294 research outputs found
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NodeMD: Diagnosing Node-Level Faults in Remote Wireless Systems ; CU-CS-1017-06
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Run-time fault diagnosis in wireless sensor systems
Software failures in wireless sensor systems are notoriously difficult to debug. Resource constraints in wireless deployments substantially restrict visibility into the root causes of system and application level faults. At the same time, the high deployment cost of wireless sensor systems often far exceeds the cumulative cost of all other sensor hardware, such that software failures that completely disable a node are prohibitively expensive to repair in real-world applications, e.g. by on-site visits to replace or reset nodes. This thesis describes NodeMD, a fault management system designed to improve node debugging capabilities prior to deployment, and enable remote debugging on in-situ sensor nodes that fail. This system successfully implements lightweight run-time detection, logging, and notification of software faults on wireless mote-class devices. NodeMD introduces a debug mode that catches a failure before it completely disables a node and drops the node into a state that enables further diagnosis and correction, thus avoiding on-site redeployment. We present a detailed analysis of NodeMD on real world applications of wireless sensor systems
Photometric characterization of the Galactic star cluster Trumpler 20
We present deep UBVI photometry for Trumpler 20, a rich, intermediate-age
open cluster located at l=301.47, b=+2.22 (RA=12:39:34, DEC=-60:37:00, J2000.0)
in the fourth Galactic quadrant. In spite of its interesting properties, this
cluster has received little attenti on, probably because the line of sight to
it crosses twice the Carina spiral arm, which causes a significant
contamination of its color-magnitude diagram (CMD) by field stars, therefore
complicating seriously its interpretation. We provide more robust estimates of
the fundamental parameters of Trumpler 20, and investigate the most prominent
features of its CMD: a rich He-burning star clump, and a vertical sequence of
stars above the turnoff, which can be either blue stragglers or field stars.
Our precise photometry has allowed us to derive updated values of the age and
heliocentric distance of Trumpler 20, which we estimate to be 1.4 0.2 Gyr
and 3.0 0.3 kpc, respectively. As predicted by models, at this age the
clump has a tail towards fainter magnitudes and bluer colors, thus providing
further confirmation of the evolutionary status of stars in this particular
phase. The derived heliocentric distance places the cluster in the inter-arm
region between the Carina and Scutum arms, which naturally explains the
presence of the vertical sequence of stars (which was originally interpreted as
the cluster itself) observed in the upper part of the CMD.Most of these stars
would therefore belong to the general galactic field, while only a few of
themwould be bona fide cluster blue stragglers. Our data suggest that the
cluster metallicity is solar, and that its reddening is \textit{E(B-V)} = 0.35
0.04.Comment: 35 pages, 10 eps figures (somewhat degraded in resolution), accepted
for publication in the Astronomical Journa
New analysis in the field of open cluster Collinder 223
The present study of the open cluster Collinder 223 (Cr 223) has been mainly
depended on the photoelectric data of Claria & Lapasset (1991; hereafter CL91).
This data of CL91 has been used with the cluster's image of AAO-DSS in order to
re-investigate and improve the main parameters of Cr 223. Stellar count has
been achieved to determine the stellar density, the cluster's center and the
cluster's diameter. In addition, the luminosity function, mass function, and
the total mass of the cluster have been estimated.Comment: 12 pages, 8 figure
The intermediate-age open cluster NGC 2112
We report on CCD photometry of a field centered on the region of the
intermediate-age open cluster NGC 2112 down to V=21. Due to the smaller field
coverage, we are able to limit the effect of field star contamination which
hampered in the past precise determinations of the cluster age and distance.
This way, we provide updated estimates of NGC 2112 fundamental parameters.
Having extended the photometry to the pass-band, we are able to construct a
colour-colour diagram, from which we infer a reddening
mag. The comparison of the Colour-Magnitude Diagram (CMD) with theoretical
isochrones leads to a distance of pc, and an age of
Gyr. While the distance is in agreement with previous determinations, the age
turns out to be much better constrained and significantly lower than previous
estimates.Comment: 7 pages, 7 eps figures, in press in MNRA
Galactic Extinction from Colors and Counts of Field Galaxies in WFPC2 Frames: An Application to GRB 970228
We develop the ``simulated extinction method'' to measure average foreground
Galactic extinction from field galaxy number-counts and colors. The method
comprises simulating extinction in suitable reference fields by changing the
isophotal detection limit. This procedure takes into account selection effects,
in particular, the change in isophotal detection limit (and hence in isophotal
magnitude completeness limit) with extinction, and the galaxy color--magnitude
relation.
