588 research outputs found
Observational Constraints on Interstellar Grain Alignment
We present new multicolor photo-polarimetry of stars behind the Southern
Coalsack. Analyzed together with multiband polarization data from the
literature, probing the Chamaeleon I, Musca, rho Opiuchus, R CrA and Taurus
clouds, we show that the wavelength of maximum polarization (lambda_max) is
linearly correlated with the radiation environment of the grains. Using
Far-Infrared emission data, we show that the large scatter seen in previous
studies of lambda_max as a function of A_V is primarily due to line of sight
effects causing some A_V measurements to not be a good tracer of the extinction
(radiation field strength) seen by the grains being probed. The derived slopes
in lambda_max vs. A_V, for the individual clouds, are consistent with a common
value, while the zero intercepts scale with the average values of the ratios of
total-to-selective extinction (R_V) for the individual clouds. Within each
cloud we do not find direct correlations between lambda_max and R_V. The
positive slope in consistent with recent developments in theory and indicating
alignment driven by the radiation field. The present data cannot conclusively
differentiate between direct radiative torques and alignment driven by H_2
formation. However, the small values of lambda_max(A_V=0), seen in several
clouds, suggest a role for the latter, at least at the cloud surfaces. The
scatter in the lambda_max vs. A_V relation is found to be associated with the
characteristics of the embedded Young Stellar Objects (YSO) in the clouds. We
propose that this is partially due to locally increased plasma damping of the
grain rotation caused by X-rays from the YSOs.Comment: Accepted for publication in the Astrophysical Journa
Cambio Global. Impacto de la Actividad Humana sobre el Sistema Tierra
Esta obra, dirigida al público en general y elaborada por un equipo interdisciplinar, aborda con un lenguaje claro y asequible pero sin renunciar al rigor científico, el complejo problema del cambio global, un desafío de dimensiones colosales para la humanidad. El cambio global se define en esta obra como el conjunto de cambios en los procesos fundamentales que definen el
funcionamiento de la biosfera derivados de la actividad humana. Los autores describen las múltiples dimensiones del cambio global, sus causas y sus consecuencias. Se describe la transformación de la superficie del planeta Tierra por la actividad humana y sus impactos sobre los ciclos del agua, los
elementos y el sistema climático, incluyendo la introducción de miles de compuestos químicos sintéticos en la biosfera. Esta exposición pretende despejar la confusión que el aluvión de noticias e informaciones, a veces contradictorias, genera en la sociedad. Los autores prestan particular atención a la definición de las estrategias de adaptación y mitigación que los distintos actores de la sociedad, desde ciudadanos a
políticos, pueden desplegar para minimizar el impacto del cambio global, animándoles al mismo tiempo a aprovechar las oportunidades que estos cambios pueden conllevar
Deep X-ray survey of the young open cluster NGC 2516 with XMM-Newton
We report on a deep X-ray survey of the young (~ 140 Myr), rich open cluster
NGC 2516 obtained with the EPIC camera on board the XMM-Newton satellite. By
combining data from six observations, a high sensitivity, greater than a factor
5 with respect to recent Chandra observations, has been achieved. We detected
431 X-ray sources and 234 of them have as optical counterparts cluster stars
spanning the entire NGC 2516 Main Sequence. The X-ray luminosities of cluster
stars span the range log Lx (erg/s) = 28.4 - 30.8. The representative
temperatures span the 0.3 - 0.6 keV (3.5 - 8 MK) range for the cool component
and 1.0 - 2.0 keV (12 -- 23 MK) for the hot one; similar values are found in
other young open clusters like the Pleiades, IC 2391, and Blanco 1. While no
significant differences are found in X-ray spectra, NGC 2516 solar type stars
are definitely less luminous in X-rays than the nearly coeval Pleiades. The
comparison with a previous ROSAT survey evidence the lack of variability
amplitudes larger than a factor 2 in solar type stars in a ~11 yr time scale of
the cluster and thus activity cycles like in the Sun are probably absent or
