2,735 research outputs found
Deep Hubble Space Telescope/ACS Observations of I Zw 18: a Young Galaxy in Formation
We present V and I photometry of the resolved stars in the most
metal-deficient blue compact dwarf galaxy known, I Zw 18 (Zsun/50), using
Hubble Space Telescope/Advanced Camera for Surveys (ACS) images, the deepest
ones ever obtained for this galaxy. The resulting I vs. V-I color-magnitude
diagram (CMD) reaches limiting magnitudes V=I=29 mag. It reveals a young
stellar population of blue main-sequence (MS) stars (age <30 Myr) and blue and
red supergiants (10 Myr<age<100 Myr), but also an older evolved population of
asymptotic giant branch (AGB) stars (100 Myr<age<500 Myr). We derive a distance
to I Zw 18 in the range 12.6 Mpc - 15 Mpc from the brightness of its AGB stars,
with preferred values in the higher range. The red giant branch (RGB) stars are
conspicuous by their absence, although, for a distance of I Zw 18 <15 Mpc, our
imaging data go ~ 1-2 mag below the tip of the RGB. Thus, the most evolved
stars in the galaxy are not older than 500 Myr and I Zw 18 is a bona fide young
galaxy. Several star formation episodes can be inferred from the CMDs of the
main body and the C component. There have been respectively three and two
episodes in these two parts, separated by periods of ~ 100-200 Myr. In the main
body, the younger MS and massive post-MS stars are distributed over a larger
area than the older AGB stars, suggesting that I Zw 18 is still forming from
the inside out. In the C component, different star formation episodes are
spatially distinct, with stellar population ages decreasing from the northwest
to the southeast, also suggesting the ongoing build-up of a young galaxy.Comment: 29 pages, 13 Postscript figures, accepted for publication in the
Astrophysical Journa
A First Step Towards Automatically Building Network Representations
To fully harness Grids, users or middlewares must have some knowledge on the
topology of the platform interconnection network. As such knowledge is usually
not available, one must uses tools which automatically build a topological
network model through some measurements. In this article, we define a
methodology to assess the quality of these network model building tools, and we
apply this methodology to representatives of the main classes of model builders
and to two new algorithms. We show that none of the main existing techniques
build models that enable to accurately predict the running time of simple
application kernels for actual platforms. However some of the new algorithms we
propose give excellent results in a wide range of situations
MonALISA : A Distributed Monitoring Service Architecture
The MonALISA (Monitoring Agents in A Large Integrated Services Architecture)
system provides a distributed monitoring service. MonALISA is based on a
scalable Dynamic Distributed Services Architecture which is designed to meet
the needs of physics collaborations for monitoring global Grid systems, and is
implemented using JINI/JAVA and WSDL/SOAP technologies. The scalability of the
system derives from the use of multithreaded Station Servers to host a variety
of loosely coupled self-describing dynamic services, the ability of each
service to register itself and then to be discovered and used by any other
services, or clients that require such information, and the ability of all
services and clients subscribing to a set of events (state changes) in the
system to be notified automatically. The framework integrates several existing
monitoring tools and procedures to collect parameters describing computational
nodes, applications and network performance. It has built-in SNMP support and
network-performance monitoring algorithms that enable it to monitor end-to-end
network performance as well as the performance and state of site facilities in
a Grid. MonALISA is currently running around the clock on the US CMS test Grid
as well as an increasing number of other sites. It is also being used to
monitor the performance and optimize the interconnections among the reflectors
in the VRVS system.Comment: Talk from the 2003 Computing in High Energy and Nuclear Physics
(CHEP03), La Jolla, Ca, USA, March 2003, 8 pages, pdf. PSN MOET00
Asperity contacts at the nanoscale: comparison of Ru and Au
We develop and validate an interatomic potential for ruthenium based on the
embedded atom method framework with the Finnis/Sinclair representation. We
confirm that the new potential yields a stable hcp lattice with reasonable
lattice and elastic constants and surface and stacking fault energies. We
employ molecular dynamics simulations to bring two surfaces together; one flat
and the other with a single asperity. We compare the process of asperity
contact formation and breaking in Au and Ru, two materials currently in use in
micro electro mechanical system switches. While Au is very ductile at 150 and
300 K, Ru shows considerably less plasticity at 300 and 600 K (approximately
the same homologous temperature). In Au, the asperity necks down to a single
atom thick bridge at separation. While similar necking occurs in Ru at 600 K,
it is much more limited than in Au. On the other hand, at 300 K, Ru breaks by a
much more brittle process of fracture/decohesion with limited plastic
deformation.Comment: 10 pages, 13 figure
A spectroscopic study of component C and the extended emission around I Zw 18
Long-slit Keck II, 4m Kitt Peak, and 4.5m MMT spectrophotometric data are
used to investigate the stellar population and the evolutionary status of I Zw
18C, the faint C component of the nearby blue compact dwarf galaxy I Zw 18.
