1,795 research outputs found
Young peoplesâ reflections on what teachers think about family obligations that conflict with school: A focus on the non-normative roles of young caring and language brokering
In âWesternâ contexts school attendance is central for an âidealâ childhood. However, many young people engage with home roles that conflict with school expectations. This paper explores perceptions of that process in relation two home activities - language brokering and young caring. We interviewed 46 young people and asked them to reflect on what the teacher would think when a child had to miss school to help a family member. This paper discusses the young peopleâs overall need to keep their out-of-school lives private from their teachers
Rotation periods of late-type stars in the young open cluster IC 2602
We present the results of a monitoring campaign aimed at deriving rotation
periods for a representative sample of stars in the young (30 Myr) open cluster
IC 2602. Rotation periods were derived for 29 of 33 stars monitored. The
periods derived range from 0.2d (one of the shortest known rotation periods of
any single open cluster star) to about 10d (which is almost twice as long as
the longest period previously known for a cluster of this age). We are able to
confirm 8 previously known periods and derive 21 new ones, delineating the long
period end of the distribution. Despite our sensitivity to longer periods, we
do not detect any variables with periods longer than about 10d. The combination
of these data with those for IC 2391, an almost identical cluster, leads to the
following conclusions:
1) The fast rotators in a 30 Myr cluster are distributed across the entire
0.5 < B-V < 1.6 color range.
2) 6 stars in our sample are slow rotators, with periods longer than 6d.
3) The amplitude of variability depends on both the color and the period. The
dependence on the latter might be important in understanding the selection
effects in the currently available rotation period database and in planning
future observations.
4) The interpretation of these data in terms of theoretical models of
rotating stars suggests both that disk-interaction is the norm rather than the
exception in young stars and that disk-locking times range from zero to a few
Myr.Comment: 23 pages, 8 figures, accepted for publication in the Astrophysical
Journa
Self-Binding Transition in Bose Condensates with Laser-Induced ``Gravitation''
In our recent publication (D. O'Dell, et al, Phys. Rev. Lett. 84, 5687
(2000)) we proposed a scheme for electromagnetically generating a self-bound
Bose-Einstein condensate with 1/r attractive interactions: the analog of a Bose
star. Here we focus upon the conditions neccessary to observe the transition
from external trapping to self-binding. This transition becomes manifest in a
sharp reduction of the condensate radius and its dependence on the laser
intensity rather that the trap potential.Comment: 5 pages, 2 figures: slightly enhanced text: more explanatio
Oscillations of rotating trapped Bose-Einstein condensates
The tensor-virial method is applied for a study of oscillation modes of
uniformly rotating Bose-Einstein condensed gases, whose rigid body rotation is
supported by an vortex array. The second order virial equations are derived in
the hydrodynamic regime for an arbitrary external harmonic trapping potential
assuming that the condensate is a superfluid at zero temperature. The
axisymmetric equilibrium shape of the condensate is determined as a function of
the deformation of the trap; its domain of stability is bounded by the
constraint on the rotation rate (measured in units of the trap
frequency .) The oscillations of the axisymmetric condensate are
stable with respect to the transverse-shear, toroidal and quasi-radial modes of
oscillations, corresponding to the , surface
deformations. In non-axisymmetric traps, the equilibrium constrains the
(dimensionless) deformation in the plane orthogonal to the rotation to the
domain with . The second harmonic oscillation modes
in non-axisymmetric traps separate into two classes which have even or odd
parity with respect to the direction of the rotation axis. Numerical solutions
show that these modes are stable in the parameter domain where equilibrium
figures exist.Comment: 16 pages, including 4 figures, uses Revtex; v2 includes a treatment
of modes in unisotropic traps; PRA in pres
Macroscopic Quantum Tunneling of Ferromagnetic Domain Walls
Quantum tunneling of domain walls out of an impurity potential in a
mesoscopic ferromagnetic sample is investigated. Using improved expressions for
the domain wall mass and for the pinning potential, we find that the cross-over
temperature between thermal activation and quantum tunneling is of a different
functional form than found previously. In materials like Ni or YIG, the
crossover temperatures are around 5 mK. We also find that the WKB exponent is
typically two orders of magnitude larger than current estimates. The sources
for these discrepancies are discussed, and precise estimates for the transition
from three-dimensional to one-dimensional magnetic behavior of a wire are
given. The cross-over temperatures from thermal to quantum transitions and
tunneling rates are calculated for various materials and sample sizes.Comment: 10 pages, 2 postscript figures, REVTe
Testing the Blazar Paradigm: ASCA Observations of FSRQs with Steep Soft X-ray Spectra
We present the first observations at medium-hard X-rays with ASCA in 1998
August--November of four Flat Spectrum Radio Quasars (FSRQs), characterized by
unusually steep soft X-ray spectra (photon index, \Gamma_{0.2-2.4 keV} \sim
2-2.5), as previously measured with ROSAT. Such steep X-ray slopes are similar
to those observed in synchrotron-dominated BL Lacs and are unexpected in the
context of the recent blazar paradigm, where sources with strong emission lines
(such as FSRQs) are dominated in soft X-rays by a flat inverse Compton tail. We
find that the ASCA spectra of the four FSRQs are consistent with a power law
model with \Gamma_{2-10 keV} \sim 1.8, flatter than their ROSAT spectra. This
indicates the onset of an inverse Compton component at energies \gtrsim 2 keV,
in agreement with the blazar unification scheme. However, these objects are
still anomalous within the blazar class for their steep soft X-ray continua
which, together with non-simultaneous data at longer wavelengths, hint at the
