1,088 research outputs found
White dwarf atmosphere models with Ly- opacity in the analysis of the white dwarf cooling sequence of NGC 6397
We discuss the importance of pure hydrogen white dwarf atmosphere models with
Ly- far red wing opacity in the analysis of the white dwarf cooling
sequence of the globular cluster NGC 6397. Our recently improved atmosphere
models account for the previously missing opacity from the Ly-
hydrogen line broadened by collisions of the absorbing hydrogen atoms with
molecular and atomic hydrogen. These models are the first that well reproduce
the UV colors and spectral energy distributions of cool white dwarfs with
observed in the Galactic Disk. Fitting the observed
magnitude and color we obtained a value for the true
distance modulus, , that is in agreement with recent
analyses. We show that the stars at the end of the cooling sequence appear to
be cooler when models that account for Ly- opacity
are used. This indicates that the age of NGC 6397 derived from the white dwarf
cooling sequence using atmosphere models that do not include the correct
Ly- opacity is underestimated by Gyr. Our analysis shows
that it is essential to use white dwarf atmosphere models with Ly-
opacity for precise dating of old stellar populations from white dwarf cooling
sequences.Comment: 4 pages, 4 figures. Accepted for publication in A&
Effectiveness of a 10-day melarsoprol schedule for the treatment of late-stage human African trypanosomiasis: confirmation from a multinational study (IMPAMEL II).
BACKGROUND: Treatment of late-stage human African trypanosomiasis (HAT) with melarsoprol can be improved by shortening the regimen. A previous trial demonstrated the safety and efficacy of a 10-day treatment schedule. We demonstrate the effectiveness of this schedule in a noncontrolled, multinational drug-utilization study. METHODS: A total of 2020 patients with late-stage HAT were treated with the 10-day melarsoprol schedule in 16 centers in 7 African countries. We assessed outcome on the basis of major adverse events and the cure rate after treatment and during 2 years of follow-up. RESULTS: The cure rate 24 h after treatment was 93.9%; 2 years later, it was 86.2%. However, 49.3% of patients were lost to follow-up. The overall fatality rate was 5.9%. Of treated patients, 8.7% had an encephalopathic syndrome that was fatal 45.5% of the time. The rate of severe bullous and maculopapular eruptions was 0.8% and 6.8%, respectively. CONCLUSIONS: The 10-day treatment schedule was well implemented in the field and was effective. It reduces treatment duration, drug amount, and hospitalization costs per patient, and it increases treatment-center capacity. The shorter protocol has been recommended by the International Scientific Council for Trypanosomiasis Research and Control for the treatment of late-stage HAT caused by Trypanosoma brucei gambiense
The Masses of Population II White Dwarfs
Globular star clusters are among the first stellar populations to have formed
in the Milky Way, and thus only a small sliver of their initial spectrum of
stellar types are still burning hydrogen on the main-sequence today. Almost all
of the stars born with more mass than 0.8 M_sun have evolved to form the white
dwarf cooling sequence of these systems, and the distribution and properties of
these remnants uniquely holds clues related to the nature of the now evolved
progenitor stars. With ultra-deep HST imaging observations, rich white dwarf
populations of four nearby Milky Way globular clusters have recently been
uncovered, and are found to extend an impressive 5 - 8 magnitudes in the
faint-blue region of the H-R diagram. In this paper, we characterize the
properties of these population II remnants by presenting the first direct mass
measurements of individual white dwarfs near the tip of the cooling sequence in
the nearest of the Milky Way globulars, M4. Based on Gemini/GMOS and Keck/LRIS
multiobject spectroscopic observations, our results indicate that 0.8 M_sun
population II main-sequence stars evolving today form 0.53 +/- 0.01 M_sun white
dwarfs. We discuss the implications of this result as it relates to our
understanding of stellar structure and evolution of population II stars and for
the age of the Galactic halo, as measured with white dwarf cooling theory.Comment: Accepted for Publication in Astrophys. J. on Aug. 05th, 2009. 19
pages including 9 figures and 2 tables (journal format
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Active Device Fabrication Using Fiber Encapsulation Additive Manufacturing
Fiber Encapsulation Additive Manufacturing (FEAM) is a novel solid freeform
fabrication process in which a fiber and a matrix are co-deposited simultaneously within a single
printer along straight and curved 2-D and 3-D paths. Using a FEAM approach in which the fiber
is a metal wire and the matrix is a thermoplastic polymer, simple electromechanical devices such
as voice coils, inductive sensors, and membrane switches have been successfully produced. This
paper will present an overview of the FEAM process, describe several fabricated devices, and
discuss recent developments in controllably stopping and starting the wire, and in creating
electrical junctions between individual wires, which together enable much more complex devices
to be made.Mechanical Engineerin
An Empirical Measure of the Rate of White Dwarf Cooling in 47 Tucanae
We present an empirical determination of the white dwarf cooling sequence in
the globular cluster 47 Tucanae. Using spectral models, we determine
temperatures for 887 objects from Wide Field Camera 3 data, as well as 292
objects from data taken with the Advanced Camera for Surveys. We make the
assumption that the rate of white dwarf formation in the cluster is constant.
