15,598 research outputs found
Synthesis, solution stability, and crystal structure of aza-thia macrocyclic complexes of silver(I).
Embryo impacts and gas giant mergers II: Diversity of Hot Jupiters' internal structure
We consider the origin of compact, short-period, Jupiter-mass planets. We
propose that their diverse structure is caused by giant impacts of embryos and
super-Earths or mergers with other gas giants during the formation and
evolution of these hot Jupiters. Through a series of numerical simulations, we
show that typical head-on collisions generally lead to total coalescence of
impinging gas giants. Although extremely energetic collisions can disintegrate
the envelope of gas giants, these events seldom occur. During oblique and
moderately energetic collisions, the merger products retain higher fraction of
the colliders' cores than their envelopes. They can also deposit considerable
amount of spin angular momentum to the gas giants and desynchronize their spins
from their orbital mean motion. We find that the oblateness of gas giants can
be used to infer the impact history. Subsequent dissipation of stellar tide
inside the planets' envelope can lead to runaway inflation and potentially a
substantial loss of gas through Roche-lobe overflow. The impact of super-Earths
on parabolic orbits can also enlarge gas giant planets' envelope and elevates
their tidal dissipation rate over 100 Myr time scale. Since giant
impacts occur stochastically with a range of impactor sizes and energies, their
diverse outcomes may account for the dispersion in the mass-radius relationship
of hot Jupiters.Comment: 19 pages, 7 figures, 7 tables. Accepted for publication in MNRA
Red-Cockaded Woodpeckers and Silvicultural Practice: is Uneven-Aged? Silviculture Preferable to Even-Aged
The endangered red-cockaded woodpecker (Picoides borealis) has become a high-profile management issue in the southeastern United States. Suitable habitat consists of mature to old pine, or mixed pine-hardwood forest, with minimal hardwood midstory vegetation. Loss of habitat, detrimental silvicultural practices, and changes in the fire regime have resulted in small fragmented populations, most of which have been declining precipitously in recent decades (Costa and Escano 1989, Conner and Rudolph 1989). The current population of l0-12 thousand birds occurs across much of the original range from Virginia and Florida west to Oklahoma and Texas (James 1995). However, populations are restricted to isolated tracts of suitable habitat, primarily on public lands. Consequently, the debate over the future of this once abundant species, characteristic of fire climax pine forests, has focused primarily on management strategies for the species by the U.S. Forest Service (USFS), the agency responsible for the majority of the public forest lands in the region
Direct Urca neutrino rate in colour superconducting quark matter
If deconfined quark matter exists inside compact stars, the primary cooling
mechanism is neutrino radiation via the direct Urca processes d->u+e+antinu_e
and u+e->d+nu_e. Below a critical temperature, T_c, quark matter forms a colour
superconductor, one possible manifestation of which is a condensate of
quark Cooper pairs in an electric-charge neutralising background of electrons.
We compute the neutrino emission rate from such a phase, including charged
pair-breaking and recombination effects, and find that on a material
temperature domain below T_c the pairing-induced suppression of the neutrino
emission rate is not uniformly exponential. If gapless modes are present in the
condensed phase, the emissivity at low temperatures is moderately enhanced
above that of completely unpaired matter. The importance of charged current
pair-breaking processes for neutrino emission both in the fully gapped and
partially gapped regimes is emphasised.Comment: 5 pages, 2 figures; to appear in Phys. Rev. C (Rapid Comm.
Red-cockaded Woodpecker Colony Status and Trends On the Angelina, Davy Crockett and Sabine National Forests
Abundant hardwood midstory, colony isolation, and habitat fragmentation are believed to be the causes for severe population declines of red-cockaded woodpeckers on three national forests in eastern Texas
Student experiences of virtual reality - a case study in learning special relativity
We present a study of student learning through the use of virtual reality. A
software package is used to introduce concepts of special relativity to
students in a game-like environment where users experience the effects of
travelling at near light speeds. From this new perspective, space and time are
significantly different to that experienced in everyday life. The study
explores how students have worked with this environment and how these students
have used this experience in their study of special relativity. A mixed method
approach has been taken to evaluate the outcomes of separate implementations of
the package at two universities. Students found the simulation to be a positive
learning experience and described the subject area as being less abstract after
its use. Also, students were more capable of correctly answering concept
questions relating to special relativity, and a small but measurable
improvement was observed in the final exam
Magnetic Flux Braiding: Force-Free Equilibria and Current Sheets
We use a numerical nonlinear multigrid magnetic relaxation technique to
investigate the generation of current sheets in three-dimensional magnetic flux
braiding experiments. We are able to catalogue the relaxed nonlinear force-free
equilibria resulting from the application of deformations to an initially
undisturbed region of plasma containing a uniform, vertical magnetic field. The
deformations are manifested by imposing motions on the bounding planes to which
the magnetic field is anchored. Once imposed the new distribution of magnetic
footpoints are then taken to be fixed, so that the rest of the plasma must then
relax to a new equilibrium configuration. For the class of footpoint motions we
have examined, we find that singular and nonsingular equilibria can be
generated. By singular we mean that within the limits imposed by numerical
resolution we find that there is no convergence to a well-defined equilibrium
as the number of grid points in the numerical domain is increased. These
singular equilibria contain current "sheets" of ever-increasing current
intensity and decreasing width; they occur when the footpoint motions exceed a
certain threshold, and must include both twist and shear to be effective. On
the basis of these results we contend that flux braiding will indeed result in
significant current generation. We discuss the implications of our results for
coronal heating.Comment: 13 pages, 12 figure
Adolescent physical self-perceptions, sport/exercise and lifestyle physical activity
Purpose – Domain and sub-domain physical self-perceptions have been associated with adolescent moderate intensity physical activity although the association with different types of adolescent moderate intensity physical activity remains unclear. This study seeks to examine the relationship between personal self-perceptions and adolescent sport/exercise and lifestyle moderate intensity physical activity frequency.
Design/methodology/approach – A total of 122, 13-to-14 year-old, English adolescents from Leeds, West Yorkshire (58 boys and 64 girls) had their personal self-perceptions, sport/exercise and lifestyle moderate intensity physical activity assessed.
Findings – No significant positive relationships were found between boys' personal self-perceptions and lifestyle moderate intensity physical activity. However, a range of weak (r?=?0.34-0.42) but significant relationships (p?<?0.01) were found between personal self perceptions and boys' sport/exercise frequency. In contrast, only perceptions of strength competence were significantly related to girls' sport/exercise frequencies (r?=?0.28; p?<?0.05), while all personal self perceptions scales were significantly related to girls' lifestyle moderate intensity physical activity (r?=?0.26-0.32; p?<?0.05).
Research limitations/implications – The use of correlation analyses by this study placed limitations on the extent to which cause-effect relationships were established. Furthermore, girls' sport/exercise was poorly distributed, which may have led to the non-significant relationship found between this activity type and personal self-perceptions. The presence of a significant relationship between these two variables should therefore not be discounted.
Originality/value – This study seems to be the first to investigate and identify variations in the personal self-perceptions – moderate intensity physical activity relationship relative to activity type. Although more research is required, findings have implications for practitioners aiming to tailor physical activity interventions to this group and researchers aiming to match specific correlates to different types of adolescent physical activity
Three-dimensional rotational averages in radiation-molecule interactions: an irreducible cartesian tensor formulation
- …