1,814 research outputs found
Dependable Distributed Computing for the International Telecommunication Union Regional Radio Conference RRC06
The International Telecommunication Union (ITU) Regional Radio Conference
(RRC06) established in 2006 a new frequency plan for the introduction of
digital broadcasting in European, African, Arab, CIS countries and Iran. The
preparation of the plan involved complex calculations under short deadline and
required dependable and efficient computing capability. The ITU designed and
deployed in-situ a dedicated PC farm, in parallel to the European Organization
for Nuclear Research (CERN) which provided and supported a system based on the
EGEE Grid. The planning cycle at the RRC06 required a periodic execution in the
order of 200,000 short jobs, using several hundreds of CPU hours, in a period
of less than 12 hours. The nature of the problem required dynamic
workload-balancing and low-latency access to the computing resources. We
present the strategy and key technical choices that delivered a reliable
service to the RRC06
X-Shooter study of accretion in -Ophiucus: very low-mass stars and brown dwarfs
We present new VLT/X-Shooter optical and NIR spectra of a sample of 17
candidate young low-mass stars and BDs in the rho-Ophiucus cluster. We derived
SpT and Av for all the targets, and then we determined their physical
parameters. All the objects but one have M*<0.6 Msun, and 8 have mass below or
close to the hydrogen-burning limit. Using the intensity of various emission
lines present in their spectra, we determined the Lacc and Macc for all the
objects. When compared with previous works targeting the same sample, we find
that, in general, these objects are not as strongly accreting as previously
reported, and we suggest that the reason is our more accurate estimate of the
photospheric parameters. We also compare our findings with recent works in
other slightly older star-forming regions to investigate possible differences
in the accretion properties, but we find that the accretion properties for our
targets have the same dependence on the stellar and substellar parameters as in
the other regions. This leads us to conclude that we do not find evidence for a
different dependence of Macc with M* when comparing low-mass stars and BDs.
Moreover, we find a similar small (1 dex) scatter in the Macc-M* relation as in
some of our recent works in other star-forming regions, and no significant
differences in Macc due to different ages or properties of the regions. The
latter result suffers, however, from low statistics and sample selection biases
in the current studies. The small scatter in the Macc-M* correlation confirms
that Macc in the literature based on uncertain photospheric parameters and
single accretion indicators, such as the Ha width, can lead to a scatter that
is unphysically large. Our studies show that only broadband spectroscopic
surveys coupled with a detailed analysis of the photospheric and accretion
properties allows us to properly study the evolution of disk accretion rates.Comment: accepted for publication in Astronomy & Astrophysics. Abstract
shortened to fit arXiv constraint
Searching for physics beyond the Standard Model in the decay B+ -> K+K+pi-
The observation potential of the decay B+ -> K+K+pi- with the ATLAS detector
at LHC is described in this paper. In the Standard Model this decay mode is
highly suppressed, while in models beyond the Standard Model it could be
significantly enhanced. To improve the selection of the K+K+pi- final state, a
charged hadron identification using Time-over-Threshold measurements in the
ATLAS Transition Radiation Tracker was developed and used.Comment: 12 pages, 4 figures, 1 tabl
An extensive VLT/X-Shooter library of photospheric templates of pre-main sequence stars
Studies of the formation and evolution of young stars and their disks rely on
the knowledge of the stellar parameters of the young stars. The derivation of
these parameters is commonly based on comparison with photospheric template
spectra. Furthermore, chromospheric emission in young active stars impacts the
measurement of mass accretion rates, a key quantity to study disk evolution.
Here we derive stellar properties of low-mass pre-main sequence stars without
disks, which represent ideal photospheric templates for studies of young stars.
