2,069 research outputs found
Individual Differences in Perceptions Impacting Teacher Retention in a Large Urban Public School District
Student achievement and school quality both suffer when there is high a rate of high teacher turnover (Adnot, Dee, Katz, & Wyckoff, 2016; Borman & Dowling, 2008) and retaining high-quality teachers has become a major challenge (TNTP, 2012). A quantitative, non-experimental research study was conducted through an electronic exit survey (Cronbachâs alpha =.811) to attempt to better understand factors which may have led K-12 classroom teachers across to leave a large urban public school district during a five-year period. Of those who participated (n=252), 79.3% cited more than one reason for departure. The top three reasons cited for leaving were: inadequate salary (55.2%), stress on the job (46.0%), and dissatisfaction with supervisor (34.5%), though no factors were found to be statistically significant. Data from this study reveal the importance of an in-depth exit survey allowing departing classroom teachers to cite more than one factor for leaving, and, the level of impact that each factor had on their decision so that the data may be monitored by school district leaders to address areas of concern if statistically significant patterns are found which may lead to a higher teacher retention rate, substantial budgetary savings and increased student achievement
Some unbounded functions of intermittent maps for which the central limit theorem holds
We compute some dependence coefficients for the stationary Markov chain whose
transition kernel is the Perron-Frobenius operator of an expanding map of
with a neutral fixed point. We use these coefficients to prove a
central limit theorem for the partial sums of , when belongs to
a large class of unbounded functions from to . We also
prove other limit theorems and moment inequalities.Comment: 16 page
Contribution to the search for binaries among Am stars - VIII. New spectroscopic orbits of eight systems and statistical study of a sample of 91 Am stars
International audienceThis paper is the last of a series devoted to the study of Am stars, with the monitoring of radial velocities of a sample of 91 objects during more than 20 yr. The purpose was to determine which stars were members of spectroscopic binaries (SBs) and study in detail those systems in order to obtain observational constraints on the origin of the Am phenomenon. In the first part, we present the results of a detailed study of eight Am stars (HD 32893, 60489, 109762, 111057, 113697, 204918, 219675 and BD+44° 4512) observed at the Haute-Provence and Cambridge observatories with CORAVEL instruments. We find that these objects are single-lined SBs whose orbital elements are determined for the first time. HD 32893 is found to be a triple spectroscopic system whose third body might be detected by speckle interferometry. Physical parameters are inferred for the primaries of those SBs. We then investigate the influence of tidal interaction and find that it has already led to the synchronism of the primaries and to the circularization of the orbits of four of those systems. In the second part of this paper, we present the main results of our whole programme and derive some statistical properties of Am stars. We give the recapitulating table of the orbital parameters found for the SBs of our whole sample and the list of those for which no evidence for radial velocity variations could be found during our monitoring. Our study shows that at least 64 per cent of Am stars are members of SBs. This rate is significantly greater than that of normal stars. Although some SBs may have been not detected, this study shows that a substantial fraction of Am stars do not belong to SBs: they are either isolated stars or members of wide binary systems. We then present some statistical properties of the orbital parameters of the SBs whose primary is an Am star, on an extended sample obtained by adding 29 Am SB orbits published by other authors. The corresponding e versus logP diagram shows a cut-off between the circular and the eccentric systems at P ~ 5.6 +/- 0.5 d, which indicates a typical age of 0.5 - 1 à 109 yr for the Am stars, which is in agreement with the values found in our previous detailed studies. A Monte Carlo analysis shows that the distribution of the mass function values f(m) is compatible with a power-law distribution N(m) ~ m-? of the masses m of the companions with ? = 0.3 +/- 0.2 or with a Gaussian distribution centred on 0.8 +/- 0.5Msolar, which indicates that the companions of Am SBs are mostly dwarf stars of type G-K-M
Ultrasonic triggering of giant magnetocaloric effect in MnAs thin films
Mechanical control of magnetic properties in magnetostrictive thin films
offers the unexplored opportunity to employ surface wave acoustics in such a
way that acoustic triggers dynamic magnetic effects. The strain-induced
modulation of the magnetic anisotropy can play the role of a high frequency
varying effective magnetic field leading to ultrasonic tuning of electronic and
magnetic properties of nanostructured materials, eventually integrated in
semiconductor technology. Here, we report about the opportunity to employ
surface acoustic waves to trigger magnetocaloric effect in
MnAs(100nm)/GaAs(001) thin films. During the MnAs magnetostructural phase
transition, in an interval range around room temperature (0{\deg}C -
60{\deg}C), ultrasonic waves (170 MHz) are strongly attenuated by the phase
coexistence (up to 150 dB/cm). We show that the giant magnetocaloric effect of
MnAs is responsible of the observed phenomenon. By a simple anelastic model we
describe the temperature and the external magnetic field dependence of such a
huge ultrasound attenuation. Strain-manipulation of the magnetocaloric effect
could be a further interesting route for dynamic and static caloritronics and
spintronics applications in semiconductor technology
Is it possible to improve the yield and grain protein concentration of organically-farmed wheat using cover crops or intercrops?
