461 research outputs found
Accretion from debris disks onto white dwarfs : Fingering (thermohaline) instability and derived accretion rates
Recent observations of a large number of DA and DB white dwarfs show evidence
of debris disks, which are the remnants of old planetary systems. The infrared
excess detected with \emph{Spitzer} and the lines of heavy elements observed in
their atmospheres with high-resolution spectroscopy converge on the idea that
planetary material accretes onto these stars. Accretion rates have been derived
by several authors with the assumption of a steady state between accretion and
gravitational settling. The results are unrealistically different for DA and DB
white dwarfs. When heavy matter is accreted onto stars, it induces an inverse
-gradient that leads to fingering (thermohaline) convection. The aim of
this letter is to study the impact of this specific process on the derived
accretion rates in white dwarfs and on the difference between DA and DB. We
solve the diffusion equation for the accreted heavy elements with a
time-dependent method. The models we use have been obtained both with the IRAP
code, which computes static models, and the La Plata code, which computes
evolutionary sequences. Computations with pure gravitational settling are
compared with computations that include fingering convection. The most
important result is that fingering convection has very important effects on DAs
but is inefficient in DBs. When only gravitational settling is taken into
account, the time-dependent computations lead to a steady state, as postulated
by previous authors. When fingering convection is added, this steady state
occurs much later. The surprising difference found in the past for the
accretion rates derived for DA and DB white dwarfs disappears. The derived
accretion rates for DAs are increased when fingering convection is taken into
account, whereas those for DBs are not modified. More precise and developed
results will be given in a forthcoming paper
Reaction to spatial novelty and exploratory strategies in baboons
Exploratory activity was examined in 4 young baboons with the aim of investigating the type of spatial coding (purely geometric and/or by taking into account the identity of the object) used for the configuration of objects. Animals were individually tested in an outdoor enclosure for their exploratory reactions
(contact time and order of spontaneous visits) to changes brought about to a configuration of different objects. Two kinds of spatial changes were made: a modification (1) of the shape of the configuration (by displacement of one object) and (2) of the spatial arrangement without changing the initial
shape (exchanging the location of two objects). In the second experiment, the effect of a spatial modification of the global geometry constituted by four identical objects was investigated. Finally, in the third experiment, a substitution of a familiar object with a novel one was performed without changing
the objects' configuration. The baboons strongly reacted to geometrical modifications of the configuration. In contrast, they were less sensitive to modifications of local features that did not affect the initial spatial configuration. Analyses of spontaneous exploratory activities revealed two types of exploratory
strategies (cyclic and back-and-forth). These data are discussed in relation to (1) the distinction between the encoding of geometric versus local spatial features and (2) the spatial function of exploratory activity
Rhesus monkeys use geometric and non geometric during a reorientation task
Rhesus monkeys (Macaca mulata) were subjected to a place finding task in a rectangular room perfectly homogeneous and without distinctive featural information. Results of Experiment 1 show that monkeys rely on the large-scale geometry of the room to retrieve a food reward. Experiments 2 and 3 indicate that subjects use also nongeometric information (colored wall) to reorient. Data of Experiments 4 and 5 suggest that monkeys do not use small angular cues but that they are sensitive to the size of the cues (Experiments 6, 7, and 8). Our findings strengthen the idea that a mechanism based on the geometry of the environment is at work in several mammalian species. In addition, the present data offer new perspectives on spatial cognition in animals that are phylogenetically close to humans. Specifically, the joint use of both geometric and landmark-based cues by rhesus monkeys tends to demonstrate that spatial processing became more flexible with evolutio
Fingering Convection in Red Giants Revisited
Fingering (thermohaline) convection has been invoked for several years as a
possible extra-mixing which could occur in Red Giant stars due to the
modification of the chemical composition induced by nuclear reactions in the
hydrogen burning zone. Recent studies show however that this mixing is not
sufficient to account for the needed surface abundances. A new prescription for
fingering convection, based on 3D numerical simulations has recently been
proposed (BGS). The resulting mixing coefficient is larger than the ones
previously given in the literature. We compute models using this new
coefficient and compare them to previous studies. We use the LPCODE stellar