We also use these spectra to constrain the impact of chromospheric emission on
the measurements of mass accretion rates. The spectra in reduced,
flux-calibrated, and corrected for telluric absorption form are made available
to the community. We derive the spectral type for our targets by analyzing the
photospheric molecular features present in their VLT/X-Shooter spectra by means
of spectral indices and comparison of the relative strength of photospheric
absorption features. We also measure effective temperature, gravity, projected
rotational velocity, and radial velocity from our spectra by fitting them with
synthetic spectra with the ROTFIT tool. The targets have negligible extinction
and spectral type from G5 to M8. We perform synthetic photometry on the spectra
to derive the typical colors of young stars in different filters. We measure
the luminosity of the emission lines present in the spectra and estimate the
noise due to chromospheric emission in the measurements of accretion luminosity
in accreting stars. We provide a calibration of the photospheric colors of
young PMS stars as a function of their spectral type in a set of standard
broad-band optical and near-infrared filters. For stars with masses of ~
1.5Msun and ages of ~1-5 Myr, the chromospheric noise converts to a limit of
measurable mass accretion rates of ~ 3x10^-10 Msun/yr.Comment: Accepted for publication on Astronomy & Astrophysics. The spectra of
the photospheric templates will be uploaded to Vizier, but are already
available on request. Abstract shortened for arxiv constraints. Language
edited versio
Urbanization effect on trends in sunshine duration in China
There is an ongoing debate on whether the observed decadal variations in surface solar radiation, known as "dimming" and "brightening" periods, are a large-scale or solely local phenomenon. We investigated this issue using long-term sunshine duration records from China, which experienced a rapid increase in urbanization during the past decades. Over the period 1960â2013, 172 pairs of urban and nearby rural stations were analyzed. Urban and rural sunshine duration trends show similar spatial patterns during a dimming phase (1960â1989) and a subsequent period during which trends were leveling off (1990â2013). This indicates that rather than local effects, the trends in sunshine duration are on more of a national or regional scale in China. Nevertheless, in the dimming phase, the declining rate of sunshine duration in rural areas is around two-thirds of that in urban areas. The ratio of rural to urban dimming generally increases from a minimum of 0.39 to a maximum of 0.87 with increasing indices of urbanization calculated based on the year 2013. It reaches a maximum when the urbanization level exceeds 50âŻ%, the urban population exceeds 20âŻmillion, or the population density becomes higher than 250âŻpersonâŻkmâ2. After the transition into the leveling-off period, sunshine duration trends are no longer significantly affected by urbanization. Meanwhile, the number of laws and regulations related to air pollution and investment in pollution treatment have been increasing in China.ISSN:0992-7689ISSN:0939-4176ISSN:1432-057
Mapping young stellar populations towards Orion with Gaia DR1
We use the first data release of the Gaia mission to explore the three
dimensional arrangement and the age ordering of the many stellar groups towards
the Orion OB association, aiming at a new classification and characterization
of the stellar population. We make use of the parallaxes and proper motions
provided in the Tycho Gaia Astrometric Solution (TGAS) sub-set of the Gaia
catalogue, and of the combination of Gaia and 2MASS photometry. In TGAS we find
evidence for the presence of a young population, at a parallax , loosely distributed around some known clusters: 25 Ori,
Ori and Ori, and NGC 1980 ( Ori). The low mass
counterpart of this population is visible in the color-magnitude diagrams
constructed by combining Gaia and 2MASS photometry. We study the density
distribution of the young sources in the sky. We find the same groups as in
TGAS, and also some other density enhancements that might be related to the
recently discovered Orion X group, the Orion dust ring, and to the
Ori complex. We estimate the ages of this population and we infer the presence
of an age gradient going from 25 Ori (13-15 Myr) to the ONC (1-2 Myr). We
confirm this age ordering by repeating the Bayesian fit using the Pan-STARRS1
data. The estimated ages towards the NGC 1980 cluster span a broad range of
values. This can either be due to the presence of two populations coming from
two different episodes of star formation or to a large spread along the line of
sight of the same population. Our results form the first step towards using the
Gaia data to unravel the complex star formation history of the Orion region in
terms of the different star formation episodes, their duration, and their
effects on the surrounding interstellar medium.Comment: 17 pages, 17 figure
Time-resolved photometry of the young dipper RX~J1604.3-2130A:Unveiling the structure and mass transport through the innermost disk
Context. RX J1604.3-2130A is a young, dipper-type, variable star in the Upper Scorpius association, suspected to have an inclined inner disk, with respect to its face-on outer disk. Aims. We aim to study the eclipses to constrain the inner disk properties. Methods. We used time-resolved photometry from the Rapid Eye Mount telescope and Kepler 2 data to study the multi-wavelength variability, and archival optical and infrared data to track accretion, rotation, and changes in disk structure. Results. The observations reveal details of the structure and matter transport through the inner disk. The eclipses show 5 d quasi-periodicity, with the phase drifting in time and some periods showing increased/decreased eclipse depth and frequency. Dips are consistent with extinction by slightly processed dust grains in an inclined, irregularly-shaped inner disk locked to the star through two relatively stable accretion structures. The grains are located near the dust sublimation radius (similar to 0.06 au) at the corotation radius, and can explain the shadows observed in the outer disk. The total mass (gas and dust) required to produce the eclipses and shadows is a few % of a Ceres mass. Such an amount of mass is accreted/replenished by accretion in days to weeks, which explains the variability from period to period. Spitzer and WISE infrared variability reveal variations in the dust content in the innermost disk on a timescale of a few years, which is consistent with small imbalances (compared to the stellar accretion rate) in the matter transport from the outer to the inner disk. A decrease in the accretion rate is observed at the times of less eclipsing variability and low mid-IR fluxes, confirming this picture. The v sin i = 16 km s(-1) confirms that the star cannot be aligned with the outer disk, but is likely close to equator-on and to be aligned with the inner disk. This anomalous orientation is a challenge for standard theories of protoplanetary disk formation.Science & Technology Facilities Council (STFC): ST/S000399/1.
