1,753 research outputs found
The impact of pollution on stellar evolution models
An approach is introduced for incorporating the concept of stellar pollution
into stellar evolution models. The approach involves enhancing the metal
content of the surface layers of stellar models. In addition, the surface
layers of stars in the mass range of 0.5-2.0 Solar masses are mixed to an
artificial depth motivated by observations of lithium abundance. The behavior
of polluted stellar evolution models is explored assuming the pollution occurs
after the star has left the fully convective pre main sequence phase. Stellar
models polluted with a few Earth masses of iron are significantly hotter than
stars of the same mass with an equivalent bulk metallicity. Polluted stellar
evolution models can successfully reproduce the metal-rich, parent star tau
Bootis and suggest a slightly lower mass than standard evolution models.
Finally, the possibility that stars in the Hyades open cluster have accreted an
average of 0.5 Earth masses of iron is explored. The results indicate that it
is not possible to rule out stellar pollution on this scale from the scatter of
Hyades stars on a color-magnitude diagram. The small amount of scatter in the
observational data set does rule out pollution on the order of 1.5 Earth masses
of iron. Pollution effects at the low level of 0.5 Earth masses of iron do not
produce substantial changes in a star's evolution.Comment: 14 pages, 3 figures, AASTeX, to appear in the 10/10/03 issue of Ap
A Novel Stochastic Multi-Scale Model of Francisella tularensis Infection to Predict Risk of Infection in a Laboratory
We present a multi-scale model of the within-phagocyte, within-host and population-level infection dynamics of Francisella tularensis, which extends the mechanistic one proposed by Wood et al. (2014). Our multi-scale model incorporates key aspects of the interaction between host phagocytes and extracellular bacteria, accounts for inter-phagocyte variability in the number of bacteria released upon phagocyte rupture, and allows one to compute the probability of response, and mean time until response, of an infected individual as a function of the initial infection dose. A Bayesian approach is applied to parameterize both the within-phagocyte and within-host models using infection data. Finally, we show how dose response probabilities at the individual level can be used to estimate the airborne propagation of Francisella tularensis in indoor settings (such as a microbiology laboratory) at the population level, by means of a deterministic zonal ventilation model
C, S, Zn and Cu abundances in planet-harbouring stars
We present a detailed and uniform study of C, S, Zn and Cu abundances in a
large set of planet host stars, as well as in a homogeneous comparison sample
of solar-type dwarfs with no known planetary-mass companions. Carbon abundances
were derived by {EW} measurement of two C I optical lines, while spectral
syntheses were performed for S, Zn and Cu. We investigated possible differences
in the behaviours of the volatiles C, S and Zn and in the refractory Cu in
targets with and without known planets in order to check possible anomalies due
to the presence of planets. We found that the abundance distributions in stars
with exoplanets are the high [Fe/H] extensions of the trends traced by the
comparison sample. All volatile elements we studied show [X/Fe] trends
decreasing with [Fe/H] in the metallicity range -0.8<[Fe/H]<0.5, with
significantly negative slopes of -0.39+-0.04 and -0.35+-0.04 for C and S,
respectively. A comparison of our abundances with those available in the
literature shows good agreement in most cases.Comment: 28 pages, 13 figures, accepted for publication in A&
Blogging in the physics classroom: A research-based approach to shaping students' attitudes towards physics
Even though there has been a tremendous amount of research done in how to
help students learn physics, students are still coming away missing a crucial
piece of the puzzle: why bother with physics? Students learn fundamental laws
and how to calculate, but come out of a general physics course without a deep
understanding of how physics has transformed the world around them. In other
words, they get the "how" but not the "why". Studies have shown that students
leave introductory physics courses almost universally with decreased
expectations and with a more negative attitude. This paper will detail an
experiment to address this problem: a course weblog or "blog" which discusses
real-world applications of physics and engages students in discussion and
thinking outside of class. Specifically, students' attitudes towards the value
of physics and its applicability to the real-world were probed using a
26-question Likert scale survey over the course of four semesters in an
introductory physics course at a comprehensive Jesuit university. We found that
students who did not participate in the blog study generally exhibited a
deterioration in attitude towards physics as seen previously. However, students
who read, commented, and were involved with the blog maintained their initially
positive attitudes towards physics. Student response to the blog was
overwhelmingly positive, with students claiming that the blog made the things
we studied in the classroom come alive for them and seem much more relevant.Comment: 20 pages, 6 figure
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To be or not to be an auctioneer: Some thoughts on the legal nature of online eBay auctions and the protection of consumers
This paper discusses the legal classification of online “eBay” auctions. The discussion has key implications on the scope of consumer protection law as sale by auctions are, for example, excluded from the scope of the Consumer Protection (Distance Selling) Regulations 2000. The paper uncovers that online “eBay” auctions cannot always be considered as traditional auctions and that eBay, as an intermediary, is not to be considered as an auctioneer. This creates difficulties associated with a distributive application of consumer protection laws such as the Consumer Protection (Distance Selling) Regulations 2000. Another set of difficulties is associated with a lenient legal regime applicable to the liability of eBay under the Electronic Commerce (EC Directive) Regulations 2002 . The paper concludes that there is an urgent need to clarify the legal classification of online auctions and to rethink the liability of online auction sites to better protect consumers
Stochastic dynamics of Francisella tularensis infection and replication
We study the pathogenesis of Francisella tularensis infection with an experimental mouse model, agent-based computation and mathematical analysis. Following inhalational exposure to Francisella tularensis SCHU S4, a small initial number of bacteria enter lung host cells and proliferate inside them, eventually destroying the host cell and releasing numerous copies that infect other cells. Our analysis of disease progression is based on a stochastic model of a population of infectious agents inside one host cell, extending the birth-and-death process by the occurrence of catastrophes: cell rupture events that affect all bacteria in a cell simultaneously. Closed expressions are obtained for the survival function of an infected cell, the number of bacteria released as a function of time after infection, and the total bacterial load. We compare our mathematical analysis with the results of agent-based computation and, making use of approximate Bayesian statistical inference, with experimental measurements carried out after murine aerosol infection with the virulent SCHU S4 strain of the bacterium Francisella tularensis, that infects alveolar macrophages. The posterior distribution of the rate of replication of intracellular bacteria is consistent with the estimate that the time between rounds of bacterial division is less than 6 hours in vivo
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Orthographic facilitation in oral vocabulary acquisition
An experiment investigated whether exposure to orthography facilitates oral vocabulary learning. A total of 58 typically developing children aged 8-9 years were taught 12 nonwords. Children were trained to associate novel phonological forms with pictures of novel objects. Pictures were used as referents to represent novel word meanings. For half of the nonwords children were additionally exposed to orthography, although they were not alerted to its presence, nor were they instructed to use it. After this training phase a nonword-picture matching posttest was used to assess learning of nonword meaning, and a spelling posttest was used to assess learning of nonword orthography. Children showed robust learning for novel spelling patterns after incidental exposure to orthography. Further, we observed stronger learning for nonword-referent pairings trained with orthography. The degree of orthographic facilitation observed in posttests was related to children's reading levels, with more advanced readers showing more benefit from the presence of orthography
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