12,367 research outputs found
Computing on Masked Data to improve the Security of Big Data
Organizations that make use of large quantities of information require the
ability to store and process data from central locations so that the product
can be shared or distributed across a heterogeneous group of users. However,
recent events underscore the need for improving the security of data stored in
such untrusted servers or databases. Advances in cryptographic techniques and
database technologies provide the necessary security functionality but rely on
a computational model in which the cloud is used solely for storage and
retrieval. Much of big data computation and analytics make use of signal
processing fundamentals for computation. As the trend of moving data storage
and computation to the cloud increases, homeland security missions should
understand the impact of security on key signal processing kernels such as
correlation or thresholding. In this article, we propose a tool called
Computing on Masked Data (CMD), which combines advances in database
technologies and cryptographic tools to provide a low overhead mechanism to
offload certain mathematical operations securely to the cloud. This article
describes the design and development of the CMD tool.Comment: 6 pages, Accepted to IEEE HST Conferenc
The properties of SCUBA cores in the Perseus molecular cloud: the bias of clump-finding algorithms
We present a new analysis of the properties of star-forming cores in the
Perseus molecular cloud, identified in SCUBA 850 micron data. Our goal is to
determine which core properties can be robustly identified and which depend on
the extraction technique. Four regions in the cloud are examined: NGC1333,
IC348/HH211, L1448 and L1455. We identify clumps of dust emission using two
popular automated algorithms, CLFIND and GAUSSCLUMPS, finding 85 and 122 clumps
in total respectively. Some trends are true for both populations: clumps become
increasingly elongated over time and are consistent with constant surface
brightness objects, with an average brightness ~4 to 10 times larger than the
surrounding molecular cloud; the clump mass distribution (CMD) resembles the
stellar intial mass function, with a slope alpha = -2.0+/-0.1 for CLFIND and
alpha = -3.15+/-0.08 for GAUSSCLUMPS, which straddle the Salpeter value. The
mass at which the slope shallows (similar for both algorithms at M~6 Msun)
implies a star-forming efficiency of between 10 and 20 per cent. Other trends
reported elsewhere depend on the clump-finding technique: we find protostellar
clumps are both smaller (for GAUSSCLUMPS) and larger (for CLFIND) than their
starless counterparts; the functional form, best-fitting to the CMD, is
different for the two algorithms. The GAUSSCLUMPS CMD is best-fitted with a
log-normal distribution, whereas a broken power law is best for CLFIND; the
reported lack of massive starless cores in previous studies can be seen in the
CLFIND but not the GAUSSCLUMPS data. Our approach highlights similarities and
differences between the clump populations, illustrating the caution that must
be exercised when comparing results from different studies and interpreting the
properties of continuum cores.Comment: 19 pages, 17 figures, accepted for publication by MNRA
Cosmological interpretation of the color-magnitude diagrams of galaxy clusters
We investigate the color-magnitude diagram (CMD) of cluster galaxies in the
hierarchical -CDM cosmological scenario using both single stellar
populations and simple galaxy models. First, we analyze the effect of bursts
and mergers and companion chemical pollution and rejuvenation of the stellar
content on the integrated light emitted by galaxies. The dispersion of the
galaxy magnitudes and colors on the plane is mainly due to mixing
of ages and metallicities of the stellar populations, with mergers weighting
more than bursts of similar mass fractions. The analysis is made using the
Monte-Carlo technique applied to ideal model galaxies reduced to single stellar
populations with galaxy-size mass to evaluate mass, age and metallicity of each
object. We show that separately determining the contributions by bursts and
mergers leads to a better understanding of observed properties of CMD of
cluster galaxies. Then we repeat the analysis using suitable chemo-photometric
models of galaxies whose mass is derived from the cosmological predictions of
the galaxy content of typical clusters. Using the halo mass function and the
Monte-Carlo technique, we derive the formation redshift of each galaxy and its
photometric history. These are used to simulate the CMD of the cluster
galaxies. The main conclusion is that most massive galaxies have acquired the
