969 research outputs found
How the 52-week high and low affect beta and volatility
Academic Session 14 - Capital Market and Investment Strategy IIIWe provide a new perspective on stock price behavior around 52-week highs and lows. Instead of focusing on noisy measurements of abnormal returns (alpha), our main focus is to analyze whether a stock’s beta, return volatility and option-implied volatility change (i) when stock prices approach their 52-week high or low, and (ii) when stock prices break through these highs or lows. We find that betas and volatilities decrease when approaching a high or low, and that volatilities increase after breakthroughs. The effects are economically large and very significant, and consistent across stock and stock-option markets. Among several explanations for our findings, we find most support for the anchoring theory.postprintThe 8th NTU International Conference on Economics, Finance and Accounting (2010 IEFA), Taiwan, 21-23 June 2010
Theoretical wind clumping predictions from 2D LDI models of O-star winds at different metallicities
Hot, massive (OB) stars experience strong line-driven stellar winds and mass
loss. As the majority of efficient driving lines are metallic, the amount of
wind driving and mass loss is dependent on the stellar metallicity Z. In
addition, line-driven winds are intrinsically inhomogeneous and clumpy.
However, to date, neither theoretical nor empirical studies of line-driven
winds have investigated how such wind clumping may also depend on Z. We
theoretically investigated the degree of wind clumping due to the
line-deshadowing instability (LDI) as a function of Z. We performed
two-dimensional hydrodynamic simulations of the LDI with an assumed
one-dimensional radiation line force for a grid of O-star wind models with
fixed luminosity, but with different metal contents by varying the accumulative
line strength Qbar describing the total ensemble of driving lines. We find
that, for this fixed luminosity, the amount of wind clumping decreases with
metallicity. The decrease is clearly seen in the statistical properties of our
simulations, but is nonetheless rather weak; a simple power-law fit for the
dependence of the clumping factor f_cl = /^2 on metallicity yields
f_cl ~ Z^(0.15 +/- 0.01). This implies that empirically derived power-law
dependencies of mass-loss rate Mdot on metallicity -- which were previously
inferred from spectral diagnostics effectively depending on Mdot*sqrt(f_cl) but
without having any constraints on f_cl(Z) -- should be only modestly altered by
clumping. We expect that this prediction can be directly tested using new data
from the Hubble Space Telescope Ultraviolet Legacy Library of Young Stars as
Essential Standards (ULLYSES) project.Comment: accepted for publication in A&
Theoretical wind clumping predictions of OB supergiants from line-driven instability simulations across the bi-stability jump
(Abridged) The behaviour of mass loss across bi-stability jump is a key
uncertainty in models of massive stars. While an increase in mass loss is
theoretically predicted, this has so far not been observationally confirmed.
However, radiation-driven winds of massive stars are known to exhibit clumpy
structures triggered by the line-deshadowing instability (LDI). Wind clumping
affects empirical mass-loss rates inferred from density square-dependent
spectral diagnostics. If clumping properties differ significantly for O and B
supergiants across the bi-stability jump, this may help alleviate discrepancies
between theory and observations. We investigate with analytical and numerical
tools how the onset of clumpy structures behaves in the winds of O supergiants
(OSG) and B supergiants (BSG) across the bi-stability jump. We derive a scaling
relation for the linear growth rate of the LDI for a single optically thick
line and apply it in both regimes. We run 1D time-dependent line-driven
instability simulations to study the non-linear evolution of the LDI in clumpy
OSG and BSG winds. Linear perturbation analysis for a single line shows that
the LDI linear growth rate scales strongly with stellar effective temperature
and terminal wind speed. This implies significantly lower growth rates for
(cooler, slower) BSG winds than for OSG winds. This is confirmed by the
non-linear simulations, which show significant differences in OSG and BSG wind
structure formation, with the latter characterized by significantly weaker
clumping factors and lower velocity dispersions. This suggests that lower
correction factors due to clumping should be employed when deriving empirical
mass-loss rates for BSGs on the cool side of the bi-stability jump. Moreover,
the non-linear simulations provide a theoretical background toward explaining
the general lack of observed intrinsic X-ray emission in (single) B star winds.Comment: 10 pages, 5 figures, accepted for publication in A&
Stepwise evolution of the Sec machinery in Proteobacteria
The Sec machinery facilitates the translocation of proteins across and into biological membranes. In several of the Proteobacteria, this machinery contains accessory features that are not present in any other bacterial division. The genomic distribution of these features in the context of bacterial phylogeny suggests that the Sec machinery has evolved in discrete steps. The canonical Sec machinery was initially supplemented with SecB; subsequently, SecE was extended with two transmembrane segments and, finally, SecM was introduced. The Sec machinery of Escherichia coli and other Enterobacteriales represents the end product of this stepwise evolution.</p