Hydrogen H and H emission lines are detected in the spectra of I
Zw 18C, implying that ionizing massive stars are present. High signal-to-noise
Keck II spectra of different regions in I Zw 18C reveal H, H
and higher order hydrogen lines in absorption. Several techniques are used to
constrain the age of the stellar population in I Zw 18C. Ages derived from two
different methods, one based on the equivalent widths of the H,
H emission lines and the other on H, H absorption lines
are consistent with a 15 Myr instantaneous burst model. We find that a small
extinction in the range = 0.20 -- 0.65 mag is needed to fit the observed
spectral energy distribution of I Zw 18C with that model. In the case of
constant star formation, all observed properties are consistent with stars
forming continuously between ~ 10 Myr and < 100 Myr ago. We use all available
observational constraints for I Zw 18C, including those obtained from Hubble
Space Telescope color-magnitude diagrams, to argue that the distance to I Zw 18
should be as high as ~ 15 Mpc. The deep spectra also reveal extended ionized
gas emission around I Zw 18. H emission is detected as far as 30" from
it. To a B surface brightness limit of ~ 27 mag arcsec we find no
observational evidence for extended stellar emission in the outermost regions,
at distances > 15" from I Zw 18.Comment: 38 pages, 11 Postscript figures, accepted for publication in the
Astrophysical Journa
Self-Organized Criticality Effect on Stability: Magneto-Thermal Oscillations in a Granular YBCO Superconductor
We show that the self-organized criticality of the Bean's state in each of
the grains of a granular superconductor results in magneto-thermal oscillations
preceding a series of subsequent flux jumps. We find that the frequency of
these oscillations is proportional to the external magnetic field sweep rate
and is inversely proportional to the square root of the heat capacity. We
demonstrate experimentally and theoretically the universality of this
dependence that is mainly influenced by the granularity of the superconductor.Comment: submitted to Physical Review Letters, 4 pages, RevTeX, 4 figures
available as uufile
Stimulation magnétique transcrânienne répétée : efficacité et tolérance dans le traitement des dépressions chez le sujet âgé
International audienceAu fur et à mesure des études, les données suggérant l'efficacité de la Stimulation Magnétique Transcranienne répétée dans le traitement des dépressions chez les patients âgés se complètent, en dépit de quelques résultats contradictoires. Plusieurs paramètres pourraient être associés avec une efficacité antidépressive de la stimulation haute fréquence du cortex préfrontal dorso-latéral gauche : intensité de la stimulation supérieure à 90 %, voire 110 % du seuil moteur, nombre de stimulations par session supérieur à 1 000, nombre total de jours de traitement supérieur à dix. En revanche, un âge plus élevé où l'existence de certaines lésions cérébrales semblent liées à une moindre réponse. Enfin, la tolérance de la SMTr, notamment au niveau des fonctions cognitives, paraît excellente, y compris chez des patients fragiles, notamment du fait d'une maladie cérébro-vasculaire
Luminous Blue Variable Stars In The Two Extremely Metal-Deficient Blue Compact Dwarf Galaxies DDO 68 and PHL 293B
We present photometric and spectroscopic observations of two luminous blue
variable (LBV) stars in two extremely metal-deficient blue compact dwarf (BCD)
galaxies, DDO 68 with 12+logO/H = 7.15 and PHL 293B with 12+logO/H = 7.72.
These two BCDs are the lowest-metallicity galaxies where LBV stars have been
detected, allowing to study the LBV phenomenon in the extremely low metallicity
regime, and shedding light of the evolution of the first generation of massive
stars born from primordial gas. We find that the strong outburst of the LBV
star in DDO 68 occurred sometime between February 2007 and January 2008. We
have compared the properties of the broad line emission in low-metallicity LBVs
with those in higher metallicity LBVs. We find that, for the LBV star in DDO
68, broad emission with a P Cygni profile is seen in both H and He I emission
lines. On the other hand, for the LBV star in PHL 293B, P Cygni profiles are
detected only in H lines. For both LBVs, no heavy element emission line such as
Fe II was detected. The Halpha luminosities of LBV stars in both galaxies are
comparable to the one obtained for the LBV star in NGC 2363 (Mrk 71) which has
a higher metallicity 12+logO/H = 7.89. On the other hand, the terminal
velocities of the stellar winds in both low-metallicity LBVs are high, ~800
km/s, a factor of ~4 higher than the terminal velocities of the winds in
high-metallicity LBVs. This suggests that stellar winds at low metallicity are
driven by a different mechanism than the one operating in high-metallicity
winds.Comment: 26 pages, 5 figures, accepted for publication in the Astrophysical
Journa
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