red color they show today in very early epochs and remained the same ever
since. The simulations nicely reproduce the Red Sequence, the Green Valley and
the Blue Cloud, the three main regions of the CMD in which galaxies crowd.Comment: Accepted for publication in Ap
The star formation history of the SMC star cluster NGC419
The rich SMC star cluster NGC419 has recently been found to present both a
broad main sequence turn-off and a dual red clump of giants, in the sharp
colour-magnitude diagrams (CMD) derived from the High Resolution Channel of the
Advanced Camera for Surveys on board the Hubble Space Telescope. In this work,
we apply to the NGC419 data the classical method of star formation history
(SFH) recovery via CMD reconstruction, deriving for the first time this
function for a star cluster with multiple turn-offs. The values for the cluster
metallicity, reddening, distance and binary fraction, were varied within the
limits allowed by present observations. The global best-fitting solution is an
excellent fit to the data, reproducing all the CMD features with striking
accuracy. The corresponding star formation rate is provided together with
estimates of its random and systematic errors. Star formation is found to last
for at least 700 Myr, and to have a marked peak at the middle of this interval,
for an age of 1.5 Gyr. Our findings argue in favour of multiple star formation
episodes (or continued star formation) being at the origin of the multiple main
sequence turn-offs in Magellanic Cloud clusters with ages around 1 Gyr. It
remains to be tested whether alternative hypotheses, such as a main sequence
spread caused by rotation, could produce similarly good fits to the data.Comment: 10 pages, MNRAS in pres
The Evolution of L and T Dwarfs in Color-Magnitude Diagrams
We present new evolution sequences for very low mass stars, brown dwarfs and
giant planets and use them to explore a variety of influences on the evolution
of these objects. We compare our results with previous work and discuss the
causes of the differences and argue for the importance of the surface boundary
condition provided by atmosphere models including clouds.
The L- to T-type ultracool dwarf transition can be accommodated within the
Ackerman & Marley (2001) cloud model by varying the cloud sedimentation
parameter. We develop a simple model for the evolution across the L/T
transition. By combining the evolution calculation and our atmosphere models,
we generate colors and magnitudes of synthetic populations of ultracool dwarfs
in the field and in galactic clusters. We focus on near infrared color-
magnitude diagrams (CMDs) and on the nature of the ``second parameter'' that is
responsible for the scatter of colors along the Teff sequence. Variations in
metallicity and cloud parameters, unresolved binaries and possibly a relatively
young population all play a role in defining the spread of brown dwarfs along
the cooling sequence. We find that the transition from cloudy L dwarfs to
cloudless T dwarfs slows down the evolution and causes a pile up of substellar
objects in the transition region, in contradiction with previous studies. We
apply the same model to the Pleiades brown dwarf sequence. Taken at face value,
the Pleiades data suggest that the L/T transition occurs at lower Teff for
lower gravity objects. The simulated populations of brown dwarfs also reveal
that the phase of deuterium burning produces a distinctive feature in CMDs that
should be detectable in ~50-100 Myr old clusters.Comment: Accepted for publication in the ApJ. 52 pages including 20 figure
The Magellanic Bridge: The Nearest Purely Tidal Stellar Population
We report on observations of the stellar populations in twelve fields
spanning the region between the Magellanic Clouds, made with the Mosaic-II
camera on the 4-meter telescope at the Cerro-Tololo Inter-American Observatory.
The two main goals of the observations are to characterize the young stellar
population (which presumably formed in situ in the Bridge and therefore
represents the nearest stellar population formed from tidal debris), and to
search for an older stellar component (which would have been stripped from
either Cloud as stars, by the same tidal forces which formed the gaseous
Bridge). We determine the star-formation history of the young inter-Cloud
population, which provides a constraint on the timing of the gravitational
interaction which formed the Bridge. We do not detect an older stellar
population belonging to the Bridge in any of our fields, implying that the
material that was stripped from the Clouds to form the Magellanic Bridge was
very nearly a pure gas.Comment: 19 pages, 9 figures. Accepted to Ap
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