309 research outputs found
Scandium: A key element for understanding Am stars
{\rm Context.} Atomic diffusion is believed to cause the abundance anomalies
observed in AmFm stars. However, the detailed process has still not been
well-established. For instance, two possible scenarios for the diffusion theory
are presently envisaged. They differ mainly by the depth from which the
abundance anomalies emanate. The first scenario predicts that the abundances
are modified in the superficial regions of the star, just below the hydrogen
convection zone. The second scenario predicts that a much deeper extension of
the mixing zone exists due to the convection caused by Fe accumulation in
regions below the hydrogen convection zone. {\rm Aims.} We calculate much more
accurate radiative accelerations of Sc than previously, to better understand
the observed abundance anomalies of this element. We believe that it is a key
element to use as a diagnostic tool for understanding AmFm stars.
{\rmMethods.} The method employed to obtain these radiative accelerations is
based on an interpolation from the parameters of the so-called SVP parametric
method.
{\rm Results.} The radiative accelerations, shown here in a typical Am
stellar model, are discussed in light of the observed anomalies of Ca and Sc.
Our results suggest that the deeper mixing scenario is not entirely
satisfactory: the mixing zone should be deeper than what is predicted by recent
models to account for observed Sc underabundances. Our results seem more
compatible with the scenario where the abundances anomalies are created in the
superficial regions. However, only detailed evolutionary modelling with mass
loss and diffusion of all important species, including Ca and Sc, with accurate
radiative accelerations, will be able to give more insight into where the
source of these anomalies occur in AmFm stars.Comment: 6 pages, 3 figures, accepted for publication in A&
A generic coupling between internal states and activity leads to activation fronts and criticality in active systems
We investigate a generic coupling between the particle's internal state and
self-propulsion to study the onset of collective motion. Our analysis reveals
such a coupling renders an otherwise non-critical 2-state system, into an
effective 3-state system able to display scale-invariant activity avalanches,
proving the existence of an intimate connection with critical phenomena.
Furthermore, we identify three distinct propagating front regimes, including a
selfish regime. The obtained results provide insight into the way collectives
distribute, process, and respond to local environmental cues.Comment: 5 pages, 3 figure
Integrating Multimedia Techniques into CS Pedagogy
Through its grants, the National Science Foundation sponsors workshops that inform faculty of current topics in Computer Science. Such a workshop, entitled, Developing Multimedia-based Interactive Laboratory Modules for Computer Science , was given July 27 -August 6, 1998, at Illinois State University in Normal, Illinois. Each participant was expected to design and implement a small part of a laboratory module. This paper describes what some of the faculty members who participated in the workshop have done with the knowledge obtained from the workshop
AmFm and lithium gap stars: Stellar evolution models with mass loss
A thorough study of the effects of mass loss on internal and surface
abundances of A and F stars is carried out in order to constrain mass loss
rates for these stars, as well as further elucidate some of the processes which
compete with atomic diffusion. Self-consistent stellar evolution models of 1.3
to 2.5 M_sun stars including atomic diffusion and radiative accelerations for
all species within the OPAL opacity database were computed with mass loss and
compared to observations as well as previous calculations with turbulent
mixing. Models with unseparated mass loss rates between 5 x 10^-14 and 10^-13
M_sun/yr reproduce observations for many cluster AmFm stars as well as Sirius A
and o Leonis. These models also explain cool Fm stars, but not the Hyades
lithium gap. Like turbulent mixing, these mass loss rates reduce surface
abundance anomalies; however, their effects are very different with respect to
internal abundances. For most of the main sequence lifetime of an A or F star,
surface abundances in the presence of such mass loss depend on separation which
takes place between log(Delta M/M_star)= -6 and -5. The current observational
constraints do not allow us to conclude that mass loss is to be preferred over
turbulent mixing (induced by rotation or otherwise) in order to explain the
AmFm phenomenon. Internal concentration variations which could be detectable
through asteroseismic tests should provide further information. If atomic
diffusion coupled with mass loss are to explain the Hyades Li gap, the wind
would need to be separated.Comment: 27 pages, 25 figures, accepted for publication in A&
Does age influence between-rams companionship in mouflon (Ovis gmelini)?