Radiography in high mass X-ray binaries -- Micro-structure of the stellar wind through variability of the column density
In high mass X-ray binaries (HMXBs), an accreting compact object orbits a
high mass star which loses mass through a dense and inhomogeneous wind. Using
the compact object as an X-ray backlight, the time variability of the absorbing
column density in the wind can be exploited in order to shed light on the
micro-structure of the wind and obtain unbiased stellar mass loss rates for
high mass stars. We explore the impact of clumpiness on the variability of the
column density with a simplified wind model. In particular, we focus on the
standard deviation of the column density and the characteristic duration of
enhanced absorption episodes, and compare them with analytical predictions
based on the porosity length. We identified the favorable systems and orbital
phases to determine the wind micro-structure. The coherence time scale of the
column density is shown to be the self-crossing time of a clump in front of the
compact object. We provide a recipe to get accurate measurements of the size
and of the mass of the clumps, purely based on the observable time variability
of the column density. The coherence time scale grants direct access to the
size of the clumps while their mass can be deduced separately from the
amplitude of the variability. If it is due to unaccreted passing-by clumps, the
high column density variations in some HMXBs requires high mass clumps to
reproduce the observed peak-to-peak amplitude and coherence time scales. These
clump properties are hardly compatible with the ones derived from first
principles. Alternatively, other components could contribute to the variability
of the column density: larger orbital scale structures produced by a mechanism
still to be identified, or a dense environment in the immediate vicinity of the
accretor such as an accretion disk, an outflow or a spherical shell around the
magnetosphere of the accreting neutron star
School denomination, family and school composition, and educational achievements:Are there differences between religious and non-religious schools?
Despite an ever increasing process of secularization in Dutch society, the share of religious schools in the Netherlands has remained remarkably stable over the last fifty years. One explanation for this persistent attraction among parents is the sector’s good reputation regarding educational quality and achievement. This study tests whether this assumption corresponds with empirical reality. The data analyzed come from the second measurement round of the COOL5-18 cohort study conducted in 2011. A total of 19 cognitive and non-cognitive outcome measures were available from 27457 students in grades 2, 5 and 8 of 386 primary schools. The analyses compare the outcomes of public (i.e. neutral) schools with those of three types of religious schools, namely Protestant, Catholic, and Islamic schools. The results show that after controlling for differences between students in terms of social and ethnic background, and schools in terms of share of low educated and immigrant parents, no differences between public and religious schools remain. Meanwhile, Islamic schools appear to have a special potential regarding the elimination of their educational disadvantage
School denomination, family and school composition, and educational achievements:Are there differences between religious and non-religious schools?
Transparency in peer review: Exploring the content and tone of reviewers\u27 confidential comments to editors.
Mogelijkheden en beperkingen van een gebiedsgerichte uitwerking van de Nitraatrichtlijn
Wordt de landbouw onevenredig hard getroffen in die gebieden in Nederland waar aan de nitraatconcentratie in het grondwater wordt voldaan? Die vraag probeerden Alterra en de Universiteit Utrecht te beantwoorden in een studie naar de mogelijkheden die de Nitraatrichtlijn biedt om de aanwijzing van nitraatuitspoelingsgevoelige gebieden (kwetsbare zones) en bijbehorende nitraatactieprogramma's te differentiëren naar de landbouw- en milieukundige omstandigheden in de verschillende regio's. Het doel hierachter is een betere aansluiting te krijgen bij de gebiedsgerichte uitwerking van de Kaderrichtlijn Water. De resultaten zijn gebaseerd op literatuurstudie, brainstormsessies, interviews en een bijeenkomst met actoren uit het veld. Een verdere differentiatie is technisch mogelijk via aanwijzing van specifieke kwetsbare zones en het uitzonderen van andere gebieden. Verwacht wordt echter dat het uit te zonderen areaal beperkt zal zijn. Meer perspectief bied een verdergaande (gebiedsgerichte) differentiatie van gebruiksnormen die van toepassing zijn op de aangewezen kwetsbare zones. Nitraatactieprogramma's zijn verder te 'harmoniseren' met stroomgebiedsplannen van de Kaderrichtlijn Water, maar dienen wel als zodanig herkenbaar in stand te blijven
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