16,836 research outputs found
A revised HRD for individual components of binary systems from BaSeL BVRI synthetic photometry. Influence of interstellar extinction and stellar rotation
Johnson BVRI photometric data for individual components of binary systems
have been provided by ten Brummelaar et al. (2000). This is essential because
such binaries could play a critical role in calibrating the single-star stellar
evolution theory. While they derived the effective temperature from their
estimated spectral type, we infer metallicity-dependent Teffs from a minimizing
method fitting the B-V, V-R and V-I colours. For this purpose, a grid of
621,600 flux distributions were computed from the Basel Stellar Library (BaSeL
2.2) of model-atmosphere spectra, and their theoretical colours compared with
the observed photometry. As a matter of fact, the BaSeL colours show a very
good agreement with the BVRI metallicity-dependent empirical calibrations of
Alonso et al. (1996), temperatures being different by 3+-3 % in the range
4000-8000 K for dwarf stars. Before deriving the metallicity-dependent Teff
from the BaSeL models, we paid particular attention to the influence of
reddening and stellar rotation. A comparison between the MExcess code and
neutral hydrogen column density data shows a good agreement for the sample but
we point out a few directions where the MExcess model overestimates the E(B-V)
colour excess. Influence of stellar rotation on the BVRI colours can be
neglected except for 5 stars with large vsini, the maximum effect on
temperature being less than 5%. Our final results are in good agreement with
previous spectroscopic determinations available for a few primary components,
and with ten Brummelaar et al. below ~10,000 K. Nevertheless, we obtain an
increasing disagreement with their Teffs beyond 10,000 K. Finally, we provide a
revised Hertzsprung-Russell diagram for the systems with the more accurately
determined temperatures. (Abridged)Comment: 11 pages, accepted for publication in A&
Fate of the Bose insulator in the limit of strong localization and low Cooper-pair density in ultrathin films
A Bose insulator composed of a low density of strongly localized Cooper pairs develops at the two-dimensional superconductor to insulator transition (SIT) in a number of thin film systems. Investigations of ultrathin amorphous PbBi films far from the SIT described here provide evidence that the Bose insulator gives way to a second insulating phase with decreasing film thickness. At a critical film thickness dc the magnetoresistance changes sign from positive, as expected for boson transport, to negative, as expected for fermion transport, signs of local Cooper-pair phase coherence effects on transport vanish, and the transport activation energy exhibits a kink. Below dc pairing fluctuation effects remain visible in the high-temperature transport while the activation energy continues to rise. These features show that Cooper pairing persists and suggest that the localized unpaired electron states involved in transport are interspersed among regions of strongly localized Cooper pairs in this strongly localized, low Cooper-pair density phase
Collapse of the Cooper pair phase coherence length at a superconductor to insulator transition
We present investigations of the superconductor to insulator transition (SIT)
of uniform a-Bi films using a technique sensitive to Cooper pair phase
coherence. The films are perforated with a nanohoneycomb array of holes to form
a multiply connected geometry and subjected to a perpendicular magnetic field.
Film magnetoresistances on the superconducting side of the SIT oscillate with a
period dictated by the superconducting flux quantum and the areal hole density.
The oscillations disappear close to the SIT critical point to leave a
monotonically rising magnetoresistance that persists in the insulating phase.
These observations indicate that the Cooper pair phase coherence length, which
is infinite in the superconducting phase, collapses to a value less than the
interhole spacing at this SIT. This behavior is inconsistent with the gradual
reduction of the phase coherence length expected for a bosonic, phase
fluctuation driven SIT. This result starkly contrasts with previous
observations of oscillations persisting in the insulating phase of other films
implying that there must be at least two distinct classes of disorder tuned
SITs
Affiliative Subgroups in Preschool Classrooms: Integrating Constructs and Methods from Social Ethology and Sociometric Traditions
Recent studies of school-age children and adolescents have used social network analyses to characterize selection and socialization aspects of peer groups. Fewer network studies have been reported for preschool classrooms and many of those have focused on structural descriptions of peer networks, and/or, on selection processes rather than on social functions of subgroup membership. In this study we started by identifying and describing different types of affiliative subgroups (HMP- high mutual proximity, LMP- low mutual proximity, and ungrouped children) in a sample of 240 Portuguese preschool children using nearest neighbor observations. Next, we used additional behavioral observations and sociometric data to show that HMP and LMP subgroups are functionally distinct: HMP subgroups appear to reflect friendship relations, whereas LMP subgroups appear to reflect common social goals, but without strong, within-subgroup dyadic ties. Finally, we examined the longitudinal implications of subgroup membership and show that children classified as HMP in consecutive years had more reciprocated friendships than did children whose subgroup classification changed from LMP or ungrouped to HMP. These results extend previous findings reported for North American peer groups
Affiliative Subgroups in Preschool Classrooms: Integrating Constructs and Methods from Social Ethology and Sociometric Traditions
Recent studies of school-age children and adolescents have used social network analyses to characterize selection and socialization aspects of peer groups. Fewer network studies have been reported for preschool classrooms and many of those have focused on structural descriptions of peer networks, and/or, on selection processes rather than on social functions of subgroup membership. In this study we started by identifying and describing different types of affiliative subgroups (HMP- high mutual proximity, LMP- low mutual proximity, and ungrouped children) in a sample of 240 Portuguese preschool children using nearest neighbor observations. Next, we used additional behavioral observations and sociometric data to show that HMP and LMP subgroups are functionally distinct: HMP subgroups appear to reflect friendship relations, whereas LMP subgroups appear to reflect common social goals, but without strong, within-subgroup dyadic ties. Finally, we examined the longitudinal implications of subgroup membership and show that children classified as HMP in consecutive years had more reciprocated friendships than did children whose subgroup classification changed from LMP or ungrouped to HMP. These results extend previous findings reported for North American peer groups
High coercivity induced by mechanical milling in cobalt ferrite powders
In this work we report a study of the magnetic behavior of ferrimagnetic
oxide CoFe2O4 treated by mechanical milling with different grinding balls. The
cobalt ferrite nanoparticles were prepared using a simple hydrothermal method
and annealed at 500oC. The non-milled sample presented coercivity of about 1.9
kOe, saturation magnetization of 69.5 emu/g, and a remanence ratio of 0.42.
After milling, two samples attained coercivity of 4.2 and 4.1 kOe, and
saturation magnetization of 67.0 and 71.4 emu/g respectively. The remanence
ratio MR/MS for these samples increase to 0.49 and 0.51, respectively. To
investigate the influence of the microstructure on the magnetic behavior of
these samples, we used X-ray powder diffraction (XPD), transmission electron
microscopy (TEM), and vibrating sample magnetometry (VSM). The XPD analysis by
the Williamson-Hall plot was used to estimate the average crystallite size and
strain induced by mechanical milling in the samples
Surgical Treatment of Intravenous Leiomyomatosis: The Role of the IVC Filter
IntroductionIntravenous leiomyomatosis is a rare, life-threatening intravenous tumor associated with uterine leiomyomata.ReportThis report describes the case of a 45-year-old woman with a history of weakness and exertional dyspnea, and an extensive intracaval mass extending to the right side of the heart. The tumor was successfully removed in a two-stage surgical procedure with an inferior vena cava (IVC) filter deployed before the second stage. An extensive DVT was observed postoperatively.DiscussionSurgical removal is the only effective treatment for intravenous leiomyomatosis, and the rate of recurrence remains unclear. An IVC filter should be placed routinely to prevent postoperative or late (in case of recurrence) pulmonary embolism
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