13,669 research outputs found
Spin Model for Inverse Melting and Inverse Glass Transition
A spin model that displays inverse melting and inverse glass transition is
presented and analyzed. Strong degeneracy of the interacting states of an
individual spin leads to entropic preference of the "ferromagnetic" phase,
while lower energy associated with the non-interacting states yields a
"paramagnetic" phase as temperature decreases. An infinite range model is
solved analytically for constant paramagnetic exchange interaction, while for
its random exchange, analogous results based on the replica symmetric solution
are presented. The qualitative features of this model are shown to resemble a
large class of inverse melting phenomena. First and second order transition
regimes are identified
Galaxy-galaxy(-galaxy) lensing as a sensitive probe of galaxy evolution
The gravitational lensing effect provides various ways to study the mass
environment of galaxies. We investigate how galaxy-galaxy(-galaxy) lensing can
be used to test models of galaxy formation and evolution. We consider two
semi-analytic galaxy formation models based on the Millennium Run N-body
simulation: the Durham model by Bower et al. (2006) and the Garching model by
Guo et al. (2011). We generate mock lensing observations for the two models,
and then employ Fast Fourier Transform methods to compute second- and
third-order aperture statistics in the simulated fields for various galaxy
samples. We find that both models predict qualitatively similar aperture
signals, but there are large quantitative differences. The Durham model
predicts larger amplitudes in general. In both models, red galaxies exhibit
stronger aperture signals than blue galaxies. Using these aperture measurements
and assuming a linear deterministic bias model, we measure relative bias ratios
of red and blue galaxy samples. We find that a linear deterministic bias is
insufficient to describe the relative clustering of model galaxies below ten
arcmin angular scales. Dividing galaxies into luminosity bins, the aperture
signals decrease with decreasing luminosity for brighter galaxies, but increase
again for fainter galaxies. This increase is likely an artifact due to too many
faint satellite galaxies in massive group and cluster halos predicted by the
models. Our study shows that galaxy-galaxy(-galaxy) lensing is a sensitive
probe of galaxy evolution.Comment: 11 pages, 8 figures, accepted in A&
Dust from AGBs: relevant factors and modelling uncertainties
The dust formation process in the winds of Asymptotic Giant Branch stars is
discussed, based on full evolutionary models of stars with mass in the range
MMM, and metallicities .
Dust grains are assumed to form in an isotropically expanding wind, by growth
of pre--existing seed nuclei. Convection, for what concerns the treatment of
convective borders and the efficiency of the schematization adopted, turns out
to be the physical ingredient used to calculate the evolutionary sequences with
the highest impact on the results obtained. Low--mass stars with MM produce carbon type dust with also traces of silicon carbide. The
mass of solid carbon formed, fairly independently of metallicity, ranges from a
few M, for stars of initial mass M, to
M for MM; the size of dust
particles is in the range mm. On the contrary,
the production of silicon carbide (SiC) depends on metallicity. For the size of SiC grains varies in the range m, while the mass of SiC formed is
. Models of
higher mass experience Hot Bottom Burning, which prevents the formation of
carbon stars, and favours the formation of silicates and corundum. In this case
the results scale with metallicity, owing to the larger silicon and aluminium
contained in higher--Z models. At Z= we find that the most
massive stars produce dust masses M, whereas models of
smaller mass produce a dust mass ten times smaller. The main component of dust
are silicates, although corundum is also formed, in not negligible quantities
().Comment: Paper accepted for publication in Monthly Notices of the Royal
Astronomical Society Main Journal (2014 January 4
AGB and SAGB stars: modelling dust production at solar metallicity
We present dust yields for asymptotic giant branch (AGB) and
super--asymptotic giant branch (SAGB) stars of solar metallicity. Stars with
initial mass reach the carbon
star stage during the AGB phase and produce mainly solid carbon and SiC. The
size and the amount of the carbon particles formed follows a positive trend
with themass of the star; the carbon grains with the largest size (m) are produced by AGB stars with ,
as these stars are those achieving the largest enrichment of carbon in the
surface regions. The size of SiC grains, being sensitive to the surface silicon
abundance, keeps around m. The mass of carbonaceous
dust formed is in the range , whereas the
amount of SiC produced is . Massive
AGB/SAGB stars with experience HBB, that inhibits
formation of carbon stars. The most relevant dust species formed in these stars
are silicates and alumina dust, with grain sizes in the range m and m, respectively. The
mass of silicates produced spans the interval and increases with the initial
mass of the star.Comment: Accepted for publication in MNRA
Dust formation in the winds of AGBs: the contribution at low metallicities
We present new models for the evolution of stars with mass in the range 1Msun
< M < 7.5Msun, followed from the pre-main-sequence through the asymptotic giant
branch phase. The metallicity adopted is $Z=3*10^{-4} (which, with an
alpha-enhancement of +0.4, corresponds to [Fe/H]=-2). Dust formation is
described by following the growth of dust grains of various types as the wind
expands from the stellar surface.