Durant le rut, les mouflons mâles (Ovis gmelini), quel que soit leur âge, s'associent de façon non sélective avec les autres classes d'âge et de sexe. Ceci est vrai tout au long de l'année pour les béliers d'un an, avec cependant
une préférence pour leurs pairs en été. En dehors du rut, on a observé une tendance croissante avec l'âge des béliers à éviter les brebis et les agneaux et une préférence marquée pour la compagnie de béliers d'âge similaire. Les indices de préférence étaient plus faibles au printemps qu'en
été, vraisemblablement en raison de la formation de grands rassemblements durant la période de plus grande croissance de la végétatio
Chemical composition of A and F dwarf members of the Coma Berenices open cluster
Abundances of 18 chemical elements have been derived for 11 A (normal and Am)
and 11 F dwarfs members of the Coma Berenices open cluster in order to set
constraints on evolutionary models including transport processes (radiative and
turbulent diffusion)calculated with the Montreal code. A spectral synthesis
iterative procedure has been applied to derive the abundances from selected
high quality lines in high resolution high signal-to-noise echelle spectra
obtained with ELODIE at the Observatoire de Haute Provence. The chemical
pattern found for the A and F dwarfs in Coma Berenices is reminiscent of that
found in the Hyades and the UMa moving group. In graphs representing the
abundances [X/H] versus the effective temperature, the A stars often display
abundances much more scattered around their mean values than the F stars do.
Large star-to-star variations are detected for A stars in their abundances
which we interpret as evidence of transport processes competing with radiative
diffusion. The F stars have solar abundances for almost all elements except for
Mg, Si, V and Ba. The derived abundances patterns, [X/H] versus atomic number,
for the slow rotator HD108642 (A2m) and the moderately fast rotator HD106887
(A4m) were compared to the predictions of self consistent evolutionary model
codes including radiative and different amounts of turbulent diffusion. None of
the models reproduces entirely the overall shape of the abundance pattern.
While part of the discrepancies between derived and predicted abundances may be
accounted for by non-LTE effects, the inclusion of competing processes such as
rotational mixing in the radiative zones of these stars seems necessary to
improve the agreement between observed and predicted abundance patterns.Comment: 25 pages, 20 figure
Behaviour of ewes (Ovis gmelini musimon x Ovis sp.) during the rut
It was tested whether male courtship induces changes in activities in female mouflon
(Ovis gmelini musimon x Ovis sp.) due to the promiscuous mating system. For females in the
presence of males, we expected a higher involvement in social interactions, reduced time
spent feeding, increased time standinglscanning and moving, and a higher rate and level of
act transition and diversity. We tested these hypotheses by comparing mouflon ewe activities
in mixed-sex, mating (one estrous female with at least one adult male) and female groups
during the pre-rut (16-31/0ct.), the rut (01-30/Nov.) and the post-rut (01-15/Dec.). Females
devoted more time to feeding and less to standing/scanning from the pre- to the post-rut, in
mixed and female groups. At the same time, act transition and diversity decreased in female
groups. Females could experience reduced appetite during the oestrus period but ecological
factors (better quality and higher abundance of food) may explain these results.
Differences between mixed-sex and female groups were found only during the full-rut.
Females devoted more time to standing/scanning and Jess to feeding in mixed-sex groups.
