination should be considered in the few countries who have not yet implemented it, to decrease associated morbidity and mortality
Galactic Cosmic Rays from Superbubbles and the Abundances of Lithium, Beryllium, and Boron
In this article we study the galactic evolution of the LiBeB elements within
the framework of a detailed model of the chemical evolution of the Galaxy that
includes galactic cosmic ray nucleosynthesis by particles accelerated in
superbubbles. The chemical composition of the superbubble consists of varying
proportions of ISM and freshly supernova synthesized material. The
observational trends of 6 LiBeB evolution are nicely reproduced by models in
which GCR come from a mixture of 25% of supernova material with 75% of ISM,
except for 6 Li, for which maybe an extra source is required at low
metallicities. To account for 7 Li evolution several additional sources have
been considered (neutrino-induced nucleosynthesis, nova outbursts, C-stars).
The model fulfills the energetic requirements for GCR acceleration.Comment: 25 pages, 9 figures. Accepted for publication in the Astrophysical
Journa
The 'Forbidden' Abundance of Oxygen in the Sun
We reexamine closely the solar photospheric line at 6300 A, which is
attributed to a forbidden line of neutral oxygen, and is widely used in
analyses of other late-type stars.
We use a three-dimensional time-dependent hydrodynamical model solar
atmosphere which has been tested successfully against observed granulation
patterns and an array of absorption lines. We show that the solar line is a
blend with a Ni I line, as previously suggested but oftentimes neglected.
Thanks to accurate atomic data on the [O I] and Ni I lines we are able to
derive an accurate oxygen abundance for the Sun: log epsilon (O) = 8.69 +/-
0.05 dex, a value at the lower end of the distribution of previously published
abundances, but in good agreement with estimates for the local interstellar
medium and hot stars in the solar neighborhood. We conclude by discussing the
implication of the Ni I blend on oxygen abundances derived from the [O I] 6300
A line in disk and halo stars.Comment: 16 pages, 3 eps figures included; a more compact PostScript version
created using emulateapj.sty is available from
http://hebe.as.utexas.edu/recent_publi.html; to appear in ApJ
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