ESO fellowship.
European Union (EU): 823 823.
German Research Foundation (DFG): FOR 2634/1 TE 1024/1-1.
French National Research Agency (ANR): ANR-16-CE31-0013.
Alexander von Humboldt Foundation.
European Research Council (ERC): 678 194.
European Research Council (ERC): 742 095.
National Aeronautics & Space Administration (NASA).
National Science Foundation (NSF).
National Aeronautics & Space Administration (NASA): NNG05GF22G.
National Science Foundation (NSF): AST-0909182, AST-1 313 422
On the gas content of transitional disks: a VLT/X-Shooter study of accretion and winds
Transitional disks (TDs) are thought to be a late evolutionary stage of
protoplanetary disks with dust depleted inner regions. The mechanism
responsible for this depletion is still under debate. To constrain the models
it is mandatory to have a good understanding of the properties of the gas
content of the inner disk. Using X-Shooter broad band -UV to NIR- medium
resolution spectroscopy we derive the stellar, accretion, and wind properties
of a sample of 22 TDs. The analysis of these properties allows us to put strong
constraints on the gas content in a region very close to the star (<0.2 AU)
which is not accessible with any other observational technique. We fit the
spectra with a self-consistent procedure to derive simultaneously SpT,Av,and
mass accretion rates (Macc) of the targets. From forbidden emission lines we
derive the wind properties of the targets. Comparing our findings to values for
cTTs, we find that Macc and wind properties of 80% of the TDs in our sample,
which is strongly biased towards strongly accreting objects, are comparable to
those of cTTs. Thus, there are (at least) some TDs with Macc compatible with
those of cTTs, irrespective of the size of the dust inner hole.Only in 2 cases
Macc are much lower, while the wind properties are similar. We do not see any
strong trend of Macc with the size of the dust depleted cavity, nor with the
presence of a dusty optically thick disk close to the star. In the TDs in our
sample there is a gas rich inner disk with density similar to that of cTTs
disks. At least for some TDs, the process responsible of the inner disk
clearing should allow for a transfer of gas from the outer disk to the inner
region. This should proceed at a rate that does not depend on the physical
mechanism producing the gap seen in the dust emission and results in a gas
density in the inner disk similar to that of unperturbed disks around stars of
similar mass.Comment: Accepted on Astronomy & Astrophysics. Abstract shortened to fit arXiv
constraint
X-Shooter spectroscopy of young stellar objects in Lupus. Atmospheric parameters, membership and activity diagnostics
A homogeneous determination of basic stellar parameters of young stellar
object (YSO) candidates is needed to confirm their evolutionary stage,
membership to star forming regions (SFRs), and to get reliable values of the
quantities related to chromospheric activity and accretion. We used the code
ROTFIT and synthetic BT-Settl spectra for the determination of the atmospheric
parameters (Teff and logg), the veiling, the radial (RV) and projected
rotational velocity (vsini), from X-Shooter spectra of 102 YSO candidates in
the Lupus SFR. We have shown that 13 candidates can be rejected as Lupus
members based on their discrepant RV with respect to Lupus and/or the very low
logg values. At least 11 of them are background giants. The spectral
subtraction of inactive templates enabled us to measure the line fluxes for
several diagnostics of both chromospheric activity and accretion. We found that
all Class-III sources have H fluxes compatible with a pure
chromospheric activity, while objects with disks lie mostly above the boundary
between chromospheres and accretion. YSOs with transitional disks displays both
high and low H fluxes. We found that the line fluxes per unit surface
are tightly correlated with the accretion luminosity () derived
from the Balmer continuum excess. This rules out that the relationships between
and line luminosities found in previous works are simply due to
calibration effects. We also found that the CaII-IRT flux ratio,
, is always small, indicating an optically thick emission
source. The latter can be identified with the accretion shock near the stellar
photosphere. The Balmer decrement reaches instead, for several accretors, high
values typical of optically thin emission, suggesting that the Balmer emission
originates in different parts of the accretion funnels with a smaller optical
depth.Comment: 28 pages, 26 figures, accepted by A&
Neurological determination of death in isolated brainstem lesions: A case report to highlight the issues involved
The neurological determination of death in patients with isolated brainstem lesions or by disruption of the posterior cerebral circulation is uncommon and many intensivists may never see such a case in their career. It is also the only major difference between the âwhole brainâ and âbrain stemâ formulations for the neurological determination of death. We present a case of a patient with infarction of the structures supplied by the posterior cerebral circulation in whom death was diagnosed using neurological criteria, to illustrate the issues involved. We also suggest that international consensus may be achieved if ancillary tests, such as CT angiography, are made mandatory in this situation o demonstrate loss of blood flow in the anterior cerebral circulation as well the posterior circulation
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