The objective of our work was to investigate innovative sustainable cropping systems to produce regular yields of wheat with a satisfactory grain protein concentration in organic farming systems. Achieving good production levels in stockless French organic farming systems is a major challenge due to strong N limitation. Our approach is mainly based on a better valorisation of the natural nitrogen resources from soil mineralisation and symbiotic fixation of legumes, and not by an increase in the use of organic fertiliser. Two experiments were carried out in southwestern France where winter wheat and durum wheat were grown for their use in human consumption (bread and pasta, respectively). On one hand, cover crops were sown in summer and were incorporated in early November just before the wheat was sown. On the other hand, wheat was cultivated in mixture (intercropping) with a grain legume such as winter pea or fababean. The cover crops were found to be effective in the case of rainy winter years i) because of their role as a nitrate catch crop to mitigate nitrate leaching and ii) because they made it possible to increase the yield and the protein concentration of wheat grains by increasing available N (role of green manure). In the case of intercropping, wheat yield was reduced in comparison to a wheat crop alone, as expected, but the protein concentration was significantly increased and the whole grain yield (wheat + legume) was increased. Thus, in organic farming, intercrops seem to be more effective for enhancing natural nitrogen resources. However, it is still necessary: (i) to optimise the technical sequences of these two farming systems, and; ii) to determine the role of intercrops within rotations and to analyse their effect for both pests and diseases management, which is crucial in organic farming systems
Daily and seasonal variations in the spatial distribution of zooplankton populations in relation to the physical structure in the Ligurian Sea Front
Results from five hydrographic and biological surveys at different seasons across the Ligurian Sea front, using horizontal continuous measurements and vertical profiles are presented. The vertical circulation across the front is described, and two divergences and one convergence are identified as permanent features from data. The key to find their location for each survey is given. The spatial patterns of 14 zooplankton taxa along the transect are established using variance analysis, principal component and correspondence analyses. The spatial distribution of each taxon is related to the physical structure, and the convection cells evidenced by the scheme appear as different biotopes. The daily variability of the spatial distributions is negligible compared to the annual variability. Distinctions are made between coastal species always inhabiting the peripheral (coastal zone) and others with distinct coastal, frontal and offshore distributions varying with the season. For the latter species the frontal zone is a preferential biotope during their seasonal growth period. Consequently the accumulation of organisms near the convergence cannot be created by only the dynamic causes. The role of biological processes in the observed distributions is discussed
Contribution to the search for binaries among Am stars - VII. Orbital elements of seven new spectroscopic binaries, implications on tidal effects
International audienceWe present the results of a radial-velocity study of seven Am stars (HD 3970, 35035, 93946, 151746, 153286, 204751 and 224002) observed at the Observatoire de Haute-Provence (OHP) and the Cambridge Observatories with CORAVEL instruments. We find that these systems are single-lined spectroscopic binaries whose orbital elements are determined for the first time. Among this sample, HD 35035 and 153286 have long periods, with P = 2.8 and 9.5yr, respectively, which is rather unusual for Am stars. Four systems have orbits with large eccentricities (with e >= 0.4). Physical parameters are inferred from this study for the primaries of those systems. We then investigate the influence of tidal interaction, which has already led to the synchronism of the primaries and/or to the circularization of the orbits of some systems belonging to this sample. We extend this study to the list of 33 objects studied in this series of papers and derive values of the critical fractional radii r = R/a for circularization and synchronization of Am-type binaries. We find that the stars with r >~ 0.15 are orbiting on circular orbits and that synchronism is likely for all components with r >~ 0.20
Contribution to the search for binaries among Am stars - III. HD 7119: a double-lined spectroscopic binary and a triple system
Radial velocity observations of HD 7119 with the CORAVEL instrument at Observatoire de Haute-Provence are reported. Known as an AmÎŽÎŽ Del metallic-line star, HD 7119 was included in our spectroscopic survey of Am-type stars, the purpose of which was to determine the frequency of binaries in this stellar family. This object is found to be a double-lined spectroscopic binary with a variable value of V0, the systematic velocity of the centre of gravity of the pair. The variation of this parameter is interpreted in terms of the orbital motion of an unseen third body with a much longer period. The orbital elements were derived for both the short- and the long-period orbits. These orbits can be considered to be well determined since these observations were performed on a regular basis over the 1992-1998 period, covering more than 320 orbital cycles for the short-period (P= 6.76 d) and 1.3 cycle for the long-period orbit (P⌠1700 d). As deduced from the ratio of the correlation dip areas, the magnitude difference of the components of the short-period system is 0.7 mag. Combined with the Hipparcos data, this value leads to visual absolute magnitudes of 0.5 and 1.2 for the primary and secondary components, respectively. Such magnitudes are consistent with evolved ÎŽ Del-type stars. The third body could be a cool dwarf star with a minimum mass of 0.5 Mâ, located at ⌠0.016 arcsec of the main system. Consequently, it cannot be the visual companion detected by Couteau with a separation of 3.35 arcsec. If this latter visual component were a physical component (rather than an optical one), HD 7119 would be a quadruple syste
Contribution to the search for binaries among Am stars â V. Orbital elements of eight short-period spectroscopic binaries
We present the results of a radial-velocity study of eight Am stars (HD 341, 55822, 61250, 67317, 93991, 162950, 224890 and 225137) observed at Observatoire de Haute-Provence with the CORAVEL instrument. We find that these systems are single-line spectroscopic binaries whose orbital elements are determined for the first tim
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