1,168 research outputs found
A spectroscopic survey of the youngest field stars in the solar neighbourhood. I. The optically bright sample
We present the first results of a ground-based programme conducted on 1-4m
class telescopes. Our sample consists of 1097 active and presumably young
stars, all of them being optical counterparts of RASS X-ray sources in the
northern hemisphere. We concentrate on the 704 optically brightest
(V_Ticho<=9.5 mag) candidates. We acquired high-res spectroscopy in the
Halpha/Li spectral regions for 426 of such stars without relevant literature
data. We describe the sample and the observations and we start to discuss its
physical properties. We used a cross-correlation technique and other tools to
derive accurate radial/rotational velocities and to perform a spectral
classification for both single and SB2 stars. The spectral subtraction
technique was used to derive chromospheric activity levels and Li abundances.
We estimated the fraction of young single stars and multiple systems in stellar
soft X-ray surveys and the contamination by more evolved systems, like RS
CVn's. We classified stars on the basis of Li abundance and give a glimpse of
their sky distribution. The sample appears to be a mixture of young
Pleiades-/Hyades- like stars plus an older Li-poor population (~1-2 Gyr). 7
stars with Li abundance compatible with the age of IC 2602 (~30 Myr) or younger
were detected as well, although 2 appear to be Li-rich giants. The discovery of
a large number of Li-rich giants is another outcome of this survey. The
contamination of soft X-ray surveys by old systems in which the activity level
is enhanced by tidal synchronisation is not negligible, especially for K-type
stars. 5 stars with Li content close to the primordial abundance are probably
associated with known moving groups in the solar neighbourhood. Some of them
are PTTS candidates according to their positions in the HR diagram.Comment: 16 pages, 12 figures, 6 tables; 2 figures and 2 tables in electronic
form only. Paper accepted by Astronomy and Astrophysic
A new management method for wireless sensor networks
International audienceThe Wireless Sensor Networks (WSN) with their constant evolution, need new management methods to be monitored efficiently by taking into account the context and their constraints such as energy consumption, reliability and remote monitoring. WSN has diverse application domains: smart home, smart care, environmental data collection etc. In order to manage a large scale WSN, several Wireless sensor network Management Tools (WMTs) are developed. Some of them use SNMP protocol like because it is impossible to implement the full compliance classical SNMP standard on each wireless sensor node. Therefore, it is important to develop a new WMT with a restricted SNMP standard dedicated to WSN applications. In this paper, we present a new WMT named LiveNCM: LiveNode Non invasive Context-aware, and modular Management tool. LiveNCM is divided into two main parts: one is centralized on the fixed network structure and another one, distributed on each node. Each part introduces the concept of non-invasive context-aware to reduce data exchanges and diagnoses the wireless sensor node state with few messages. Moreover, nodes are based on a configurable modular architecture enables to adapt to an application and to a local node constraints. LiveNCM is implemented on the LiveNode platform to validate the energy consumption and on a UNIX system to validate the adopted SNMP sub-agent. Ultimately, a decreasing data exchange and an improvement in the energy consumption in the entire WSN were observed. An implementation of the proposed management method is presented
The BTC40 Survey for Quasars at 4.8 < z < 6
The BTC40 Survey for high-redshift quasars is a multicolor search using
images obtained with the Big Throughput Camera (BTC) on the CTIO 4-m telescope
in V, I, and z filters to search for quasars at redshifts of 4.8 < z < 6. The
survey covers 40 sq. deg. in B, V, & I and 36 sq. deg. in z. Limiting
magnitudes (3 sigma) reach to V = 24.6, I = 22.9 and z = 22.9. We used the
(V-I) vs. (I-z) two-color diagram to select high-redshift quasar candidates
from the objects classified as point sources in the imaging data. Follow-up
spectroscopy with the AAT and CTIO 4-m telescopes of candidates having I < 21.5
has yielded two quasars with redshifts of z = 4.6 and z = 4.8 as well as four
emission line galaxies with z = 0.6. Fainter candidates have been identified
down to I = 22 for future spectroscopy on 8-m class telescopes.Comment: 27 pages, 8 figures; Accepted for publication in the Astronomical
Journa
Estimates of climate system properties incorporating recent climate change
Historical time series of surface temperature and ocean heat content changes
are commonly used metrics to diagnose climate change and estimate properties
of the climate system. We show that recent trends, namely the slowing of
surface temperature rise at the beginning of the 21st century and the
acceleration of heat stored in the deep ocean, have a substantial impact on
these estimates. Using the Massachusetts Institute of Technology Earth System
Model (MESM), we vary three model parameters that influence the behavior of
the climate system: effective climate sensitivity (ECS), the effective ocean
diffusivity of heat anomalies by all mixing processes (Kv), and the net
anthropogenic aerosol forcing scaling factor. Each model run is compared to
observed changes in decadal mean surface temperature anomalies and the trend
in global mean ocean heat content change to derive a joint probability
distribution function for the model parameters. Marginal distributions for
individual parameters are found by integrating over the other two parameters.