Models with mass M>3Msun experience Hot Bottom Burning, thus maintaining the
surface C/O below unity. Unlike higher Z models, the scarcity of silicon
available in the envelope prevents the formation of silicates in meaningful
quantities, sufficient to trigger the acceleration of the wind via radiation
pressure on the dust grains formed. No silicate formation occurs below a
threshold metallicity of Z=10^{-3}.
Low--mass stars, with M< 2.5Msun become carbon stars, forming solid carbon
dust in their surroundings. The total dust mass formed depends on the uncertain
extent of the inwards penetration of the convective envelope during the Third
Dredge--Up episodes following the Thermal Pulses. Carbon grains have sizes 0.08
micron < a_C < 0.12 micron and the total amount of dust formed (increasing with
the mass of the star) is M_C=(2-6)*10^{-4}Msun.
Our results imply that AGB stars with Z=3*10^{-4} can only contribute to
carbon dust enrichment of the interstellar medium on relatively long
timescales, > 300 Myr, comparable to the evolutionary time of a 3Msun star. At
lower metallicities the scarcity of silicon available and the presence of Hot
Bottom Burning even in M< 2Msun, prevents the formation of silicate and carbon
grains. We extrapolate our conclusion to more metal--poor environments, and
deduce that at Z < 10^{-4} dust enrichment is mostly due to metal condensation
in supernova ejecta.Comment: 13 pages, 8 figures, accepted for publication on MNRA
Early education pilot for two year old children : evaluation
This report provides the findings of the evaluation of the early education pilot for disadvantaged two year old children (the pilot). This study aimed to assess the impact of the pilot by looking at: how well the pilot was targeted, parentsâ experiences of taking up a pilot place, the quality of the pilot settings, the impact on the childrenâs behaviour, and parentsâ views and experiences of using a pilot place.
The pilot provided free early years education to over 13,500 disadvantaged two year olds between 2006 and 2008. The main purpose of the pilot was to improve childrenâs social and cognitive outcomes, e.g. their social confidence and independence, and their verbal skills and reasoning ability.
Additional aims were to have a positive impact on childrenâs parents and wider family e.g. on the relationship between parents and their children, or on parentâs emotional wellbeing. The funding offered these children 7.5 or in a small number of local authorities 12.5 hours of early years education per week for 38 weeks of the year.
The pilot places were available in a variety of early years settings e.g. nurseries, play groups and with childminders, but all were required to operate the Birth to Three Matters curriculum.© National Centre for Social Research 2009. The full text of this report is not available in ORA. You may be able to access the report at https://www.gov.uk/government/publications/early-education-pilot-for-2-year-old-children-evaluation (URL checked 26 March 2014) or via the publication website link above
"Decessit sine prole" - childlessness, celibacy, and survival of the richest in pre-industrial England
In explaining England's early industrial development, previous research has highlighted that wealthy pre-industrial elites had more surviving offspring than their poorer counter- parts. Thus, entrepreneurial traits spread and helped England grow rich. We contest this view, showing that lowerclass reproduction rates were no different from the elites when accounting for singleness and childlessness. Elites married less and were more often childless. Many died without descendants (decessit sine prole). We find that the middle classes had the highest reproduction and argue that this advantage was instrumental to England's economic success because the middle class invested most strongly in human capital
Childlessness, celibacy and net fertility in pre-industrial England: the middle-class evolutionary advantage
This paper reconsiders the fertility of historical social groups by accounting for singleness and childlessness. We find that the middle class had the highest reproductive success during England's early industrial development. In light of the greater propensity of the middle class to invest in human capital, the rise in the prevalence of these traits in the population could have been instrumental to England's economic success. Unlike earlier results about the survival of the richest, the paper shows that the reproductive success of the rich (and also the poor) were lower than that of the middle class, once accounting for singleness and childlessness. Hence, the prosperity of England over this period can be attributed to the increase in the prevalence of middle-class traits rather than those of the upper (or lower) class
Burmese amber fossils bridge the gap in the Cretaceous record of polypod ferns
publisher: Elsevier articletitle: Burmese amber fossils bridge the gap in the Cretaceous record of polypod ferns journaltitle: Perspectives in Plant Ecology, Evolution and Systematics articlelink: http://dx.doi.org/10.1016/j.ppees.2016.01.003 content_type: article copyright: Copyright © 2016 Elsevier GmbH. All rights reserved.Copyright © 2016 Elsevier GmbH. All rights reserved. This document is the authors' final accepted version of the journal article. You are advised to consult the publisher's version if you wish to cite from it
- âŠ