possibility that the synchrotron emission extends to soft X-rays. This would
imply an anomalously high synchotron peak frequency for a quasar with luminous
broad lines, challenging current blazar unification schemes. Alternatively, a
plausible explanation for the steep optical-to-soft X-ray continua of the four
FSRQs is thermal emission from the accretion disk, similar to the blazars
3C~273 and 3C~345. In the Appendix, we present fits to the SIS data in an
effort to contribute to the ongoing calibration of the the time-dependence of
the SIS response at low energies.Comment: accepted for publication in The Astrophysical Journa
Ten-Micron Observations of Nearby Young Stars
(abridged) We present new 10-micron photometry of 21 nearby young stars
obtained at the Palomar 5-meter and at the Keck I 10-meter telescopes as part
of a program to search for dust in the habitable zone of young stars. Thirteen
of the stars are in the F-K spectral type range ("solar analogs"), 4 have B or
A spectral types, and 4 have spectral type M. We confirm existing IRAS
12-micron and ground-based 10-micron photometry for 10 of the stars, and
present new insight into this spectral regime for the rest. Excess emission at
10 micron is not found in any of the young solar analogs, except for a possible
2.4-sigma detection in the G5V star HD 88638. The G2V star HD 107146, which
does not display a 10-micron excess, is identified as a new Vega-like
candidate, based on our 10-micron photospheric detection, combined with
previously unidentified 60-micron and 100-micron IRAS excesses. Among the
early-type stars, a 10-micron excess is detected only in HD 109573A (HR 4796A),
confirming prior observations; among the M dwarfs, excesses are confirmed in AA
Tau, CD -40 8434, and Hen 3-600A. A previously suggested N band excess in the
M3 dwarf CD -33 7795 is shown to be consistent with photospheric emission.Comment: 40 pages, 4 figures, 5 tables. To appear in the January 1, 2004 issue
of Ap
Production of a chromium Bose-Einstein condensate
The recent achievement of Bose-Einstein condensation of chromium atoms [1]
has opened longed-for experimental access to a degenerate quantum gas with
long-range and anisotropic interaction. Due to the large magnetic moment of
chromium atoms of 6 {}B, in contrast to other Bose- Einstein condensates
(BECs), magnetic dipole-dipole interaction plays an important role in a
chromium BEC. Many new physical properties of degenerate gases arising from
these magnetic forces have been predicted in the past and can now be studied
experimentally. Besides these phenomena, the large dipole moment leads to a
breakdown of standard methods for the creation of a chromium BEC. Cooling and
trapping methods had to be adapted to the special electronic structure of
chromium to reach the regime of quantum degeneracy. Some of them apply
generally to gases with large dipolar forces. We present here a detailed
discussion of the experimental techniques which are used to create a chromium
BEC and alow us to produce pure condensates with up to {} atoms in an
optical dipole trap. We also describe the methods used to determine the
trapping parameters.Comment: 17 pages, 9 figure
The influence of transition metal solutes on dislocation core structure and values of Peierls stress and barrier in tungsten
Several transition metals were examined to evaluate their potential for
improving the ductility of tungsten. The dislocation core structure and Peierls
stress and barrier of screw dislocations in binary
tungsten-transition metal alloys (WTM) were investigated using
first principles electronic structure calculations. The periodic quadrupole
approach was applied to model the structure of dislocation. Alloying
with transition metals was modeled using the virtual crystal approximation and
the applicability of this approach was assessed by calculating the equilibrium
lattice parameter and elastic constants of the tungsten alloys. Reasonable
agreement was obtained with experimental data and with results obtained from
the conventional supercell approach. Increasing the concentration of a
transition metal from the VIIIA group, i.e. the elements in columns headed by
Fe, Co and Ni, leads to reduction of the elastic constant and
increase of elastic anisotropy A=. Alloying W with a group
VIIIA transition metal changes the structure of the dislocation core from
symmetric to asymmetric, similar to results obtained for WRe
alloys in the earlier work of Romaner {\it et al} (Phys. Rev. Lett. 104, 195503
(2010))\comments{\cite{WRECORE}}. In addition to a change in the core symmetry,
the values of the Peierls stress and barrier are reduced. The latter effect
could lead to increased ductility in a tungsten-based
alloy\comments{\cite{WRECORE}}. Our results demonstrate that alloying with any
of the transition metals from the VIIIA group should have similar effect as
alloying with Re.Comment: 12 pages, 8 figures, 3 table
Bivariate genetic modelling of the response to an oral glucose tolerance challenge: A gene x environment interaction approach
AIMS/HYPOTHESIS: Twin and family studies have shown the importance of genetic factors influencing fasting and 2 h glucose and insulin levels. However, the genetics of the physiological response to a glucose load has not been thoroughly investigated. METHODS: We studied 580 monozygotic and 1,937 dizygotic British female twins from the Twins UK Registry. The effects of genetic and environmental factors on fasting and 2 h glucose and insulin levels were estimated using univariate genetic modelling. Bivariate model fitting was used to investigate the glucose and insulin responses to a glucose load, i.e. an OGTT. RESULTS: The genetic effect on fasting and 2 h glucose and insulin levels ranged between 40% and 56% after adjustment for age and BMI. Exposure to a glucose load resulted in the emergence of novel genetic effects on 2 h glucose independent of the fasting level, accounting for about 55% of its heritability. For 2 h insulin, the effect of the same genes that already influenced fasting insulin was amplified by about 30%. CONCLUSIONS/INTERPRETATION: Exposure to a glucose challenge uncovers new genetic variance for glucose and amplifies the effects of genes that already influence the fasting insulin level. Finding the genes acting on 2 h glucose independently of fasting glucose may offer new aetiological insight into the risk of cardiovascular events and death from all causes
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