9,781 research outputs found
Magnetic Braking and Viscous Damping of Differential Rotation in Cylindrical Stars
Differential rotation in stars generates toroidal magnetic fields whenever an
initial seed poloidal field is present. The resulting magnetic stresses, along
with viscosity, drive the star toward uniform rotation. This magnetic braking
has important dynamical consequences in many astrophysical contexts. For
example, merging binary neutron stars can form "hypermassive" remnants
supported against collapse by differential rotation. The removal of this
support by magnetic braking induces radial fluid motion, which can lead to
delayed collapse of the remnant to a black hole. We explore the effects of
magnetic braking and viscosity on the structure of a differentially rotating,
compressible star, generalizing our earlier calculations for incompressible
configurations. The star is idealized as a differentially rotating, infinite
cylinder supported initially by a polytropic equation of state. The gas is
assumed to be infinitely conducting and our calculations are performed in
Newtonian gravitation. Though highly idealized, our model allows for the
incorporation of magnetic fields, viscosity, compressibility, and shocks with
minimal computational resources in a 1+1 dimensional Lagrangian MHD code. Our
evolution calculations show that magnetic braking can lead to significant
structural changes in a star, including quasistatic contraction of the core and
ejection of matter in the outermost regions to form a wind or an ambient disk.
These calculations serve as a prelude and a guide to more realistic MHD
simulations in full 3+1 general relativity.Comment: 20 pages, 19 figures, 3 tables, AASTeX, accepted by Ap
Law, Urban Development, and the Poor in Developing Countries
In this Article, the potential role of legal services for the urban poor in achieving levels and forms of urban development more satisfactory to their welfare will be considered. It will be seen that the inadequacy of urban development policies is a manifestation of the fundamental problems of political development confronting the poor countries and that only under uncommonly favorable political conditions can legal services prove effective in securing more favorable environmental conditions for the urban poor. This Article is directly concerned with urban squatters
A first direct measurement of the intergalactic medium temperature around a quasar at z=6
The thermal state of the intergalactic medium (IGM) provides an indirect
probe of both the HI and HeII reionisation epochs. Current constraints on the
IGM temperature from the Lya forest are restricted to the redshift range
2<z<4.5, limiting the ability to probe the thermal memory of HI reionisation
toward higher redshift. In this work, we present the first direct measurement
of the IGM temperature around a z=6 quasar by analysing the Doppler widths of
Lya absorption lines in the proximity zone of SDSS J0818+1722. We use a high
resolution (R= 40000) Keck/HIRES spectrum in combination with detailed
numerical modelling to obtain the temperature at mean density,
T_0=23600\pm^5000_6900K (\pm^9200_9300K) at 68 (95) per cent confidence
assuming a prior probability 13500K<T_0<38500 K following HI and HeII
reionisation. This enables us to place an upper limit on the redshift of HI
reionisation, z_H, within 33 comoving Mpc of SDSS J0818+1722. If the quasar
reionises the HeII in its vicinity, then in the limit of instantaneous
reionisation we infer z_H<9.0 (11.0) at 68 (95) per cent confidence assuming
photoheating is the dominant heat source and that HI reionisation is driven by
ionising sources with soft spectra, typical of population II stars. If the HI
and HeII in the IGM around SDSS J0818+1722 are instead reionised simultaneously
by a population of massive metal-free stars, characterised by very hard
ionising spectra, we obtain a tighter upper limit of z_H<8.4 (9.4). Initiating
reionisation at higher redshifts produces temperatures which are too low with
respect to our constraint unless the HI ionising sources or the quasar itself
have spectra significantly harder than typically assumed.Comment: 15 pages, 9 figures, accepted to MNRA
Increasing Hospital Admissions for Pneumonia, England
This rise in recorded incidence from 2001 to 2005 was particularly marked among the elderly
Developing Leadership Dispositions for Preparing Urban School Leaders in Chronically Low-Performing Schools
This study originated as an institutionally sponsored research residency conducted using utilization-focused evaluation (Patton, 2008) to investigate the nature of dispositional changes in candidates pursuing National Louis University’s M.Ed. and Ed.S. degrees leading to educational leadership (EDL) state certification in one large urban school district in a Southern state. The EDL program organized learning objectives intending to develop a specific knowledge base, an operational competency set, and, as this study’s focus, leadership dispositions related to preparing assistant principals and principals to lead effectively in chronically low-performing (CLP) schools. The inquiry cross-analyzed data at the intersection of the graduate program’s disposition-related learning objectives with 13 leadership dispositions identified in The Haberman Educational Foundation Star Urban Administrator Pre-Screener. EDL program faculty administered the pre-screener to EDL program candidates twice, yielding 187 matched pairs in pre-program and post-program administrations. Statistical analyses yielded a significant difference (improvement) in overall Haberman scores at the .01 alpha level, as well as an effect size considered (Cohen, 1969) to be a medium effect size. Faculty triangulated the data with interviews of alumni and faculty with experience in leading CLP schools, who affirmed that focus on leadership dispositions serves an important developmental role in an EDL program. The study appears to validate the measurable presence and dynamic changes in EDL candidate dispositions as an element of a graduate program focused on developing effective leaders of CLP schools
- …