different by period and amplitude in young stars.Comment: 37 pages, 13 figures, to be published on A&
Identification of particles with Lorentz factor up to with Transition Radiation Detectors based on micro-strip silicon detectors
This work is dedicated to the study of a technique for hadron identification
in the TeV momentum range, based on the simultaneous measurement of the
energies and of the emission angles of the Transition Radiation (TR) X-rays
with respect to the radiating particles. A detector setup has been built and
tested with particles in a wide range of Lorentz factors (from about to
about crossing different types of radiators. The measured
double-differential (in energy and angle) spectra of the TR photons are in a
reasonably good agreement with TR simulation predictions.Comment: 31 pages, 12 figures, paper published on Nuclear Instruments &
Methods
Stellar Rotation in Young Clusters. I. Evolution of Projected Rotational Velocity Distributions
Open clusters offer us the means to study stellar properties in samples with
well-defined ages and initial chemical composition. Here we present a survey of
projected rotational velocities for a large sample of mainly B-type stars in
young clusters to study the time evolution of the rotational properties of
massive stars. The survey is based upon moderate resolution spectra made with
the WIYN 3.5 m and CTIO 4 m telescopes and Hydra multi-object spectrographs,
and the target stars are members of 19 young open clusters with an age range of
approximately 6 to 73 Myr. We made fits of the observed lines He I 4026, 4387,
4471 and Mg II 4481 using model theoretical profiles to find projected
rotational velocities for a total of 496 OB stars. We find that there are fewer
slow rotators among the cluster B-type stars relative to nearby B stars in the
field. We present evidence consistent with the idea that the more massive B
stars (M > 9 solar masses) spin down during their main sequence phase. However,
we also find that the rotational velocity distribution appears to show an
increase in the numbers of rapid rotators among clusters with ages of 10 Myr
and higher. These rapid rotators appear to be distributed between the zero age
and terminal age main sequence locations in the Hertzsprung-Russell diagram,
and thus only a minority of them can be explained as the result of a spin up at
the terminal age main sequence due to core contraction. We suggest instead that
some of these rapid rotators may have been spun up through mass transfer in
close binary systems.Comment: 33 pages, 11 figures, accepted by Ap
Stellar Rotation in Young Clusters. II. Evolution of Stellar Rotation and Surface Helium Abundance
We derive the effective temperatures and gravities of 461 OB stars in 19
young clusters by fitting the H-gamma profile in their spectra. We use
synthetic model profiles for rotating stars to develop a method to estimate the
polar gravity for these stars, which we argue is a useful indicator of their
evolutionary status. We combine these results with projected rotational
velocity measurements obtained in a previous paper on these same open clusters.
We find that the more massive B-stars experience a spin down as predicted by
the theories for the evolution of rotating stars. Furthermore, we find that the
members of binary stars also experience a marked spin down with advanced
evolutionary state due to tidal interactions. We also derive non-LTE-corrected
helium abundances for most of the sample by fitting the He I 4026, 4387, 4471
lines. A large number of helium peculiar stars are found among cooler stars
with Teff < 23000 K. The analysis of the high mass stars (8.5 solar masses < M
< 16 solar masses) shows that the helium enrichment process progresses through
the main sequence (MS) phase and is greater among the faster rotators. This
discovery supports the theoretical claim that rotationally induced internal
mixing is the main cause of surface chemical anomalies that appear during the
MS phase. The lower mass stars appear to have slower rotation rates among the
low gravity objects, and they have a large proportion of helium peculiar stars.
We suggest that both properties are due to their youth. The low gravity stars
are probably pre-main sequence objects that will spin up as they contract.
