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

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

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

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$\alpha$ and H$\beta$ 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$\gamma$, H$\delta$ 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$\alpha$, H$\beta$ emission lines and the other on H$\gamma$, H$\delta$ absorption lines are consistent with a 15 Myr instantaneous burst model. We find that a small extinction in the range $A_V$ = 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$\alpha$ emission is detected as far as 30" from it. To a B surface brightness limit of ~ 27 mag arcsec$^{-2}$ 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

The effect of collisional enhancement of Balmer lines on the determination of the primordial helium abundance

This paper describes a new determination of the primordial helium abundance (Y_P), based on the abundance analysis of five metal-poor extragalactic HII regions. For three regions of the sample (SBS 0335-052, I Zw 18, and H29) we present tailored photoionization models based on improved calculations with respect to previous models. In particular, we use the photoionization models to study quantitatively the effect of collisional excitation of Balmer lines on the determination of the helium abundance (Y) in the individual regions. This effect is twofold: first, the intensities of the Balmer lines are enhanced with respect to the pure recombination value, mimicking a higher hydrogen abundance; second, the observed reddening is larger than the true extinction, due to the differential effect of collisions on different Balmer lines. In addition to these effects, our analysis takes into account the following features of HII regions: (i) the temperature structure, (ii) the density structure, (iii) the presence of neutral helium, (iv) the collisional excitation of the HeI lines, (v) the underlying absorption of the HeI lines, and (vi) the optical thickness of the HeI lines. The object that shows the highest increase in Y after the inclusion of collisional effects in the analysis is SBS 0335-052, whose helium abundance has been revised by Delta Y = +0.0107. The revised Y values for the five objects in our sample yield an increase of +0.0035 in Y_P, giving Y_P = 0.2391 +/- 0.0020.Comment: 59 pages, 8 figures. AAS Latex. Accepted for publication in the Astrophysical Journa

Metaplastic woven bone in bone metastases: A Fourier-transform infrared analysis and imaging of bone quality (FTIR)

Most osteolytic tumors are in fact mixed and contain an osteoblastic component associated with the predominant osteolytic areas. This metaplastic woven bone is always evidenced by histological analysis even in the absence of radiological expression. Metaplastic bone formation reflects the activation of new osteoblasts coming from the stimulation of the dormant lining cells. Twelve patients with secondary metastases of the iliac crest evidenced by hot spots on a Tc-MBP san were diagnosed by histomorphometry on bone biopsies. Fourier Transformed InfraRed analysis and Imaging (FTIRI) was used on 4μm thick sections of undecalcified bone. The mineralization degree, carbonate substitution, crystallinity and the cross-links ratio of collagen (1660/1690cm bands) were determined. The matrix characteristics were analyzed and imaged in the pre-existing residual bone and in the metaplastic woven bone in the vicinity of the tumor cells. FTIRI provided images of the phosphate, amide and combination of peak ratio after having selected the peaks of interest. In addition, the matrix properties can be measured and compared between the old and newly-formed bones. Woven bone appeared poorly calcified with a low phosphate/amide ratio (P=0.03) crystallinity (P&lt;0.0001) and carbonate substitution (P=0.003). Collagen was less mature as evidenced by lower cross-links (P=0.01). Woven bone associated with bone metastasis appears poorly mineralized and rapidly elaborated by osteoblasts. The collagenous phase of the bone matrix has a low level of reticulation. FTIRI is a powerful tool to measure and visualize the various components of the bone matrix in human diseases