We present a first application of the method to the HST WFPC2 images of the
gamma-ray burster GRB 970228. Four different WFPC2 high-latitude fields,
including the HDF, are used as reference to measure the average extinction
towards the GRB in the F606W passband. From the counts, we derive an average
extinction of A_V = 0.5 mag, but the dispersion of 0.4 mag between the
estimates from the different reference fields is significantly larger than can
be accounted by Poisson plus clustering uncertainties. Although the counts
differ, the average colors of the field galaxies agree well. The extinction
implied by the average color difference between the GRB field and the reference
galaxies is A_V = 0.6 mag, with a dispersion in the estimated extinction from
the four reference fields of only 0.1 mag. All our estimates are in good
agreement with the value of 0.81\pm0.27 mag obtained by Burstein & Heiles, and
with the extinction of 0.78\pm0.12 measured by Schlegel et al. from maps of
dust IR emission. However, the discrepancy between the widely varying counts
and the very stable colors in these high-latitude fields is worth
investigating.Comment: 14 pages, 2 figures; submitted to the Astrophysical Journa
The extinction curve in the visible and the value of Rv
This article discusses the interstellar extinction curve in the visible and
the value of Rv. It is concluded that the visible extinction curve is likely to
be linear in the visible, and that indirect estimates of Rv from tentative
determinations of Av, infrared, or UV observations are questionable. There is
currently no evidence of any variation of Rv with direction. If Rv is close to
3, as it has been inferred from mid-infrared data, starlight in the visible is
extinguished by a factor F/F_0=(2.5exp{-2micron/lambda})^{E(B-V)} in the
visible. But if the visible wavelength range alone is considered, 4 appears as
its most natural and probable value, and F/F_0= exp{-2E(B-V)/lambda}.Comment: 4 figures. Published in Astronomische Nachrichten. Label of Fig. 1 is
corrected in the present version. Paper previously rejected by MNRAS (MJ
Barlow editor, review is available
Detailed Classification of Swift's Gamma-Ray Bursts
Earlier classification analyses found three types of gamma-ray bursts (short,
long and intermediate in duration) in the BATSE sample. Recent works have shown
that these three groups are also present in the RHESSI and the BeppoSAX
databases. The duration distribution analysis of the bursts observed by the
Swift satellite also favors the three-component model. In this paper, we extend
the analysis of the Swift data with spectral information. We show, using the
spectral hardness and the duration simultaneously, that the maximum likelihood
method favors the three-component against the two-component model. The
likelihood also shows that a fourth component is not needed.Comment: Accepted for publication in The Astrophysical Journa
Trumpler 20 - an old and rich open cluster
We show that the open cluster Trumpler 20, contrary to the earlier findings,
is actually an old Galactic open cluster. New CCD photometry and
high-resolution spectroscopy are used to derive the main parameters of this
cluster. At [Fe/H]=-0.11 for a single red giant star, the metallicity is
slightly subsolar. The best fit to the color-magnitude diagrams is achieved
using a 1.3 Gyr isochrone with convective overshoot. The cluster appears to
have a significant reddening at E(B-V)=0.46 (for B0 spectral type), although
for red giants this high reddening yields the color temperature exceeding the
spectroscopic T_eff by about 200 K. Trumpler 20 is a very rich open cluster,
containing at least 700 members brighter than M_V=+4. It may extend over the
field-of-view available in our study at 20'x20'.Comment: 7 pages, 5 figures; accepted for publication in MNRA
New fundamental parameters of the Galactic open clusters Berkeley 26, Czernik 27, Melotte 72, NGC 2479 and BH 37
We have obtained CCD UBVI_{KC} photometry down to V ~ 21.0 for the open
clusters Berkeley 26, Czernik 27, Melotte 72, NGC 2479 and BH 37. The latter
has never been studied before. Cluster stellar density profiles were obtained
from star counts in appropriate-sized boxes distributed throughout the entire
observed fields. Based on different measured indices, we estimate the ages of
Berkeley 26, Melotte 72 and NGC 2479. On the other hand, we indicate possible
solutions for the cluster fundamental parameters by matching theoretical
isochrones which reasonably reproduce the main cluster features in their CMDs.
In the case of NGC 2479, the cluster E(B-V) and E(V-I) colour excesses and
apparent distance modulus were estimated from the fit of the Zero-Age Main
Sequence (ZAMS) to the colour-colour and colour-magnitude diagrams,
respectively.Comment: MNRAS accepte
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