Stellar evolution models are then used to determine the rate at which objects
are leaving the main sequence, which must be the same as the rate at which
objects are arriving on the white dwarf sequence in our field. The result is an
empirically derived relation between temperature () and time () on
the white dwarf cooling sequence. Comparing this result to theoretical cooling
models, we find general agreement with the expected slopes between 20,000K and
30,000K and between 6,000K and 20,000K, but the transition to the Mestel
cooling rate of is found to occur at hotter
temperatures, and more abruptly than is predicted by any of these models.Comment: 10 pages, 16 figures, accepted for publication in Ap
Deep HST Imaging in NGC 6397: Stellar Dynamics
Multi-epoch observations with ACS on HST provide a unique and comprehensive
probe of stellar dynamics within NGC 6397. We are able to confront analytic
models of the globular cluster with the observed stellar proper motions. The
measured proper motions probe well along the main sequence from 0.8 to below
0.1 M as well as white dwarfs younger than one gigayear. The observed
field lies just beyond the half-light radius where standard models of globular
cluster dynamics (e.g. based on a lowered Maxwellian phase-space distribution)
make very robust predictions for the stellar proper motions as a function of
mass. The observed proper motions show no evidence for anisotropy in the
velocity distribution; furthermore, the observations agree in detail with a
straightforward model of the stellar distribution function. We do not find any
evidence that the young white dwarfs have received a natal kick in
contradiction with earlier results. Using the observed proper motions of the
main-sequence stars, we obtain a kinematic estimate of the distance to NGC 6397
of kpc and a mass of the cluster of at the photometric distance of 2.53 kpc. One of the
main-sequence stars appears to travel on a trajectory that will escape the
cluster, yielding an estimate of the evaporation timescale, over which the
number of stars in the cluster decreases by a factor of e, of about 3 Gyr. The
proper motions of the youngest white dwarfs appear to resemble those of the
most massive main-sequence stars, providing the first direct constraint on the
relaxation time of the stars in a globular cluster of greater than or about 0.7
Gyr.Comment: 25 pages, 20 figures, accepted for publication in Astrophysical
Journa
An Age Difference of 2 Gyr between a Metal-Rich and a Metal-Poor Globular Cluster
Globular clusters trace the formation history of the spheroidal components of
both our Galaxy and others, which represent the bulk of star formation over the
history of the universe. They also exhibit a range of metallicities, with
metal-poor clusters dominating the stellar halo of the Galaxy, and higher
metallicity clusters found within the inner Galaxy, associated with the stellar
bulge, or the thick disk. Age differences between these clusters can indicate
the sequence in which the components of the Galaxy formed, and in particular
which clusters were formed outside the Galaxy and later swallowed along with
their original host galaxies, and which were formed in situ. Here we present an
age determination of the metal-rich globular cluster 47 Tucanae by fitting the
properties of the cluster white dwarf population, which implies an absolute age
of 9.9 (0.7) Gyr at 95% confidence. This is about 2.0 Gyr younger than inferred
for the metal-poor cluster NGC 6397 from the same models, and provides
quantitative evidence that metal-rich clusters like 47 Tucanae formed later
than the metal-poor halo clusters like NGC 6397.Comment: Main Article: 10 pages, 4 figures; Supplementary Info 15 pages, 5
figures. Nature, Aug 1, 201
Carbon Stars and other Luminous Stellar Populations in M33
The M33 galaxy is a nearby, relatively metal-poor, late-type spiral. Its
proximity and almost face-on inclination means that it projects over a large
area on the sky, making it an ideal candidate for wide-field CCD mosaic
imaging. Photometry was obtained for more than 10^6 stars covering a 74' x 56'
field centered on M33. Main sequence (MS), supergiant branch (SGB), red giant
branch (RGB) and asymptotic giant branch (AGB) populations are identified and
classified based on broad-band V and I photometry. Narrow-band filters are used
to measure spectral features allowing the AGB population to be further divided
into C and M-star types. The galactic structure of M33 is examined using star
counts, colour-colour and colour-magnitude selected stellar populations. We use
the C to M-star ratio to investigate the metallicity gradient in the disk of
M33. The C/M-star ratio is found to increase and then flatten with increasing
galactocentric radius in agreement with viscous disk formation models. The
C-star luminosity function is found to be similar to M31 and the SMC,
suggesting that C-stars should be useful distance indicators. The ``spectacular
arcs of carbon stars'' in M33 postulated recently by Block et al. (2004) are
found in our work to be simply an extension of M33's disk.Comment: 20 pages, 20 figures. Accepted for publication in The Astronomical
Journa
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