To investigate how the inclusion of recent temperature changes affects our
estimates, we systematically include additional data by choosing periods that
end in 1990, 2000, and 2010. We find that estimates of ECS increase in
response to rising global surface temperatures when data beyond 1990 are
included, but due to the slowdown of surface temperature rise in the early
21st century, estimates when using data up to 2000 are greater than when data
up to 2010 are used. We also show that estimates of Kv increase in
response to the acceleration of heat stored in the ocean as data beyond 1990
are included. Further, we highlight how including spatial patterns of surface
temperature change modifies the estimates. We show that including latitudinal
structure in the climate change signal impacts properties with spatial
dependence, namely the aerosol forcing pattern, more than properties defined
for the global mean, climate sensitivity, and ocean diffusivity.</p
Galaxy-Galaxy Lensing by Non-Spherical Haloes I:Theoretical Considerations
We use Monte Carlo simulations to investigate the theory of galaxy-galaxy
lensing by non-spherical dark matter haloes. The simulations include a careful
accounting of the effects of multiple deflections. In a typical data set where
the mean tangential shear of sources with redshifts zs ~ 0.6 is measured with
respect to the observed symmetry axes of foreground galaxies with redshifts zl
~ 0.3, the signature of anisotropic galaxy-galaxy lensing differs substantially
from the expectation that one would have in the absence of multiple
deflections. The observed ratio of the mean tangential shears, g+/g-, is
strongly suppressed compared to the function that one would measure if the
intrinsic symmetry axes of the foreground galaxies were known. Depending upon
the characteristic masses of the lenses, the observed ratio of the mean
tangential shears may be consistent with an isotropic signal (despite the fact
that the lenses are non-spherical), or it may even be reversed from the
expected signal (i.e., the mean tangential shear for sources close to the
observed minor axes of the lenses may exceed the mean tangential shear for
sources close to the observed major axes of the lenses). These effects are
caused primarily by the fact that the lens galaxies have, themselves, been
lensed and therefore the observed symmetry axes of the lenses differ from their
intrinsic symmetry axes. The effects of lensing of the foreground galaxies on
the observed function g+/g- cannot be eliminated by the rejection of foreground
galaxies with small image ellipticities, nor by focusing the analysis on
sources that are located very close to the observed symmetry axes of the
foreground galaxies. We conclude that any attempt to use a measurement of g+/g-
to constrain the shapes of dark matter galaxy haloes must include Monte Carlo
simulations that take multiple deflections properly into account.Comment: 15 pages, 17 figures, submitted to MNRAS, full manuscript with
high-resolution version of Fig. 4 can be found at
http://firedrake.bu.edu/preprints/preprints.htm
The influence of gravitational lensing on the spectra of lensed QSOs
We consider the influence of (milli/micro)lensing on the spectra of lensed
QSOs. We propose a method for the observational detection of microlensing in
the spectra of lensed QSOs and apply it to the spectra of the three lensed QSOs
(PG 1115+080, QSO 1413+117 and QSO 0957+561) observed with Hubble Space
Telescope (HST). We find that the flux ratio between images A1 and A2 of PG
1115+080 is wavelength-dependent and shows differential magnification between
the emission lines and the continuum. We interpret this magnification as
arising from millilensing. We also find that the temporal variations in the
continuum of image C of QSO 1413+117 may be caused by microlensing, while the
temporal variation observed in QSO 0957+561 was probably an intrinsic one.Comment: 11 pages, accepted for publication in MNRA
A new approach to studying the electrical behavior and the inhomogeneities of the Schottky barrier height
A format for phylogenetic placements
We have developed a unified format for phylogenetic placements, that is,
mappings of environmental sequence data (e.g. short reads) into a phylogenetic
tree. We are motivated to do so by the growing number of tools for computing
and post-processing phylogenetic placements, and the lack of an established
standard for storing them. The format is lightweight, versatile, extensible,
and is based on the JSON format which can be parsed by most modern programming
languages. Our format is already implemented in several tools for computing and
post-processing parsimony- and likelihood-based phylogenetic placements, and
has worked well in practice. We believe that establishing a standard format for
analyzing read placements at this early stage will lead to a more efficient
development of powerful and portable post-analysis tools for the growing
applications of phylogenetic placement.Comment: Documents version 3 of the forma
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