These young objects very likely host a remnant magnetic field from their natal
cloud, and these strong fields sculpt out surface regions with unusual chemical
abundances.Comment: 50 pages 18 figures, accepted by Ap
ES&T Guest Comment: Celebrating Bidleman’s 1988 “Atmospheric Processes”
Since its 1988 appearance in ES&T, Terry F. Bidleman’s article, “Atmospheric processes: wet and dry deposition of organic compounds are controlled by their vapor-particle partitioning”, has had a notable impact on the field of contaminant science. The paper has been cited in over 600 journal articles published by authors from every continent. Far from fading into obscurity, the paper’s influence has been remarkably consistent. Citations over the last year match the annual average attained since publication
Parallaxes and proper motions for 20 open clusters as based on the new Hipparcos catalogue
A new reduction of the astrometric data as produced by the Hipparcos mission
has been published, claiming that the accuracies for nearly all stars brighter
than magnitude are improved, by up to a factor 4, compared to
the original catalogue. As correlations between the underlying abscissa
residuals have also been reduced by more than an order of magnitude to an
insignificant level, our ability to determine reliable parallaxes and proper
motions for open clusters should be improved. The new Hipparcos astrometric
catalogue is used to derive mean parallax and proper motion estimates for 20
open clusters. The HR-diagrams of the nearest clusters are compared and
combined to provide future input to sets of observational isochrones. The
positions of the cluster HR diagrams are consistent within different groups of
clusters shown for example by the near-perfect alignment of the sequences for
the Hyades and Praesepe, for Coma Ber and UMa, and for the Pleiades, NGC 2516,
and Blanco 1. The groups are mutually consistent when systematic differences in
are taken into account, where the effect of these differences on
the absolute magnitudes has been calibrated using field-star observations.Comment: 34 pages, 36 figures, 7 tables. Accepted for publication by A&
Development of novel low-mass module concepts based on MALTA monolithic pixel sensors
The MALTA CMOS monolithic silicon pixel sensors has been developed in the
Tower 180 nm CMOS imaging process. It includes an asynchronous readout scheme
and complies with the ATLAS inner tracker requirements for the HL-LHC. Several
4-chip MALTA modules have been built using Al wedge wire bonding to demonstrate
the direct transfer of data from chip-to-chip and to read out the data of the
entire module via one chip only. Novel technologies such as Anisotropic
Conductive Films (ACF) and nanowires have been investigated to build a compact
module. A lightweight flex with 17 {\mu}m trace spacing has been designed,
allowing compact packaging with a direct attachment of the chip connection pads
to the flex using these interconnection technologies. This contribution shows
the current state of our work towards a flexible, low material, dense and
reliable packaging and modularization of pixel detectors.Comment: 5 pages + 1 page references,8 figure
Restoring tumour selectivity of the bioreductive prodrug pr-104 by developing an analogue resistant to aerobic metabolism by human aldo-keto reductase 1c3
PR-104 is a phosphate ester pre-prodrug that is converted in vivo to its cognate alcohol, PR-104A, a latent alkylator which forms potent cytotoxins upon bioreduction. Hypoxia selectivity results from one-electron nitro reduction of PR-104A, in which cytochrome P450 oxidoreductase (POR) plays an important role. However, PR-104A also undergoes ‘off-target’ two-electron reduction by human aldo-keto reductase 1C3 (AKR1C3), resulting in activation in oxygenated tissues. AKR1C3 expression in human myeloid progenitor cells probably accounts for the dose-limiting myelotoxicity of PR-104 documented in clinical trials, resulting in human PR-104A plasma exposure levels 3.4- to 9.6-fold lower than can be achieved in murine models. Structure-based design to eliminate AKR1C3 activation thus represents a strategy for restoring the therapeutic window of this class of agent in humans. Here, we identified SN29176, a PR-104A analogue resistant to human AKR1C3 activation. SN29176 retains hypoxia selectivity in vitro with aerobic/hypoxic IC(50) ratios of 9 to 145, remains a substrate for POR and triggers γH2AX induction and cell cycle arrest in a comparable manner to PR-104A. SN35141, the soluble phosphate pre-prodrug of SN29176, exhibited superior hypoxic tumour log cell kill (>4.0) to PR-104 (2.5–3.7) in vivo at doses predicted to be achievable in humans. Orthologues of human AKR1C3 from mouse, rat and dog were incapable of reducing PR-104A, thus identifying an underlying cause for the discrepancy in PR-104 tolerance in pre-clinical models versus humans. In contrast, the macaque AKR1C3 gene orthologue was able to metabolise PR-104A, indicating that this species may be suitable for evaluating the toxicokinetics of PR-104 analogues for clinical development. We confirmed that SN29176 was not a substrate for AKR1C3 orthologues across all four pre-clinical species, demonstrating that this prodrug analogue class is suitable for further development. Based on these findings, a prodrug candidate was subsequently identified for clinical trials
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