136 research outputs found
Ferromagnetism in Fe-substituted spinel semiconductor ZnGaO
Motivated by the recent experimental observation of long range ferromagnetic
order at a relatively high temperature of 200K in the Fe-doped ZnGaO
semiconducting spinel, we propose a possible mechanism for the observed
ferromagnetism in this system. We show, supported by band structure
calculations, how a model similar to the double exchange model can be written
down for this system and calculate the ground state phase diagram for the two
cases where Fe is doped either at the tetrahedral position or at the octahedral
position. We find that in both cases such a model can account for a stable
ferromagnetic phase in a wide range of parameter space. We also argue that in
the limit of high Fe concentration at the tetrahedral positions a
description in terms of a two band model is essential. The two orbitals
and the hopping between them play a crucial role in stabilizing the
ferromagnetic phase in this limit. The case when Fe is doped simultaneously at
both the tetrahedral and the octahedral position is also discussed.Comment: 10 pages, 9 figures, added text, J. Phys. Cond. Mat. (to appear
Cubic boron nitride: a new prospective material for ultracold neutron application
For the first time, the neutron optical wall-potential of natural cubic boron
nitride (cBN) was measured at the ultracold neutron (UCN) source of the
research reactor TRIGA Mainz using the time-of-flight method (TOF). The samples
investigated had a wall-potential of (305 +/- 15) neV. This value is in good
agreement with the result extracted from neutron reflectometry data and
theoretical expectations. Because of its high critical velocity for UCN and its
good dielectric characteristics, cubic boron nitride coatings (isotopically
enriched) will be useful for a number of applications in UCN experiments
The UK Infrared Telescope M33 monitoring project. II. The star formation history in the central square kiloparsec
We have conducted a near-infrared monitoring campaign at the UK InfraRed
Telescope (UKIRT), of the Local Group spiral galaxy M33 (Triangulum). The main
aim was to identify stars in the very final stage of their evolution, and for
which the luminosity is more directly related to the birth mass than the more
numerous less-evolved giant stars that continue to increase in luminosity. In
this second paper of the series, we construct the birth mass function and hence
derive the star formation history. The star formation rate has varied between
~0.002 and 0.007 M yr^-1 kpc^-2. We give evidence of two epochs of a
star formation rate enhanced by a factor of a few -- one that happened \geq 6
Gyr ago and produced \geq 80% of the total mass in stars, and one around 250
Myr ago that lasted ~ 200 Myr and formed \leq 6% of the mass in stars. We
construct radial and azimuthal distributions in the image plane and in the
galaxy plane for populations associated with old first-ascent red giant branch
(RGB) stars, intermediate-age Asymptotic Giant Branch (AGB) stars and young
(massive) blue and red supergiants. We find that the RGB stars follow a
spheroidal distribution, while younger stars follow a flat-disc distribution.
The intermediate-age population displays signs of a pseudo-bulge or possibly a
bar. The inner spiral arm pattern as recorded in mid-19^th-century drawings is
confirmed. We interpret our findings as evidence for an old, pressure-supported
component and a younger disc formed 6 Gyr ago, with an accretion event
occurring 250 Myr ago giving rise to the compact nucleus in M33. Our study
provides support for recent Padova stellar evolution models except that
super-AGB stars likely reach low temperatures and thus high mass-loss rates,
supporting the super-AGB nature of the progenitors of dust-enshrouded
supernovae such as SN2008S.Comment: Accepted for publication in MNRA
Model for common growth of supermassive black holes, bulges and globular star clusters: ripping off Jeans clusters
It is assumed that a galaxy starts as a dark halo of a few million Jeans
clusters (JCs), each of which consists of nearly a trillion micro brown dwarfs,
MACHOs of Earth mass. JCs in the galaxy center heat up their MACHOs by tidal
forces, which makes them expand, so that coagulation and star formation occurs.
Being continuously fed by matter from bypassing JCs, the central star(s) may
transform into a super massive black hole. It has a fast growth during
the first mega years, and a slow growth at giga years. JCs disrupted
by a close encounter with this black hole can provide matter for the bulge.
Those that survive can be so agitated that they form stars inside them and
become globular star clusters. Thus black holes mostly arise together with
galactic bulges in their own environment and are about as old as the oldest
globular clusters. The age 13.2 Gyr of the star HE 1523-0901 puts forward that
the Galactic halo was sufficiently assembled at that moment. The star formation
rate has a maximum at black hole mass and bulge mass
. In case of merging supermassive black holes the JCs
passing near the galactic center provide ideal assistance to overcome the last
parsec.Comment: 6 pages latex. Matches published versio
The Formation of the First Massive Black Holes
Supermassive black holes (SMBHs) are common in local galactic nuclei, and
SMBHs as massive as several billion solar masses already exist at redshift z=6.
These earliest SMBHs may grow by the combination of radiation-pressure-limited
accretion and mergers of stellar-mass seed BHs, left behind by the first
generation of metal-free stars, or may be formed by more rapid direct collapse
of gas in rare special environments where dense gas can accumulate without
first fragmenting into stars. This chapter offers a review of these two
competing scenarios, as well as some more exotic alternative ideas. It also
briefly discusses how the different models may be distinguished in the future
by observations with JWST, (e)LISA and other instruments.Comment: 47 pages with 306 references; this review is a chapter in "The First
Galaxies - Theoretical Predictions and Observational Clues", Springer
Astrophysics and Space Science Library, Eds. T. Wiklind, V. Bromm & B.
Mobasher, in pres
The prognostic value of a nomogram for exercise capacity in women
BACKGROUND: Recent studies have demonstrated that exercise capacity is an independent predictor of mortality in women. Normative values of exercise capacity for age in women have not been well established. Our objectives were to construct a nomogram to permit determination of predicted exercise capacity for age in women and to assess the predictive value of the nomogram with respect to survival. METHODS: A total of 5721 asymptomatic women underwent a symptom-limited, maximal stress test. Exercise capacity was measured in metabolic equivalents (MET). Linear regression was used to estimate the mean MET achieved for age. A nomogram was established to allow the percentage of predicted exercise capacity to be estimated on the basis of age and the exercise capacity achieved. The nomogram was then used to determine the percentage of predicted exercise capacity for both the original cohort and a referral population of 4471 women with cardiovascular symptoms who underwent a symptom-limited stress test. Survival data were obtained for both cohorts, and Cox survival analysis was used to estimate the rates of death from any cause and from cardiac causes in each group. RESULTS: The linear regression equation for predicted exercise capacity (in MET) on the basis of age in the cohort of asymptomatic women was as follows: predicted MET = 14.7 - (0.13 x age). The risk of death among asymptomatic women whose exercise capacity was less than 85 percent of the predicted value for age was twice that among women whose exercise capacity was at least 85 percent of the age-predicted value (P<0.001). Results were similar in the cohort of symptomatic women. CONCLUSIONS: We have established a nomogram for predicted exercise capacity on the basis of age that is predictive of survival among both asymptomatic and symptomatic women. These findings could be incorporated into the interpretation of exercise stress tests, providing additional prognostic information for risk stratification
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