evolution code with the GNA generalized version of the mixing length theory to
compute Red Giant models and we introduce fingering convection using the BGS
prescription. The results show that, although the fingering zone now reaches
the outer dynamical convective zone, the efficiency of the mixing is not enough
to account for the observations. The fingering mixing coefficient should be
increased by two orders of magnitude for the needed surface abundances to be
reached. We confirm that fingering convection cannot be the mixing process
needed to account for surface abundances in RGB stars.Comment: Accepted for publication in Astronomy and Astrophysic
Serial mediation model predicting covid-19 vaccines acceptance in Portugal: The critical role of conspiracy theories in the wake of perceived quality of government communication and national stereotypes
Ever since the Covid-19 vaccination rollout, governments have aimed for herd immunity. Yet, many countries are far from achieving this goal mainly due to vaccine refusal. Previous research has pointed to conspiracy beliefs and the role of trust in governments as predictors of vaccine acceptance, yet a more comprehensive explanatory model is still missing. Using data from a convenience sample of 377 residents in Portugal (Mage = 33.56, SD = 13.67), the present study extends previous research by proposing a serial mediation model in the prediction of vaccine acceptance. The results confirm the critical role of conspiracy beliefs mediating the link between perceived quality of government communication and general vaccine acceptance (Model 1) as well as national stereotypes and acceptance of the Sinovac vaccine from China (Model 2). The implications are discussed considering that Portugal is currently ranked the second country in the world with the highest vaccination rate.info:eu-repo/semantics/publishedVersio
Investigating the human value “forgiveness” across 30 countries: a cross-cultural meta-analytical approach
The current study focuses on the human value of forgiveness and explores its correlates using a country-level meta-analytical approach. We investigated the importance of forgiveness using the Rokeach Value Survey with 41,975 participants from 30 countries to address the following research questions: How important is forgiveness across different countries? What contextual variables correlate with the ranked importance of forgiveness? This study provides important insights on country-level correlates of forgiveness in regard to conditions that may favor prioritizing the value forgiveness and its possible implications for societies. The results show that the value forgiveness is related to a highly developed socioeconomic and sociopolitical environment as well as to subjective well-being at the country level.info:eu-repo/semantics/acceptedVersio
On the Coupling between Helium Settling and Rotation-Induced Mixing in Stellar Radiative Zones: II- Application to light elements in population I main-sequence stars
In the two previous papers of this series, we have discussed the importance
of t he -gradients due to helium settling on rotation-induced mixing,
first in a n approximate analytical way, second in a 2D numerical simulation.
We have found that, for slowly rotating low mass stars, a process of ``creeping
paralysis" in which the circulation and the diffusion are nearly frozen may
take place below the convective zone. Here we apply this theory to the case of
lithium and beryll ium in galactic clusters and specially the Hyades. We take
into account the rota tional braking with rotation velocities adjusted to the
present observations. We find that two different cells of meridional
circulation appear on the hot side of the "lithium dip" and that the "creeping
paralysis" process occurs, not dir ectly below the convective zone, but deeper
inside the radiative zone, at the to p of the second cell. As a consequence,
the two cells are disconnected, which ma y be the basic reason for the lithium
increase with effective temperature on thi s side of the dip. On the cool side,
there is just one cell of circulation and t he paralysis has not yet set down
at the age of the Hyades; the same modelisatio n accounts nicely for the
beryllium observations as well as for the lithium ones .Comment: 13 printed pages, 10 figures. ApJ, in press (April 20, 2003
Lithium in field Am and normal A-F-type stars
Preliminary abundances of lithium and a few other elements have been obtained
for 31 field Am stars with good Hipparcos parallaxes, as well as for 36 normal
A and F stars. Radial and projected rotational velocities were determined as
well. We examine the Li abundance as a function of the stellar parameters: for
normal stars, it is clearly bimodal for Teff < 7500 K, while Am-Fm stars are
all somewhat Li-deficient in this range. The most Li-deficient stars - either
Am or normal - tend to be at least slightly evolved, but the reverse is not
true.Comment: 4 pages, 2 figures, poster presented at the conference "Element
stratification in stars, 40 years of atomic diffusion", eds. G. Alecian, O.
Richard and S. Vauclair, EAS Publication Series, in pres
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