4,250 research outputs found
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
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
An imaging and spectroscopic study of the very metal-deficient blue compact dwarf galaxy Tol 1214--277
We present a spectrophotometric study based on VLT/FORS I observations of one
of the most metal-deficient blue compact dwarf (BCD) galaxies known, Tol
1214-277 (Z ~ Zsun/25). The data show that roughly half of the total luminosity
of the BCD originates from a bright and compact starburst region located at the
northeastern tip of a faint dwarf galaxy with cometary appearance. The
starburst has ignited less than 4 Myr ago and its emission is powered by
several thousands O7V stars and ~ 170 late-type nitrogen Wolf-Rayet stars
located within a compact region with < 500 pc in diameter. For the first time
in a BCD, a relatively strong [Fe V] 4227 emission line is seen which together
with intense He II 4686 emission indicates the presence of a very hard
radiation field in Tol 1214-277. We argue that this extraordinarily hard
radiation originates from both Wolf--Rayet stars and radiative shocks in the
starburst region. The structural properties of the low-surface-brightness (LSB)
component underlying the starburst have been investigated by means of surface
photometry down to 28 B mag/sq.arcsec. We find that, for a surface brightness
level fainter than ~ 24.5 B mag/sq.arcsec, an exponential fitting law provides
an adequate approximation to its radial intensity distribution. The broad-band
colors in the outskirts of the LSB component of Tol 1214-277 are nearly
constant and are consistent with an age below one Gyr. This conclusion is
supported by the comparison of the observed spectral energy distribution (SED)
of the LSB host with theoretical SEDs.Comment: 17 pages, 11 Postscript figures, uses emulateapj.sty, to appear in
Astronomical Journa
HST observations of the cometary blue compact dwarf galaxy UGC 4483: a relatively young galaxy?
We present V and I photometry of the resolved stars in the cometary blue
compact dwarf galaxy UGC 4483 using Hubble Space Telescope Wide Field Planetary
Camera 2 (WFPC2) images. The resulting I vs. (V-I) color-magnitude diagram
(CMD) reaches limiting magnitudes V = 27.5 mag and I = 26.5 mag for photometric
errors less than 0.2 mag. It reveals not only a young stellar population of
blue main-sequence stars and blue and red supergiants, but also an older
evolved population of red giant and asymptotic giant branch stars. The measured
magnitude I = 23.65 +/- 0.10 mag of the red giant branch tip results in a
distance modulus (m-M) = 27.63 +/- 0.12, corresponding to a distance of 3.4 +/-
0.2 Mpc. The youngest stars are associated with the bright H II region at the
northern tip of the galaxy. The population of older stars is found throughout
the low-surface-brightness body of the galaxy and is considerably more spread
out than the young stellar population, suggesting stellar diffusion. The most
striking characteristics of the CMD of UGC 4483 are the very blue colors of the
red giant stars and the high luminosity of the asymptotic giant branch stars.
Both of these characteristics are consistent with either: 1) a very low
metallicity ([Fe/H] = -2.4 like the most metal-deficient globular clusters) and
an old age of 10 Gyr, or 2) a higher metallicity ([Fe/H] = -1.4 as derived from
the ionized gas emission lines) and a relatively young age of the oldest
stellar population in UGC 4483, not exceeding ~ 2 Gyr. Thus our data do not
exclude the possibility that UGC 4483 is a relatively young galaxy having
formed its first stars only ~ 2 Gyr ago.Comment: 37 pages, 15 PS figures, to appear in Ap
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
340 years of atmospheric circulation characteristics reconstructed from an eastern Antarctic Peninsula ice core
Copyright @ 2006 American Geophysical Union (AGU)Precipitation delivery mechanisms for Dolleman Island (DI), located off the east coast of the Antarctic Peninsula, are investigated using reanalysis and back trajectory data. The Southern Annular Mode (SAM) and ENSO are both shown to influence precipitation delivery and event size. Precipitation delivery variability is compared against the interannual variation of chemical data from two DI ice cores. Nitrate concentration in the cores is strongly linked with the ratio of easterly to westerly back trajectories arriving at DI, as described by a Cross-Peninsula Index (CPI) defined in this paper. This CPI is used subsequently to reconstruct the atmospheric circulation characteristics for the 340-year ice core record. The analysis highlights a period of increased easterlies during 1720–1780 and an increase in westerlies for 1950–1980, the latter concomitant with a positive SAM trend and western Peninsula warming. The reconstruction also reveals periods when polynyas may have been present in the Weddell Sea
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