Although significant, the percent of time dedicated to feeding and standing/scanning was
weakly related to the interaction frequency. So the presence of males seems to have a low
incidence on female activities, as seen through the low level of social interactions involving
females, and their low behavioural responsiveness. Mating groups were observed only during
the pre- and the full-rut. Because of small sample size (N = 8), data from the two periods were
mixed in order to compare group types. Data on mating groups suggest that oestrous ewes
feed much Jess, move more, change activity more often and are involved in social interactions
more frequently than in mixed-sex and female groups. However, as oestrus lasts only 1 -3
days, the energy costs of the rut are thought to be low for femalesL'hypothèse que la cour des mâles induit une modification des activités des femelles en raison du système reproducteur de type promiscuité a été testée chez le Mouflon (Ovis gmelini musimon x Ovis sp.). On s'attendait à ce qu'en
présence des mâles, les femelles soient plus fréquemment impliquées dans des interactions sociales et consacrent moins de temps à l'alimentation, plus de temps au « debout/observe » et au déplacement, changent plus souvent et diversifient davantage leurs actes. Nous avons donc comparé
l'activité des femelles dans des groupes mixtes, reproducteurs (une femelle en oestrus avec au moins un mâle ≥ 1,5 ans) et dans des groupes de femelles, au cours du pré-rut (16-31/oct.), du rut (01-30/nov.) et du post-rut (01-15/déc.). Du pré-rut au post-rut, tant dans les groupes mixtes que
de femelles, le temps consacré à l'alimentation s'accroît au détriment du « debout/observe », alors qu'aucune variation significative n'est détectée dans la part allouée au déplacement. Durant cette même période, la fréquence de transition des actes et la diversité comportementale diminue
dans les groupes de femelles. Ces résultats peuvent s'expliquer par une diminution de l'appétit des femelles pendant la période des oestrus mais aussi par des facteurs écologiques (amélioration de la qualité et abondance des ressources alimentaires). Des différences entre groupes mixtes et
de femelles n'ont été détectées que durant le plein rut. Les femelles passent moins de temps à se nourrir et plus de temps à observer dans les groupes mixtes. Bien que statistiquement significative, la relation entre fréquence de cour et pourcentage de temps consacré à l'alimentation et au «
debout/observe » est faible. La relation disparaît si les données relatives aux groupes reproducteurs sont omises. L'impact de la présence des mâles sur l'activité des femelles semble donc minime, ce qui peut être relié aux faibles taux d'interactions impliquant les femelles, ou à la faible
réactivité comportementale des femelles. Les groupes reproducteurs n'ont été observés que durant le pré- et le plein rut. En raison du petit échantillon (N = 8), les 2 périodes ont été rassemblées pour permettre une comparaison entre types de groupes. Dans les groupes reproducteurs, les
femelles en oestrus broutent nettement moins, se déplacent plus, changent plus d'activité et sont plus souvent impliquées dans des interactions sociales que les brebis dans les groupes mixtes ou de femelles. Les résultats suggèrent que le comportement des femelles n'est sensiblement modifié
que dans les groupes reproducteurs, pendant l'oestrus. Ce demier ne durant que de 1 à 3 jours, les coûts énergétiques du rut sont vraisemblablement faibles pour les femelles
Precise Modeling of the Exoplanet Host Star and CoRoT Main Target HD 52265
This paper presents a detailed and precise study of the characteristics of
the Exoplanet Host Star and CoRoT main target HD 52265, as derived from
asteroseismic studies. The results are compared with previous estimates, with a
comprehensive summary and discussion. The basic method is similar to that
previously used by the Toulouse group for solar-type stars. Models are computed
with various initial chemical compositions and the computed p-mode frequencies
are compared with the observed ones. All models include atomic diffusion and
the importance of radiative accelerations is discussed. Several tests are used,
including the usual frequency combinations and the fits of the \'echelle
diagrams. The possible surface effects are introduced and discussed. Automatic
codes are also used to find the best model for this star (SEEK, AMP) and their
results are compared with that obtained with the detailed method. We find
precise results for the mass, radius and age of this star, as well as its
effective temperature and luminosity. We also give an estimate of the initial
helium abundance. These results are important for the characterization of the
star-planet system.Comment: 9 pages, 6 figures, 7 tables, to be published in Astronomy and
Astrophysic
Perceived responder legitimacy and group identification predict cooperation and compliance in a mass decontamination field exercise
Emergency responders’ failure to communicate effectively during decontamination following a chemical or biological incident has been associated with increased public anxiety and reduced public compliance. In this study we applied the social identity approach to evaluating a field exercise that involved mass decontamination. Questionnaires were collected from 115 volunteers, who participated in the exercise as simulated casualties. Volunteers’ perceptions of effective responder communication predicted increased self-reported compliance with decontamination, mediated by perceived responder legitimacy and identification with other group members. Developing effective communication strategies using a social psychology perspective could improve the way in which incidents are managed
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