Thermohaline instability and rotation-induced mixing II- Yields of 3He for low- and intermediate-mass stars

Context. The 3He content of Galactic HII regions is very close to that of the Sun and the solar system, and only slightly higher than the primordial 3He abundance as predicted by the standard Big Bang nucleosynthesis. However, the classical theory of stellar evolution predicts a high production of 3He by low-mass stars, implying a strong increase of 3He with time in the Galaxy. This is the well-known "3He problem". Aims. We study the effects of thermohaline and rotation-induced mixings on the production and destruction of 3He over the lifetime of low- and intermediate-mass stars at various metallicities. Methods. We compute stellar evolutionary models in the mass range 1 to 6M\odot for four metallicities, taking into account thermohaline instability and rotation-induced mixing. For the thermohaline diffusivity we use the prescription based on the linear stability analysis, which reproduces Red Giant Branch (RGB) abundance patterns at all metallicities. Rotation-induced mixing is treated taking into account meridional circulation and shear turbulence. We discuss the effects of these processes on internal and surface abundances of 3He and on the net yields. Results. Over the whole mass and metallicity range investigated, rotation-induced mixing lowers the 3He production, as well as the upper mass limit at which stars destroy 3He. For low-mass stars, thermohaline mixing occuring beyond the RGB bump is the dominant process in strongly reducing the net 3He yield compared to standard computations. Yet these stars remain net 3He producers. Conclusions. Overall, the net 3He yields are strongly reduced compared to the standard framework predictions

Thermohaline instability and rotation-induced mixing. III - Grid of stellar models and asymptotic asteroseismic quantities from the pre-main sequence up to the AGB for low- and intermediate-mass stars at various metallicities

The availability of asteroseismic constraints for a large sample of stars from the missions CoRoT and Kepler paves the way for various statistical studies of the seismic properties of stellar populations. In this paper, we evaluate the impact of rotation-induced mixing and thermohaline instability on the global asteroseismic parameters at different stages of the stellar evolution from the Zero Age Main Sequence to the Thermally Pulsating Asymptotic Giant Branch to distinguish stellar populations. We present a grid of stellar evolutionary models for four metallicities (Z = 0.0001, 0.002, 0.004, and 0.014) in the mass range between 0.85 to 6.0 Msun. The models are computed either with standard prescriptions or including both thermohaline convection and rotation-induced mixing. For the whole grid we provide the usual stellar parameters (luminosity, effective temperature, lifetimes, ...), together with the global seismic parameters, i.e. the large frequency separation and asymptotic relations, the frequency corresponding to the maximum oscillation power {\nu}_{max}, the maximal amplitude A_{max}, the asymptotic period spacing of g-modes, and different acoustic radii. We discuss the signature of rotation-induced mixing on the global asteroseismic quantities, that can be detected observationally. Thermohaline mixing whose effects can be identified by spectroscopic studies cannot be caracterized with the global seismic parameters studied here. But it is not excluded that individual mode frequencies or other well chosen asteroseismic quantities might help constraining this mixing.Comment: 15 pages, 11 figures, accepted for publication in A&

Beryllium abundances along the evolutionary sequence of the open cluster IC 4651 - New test for hydrodynamical stellar models

[abridged] Previous analyses of lithium abundances in main sequence and red giant stars have revealed the action of mixing mechanisms other than convection in stellar interiors. Beryllium abundances in stars with lithium abundance determinations can offer valuable complementary information on the nature of these mechanisms. Our aim is to derive beryllium abundances along the whole evolutionary sequence of an open cluster, IC 4651. These Be abundances are used together with previously determined Li abundances, in the same sample stars, to investigate the mixing mechanisms in a range of stellar masses and evolutionary stages. New beryllium abundances are determined from high-resolution, high signal-to-noise UVES spectra using spectrum synthesis and model atmospheres. The careful synthetic modelling of the Be lines region is used to calculate reliable abundances in rapidly rotating stars. The observed behavior of Be and Li is compared to theoretical predictions from stellar models including rotation-induced mixing, internal gravity waves, atomic diffusion, and thermohaline mixing. Beryllium is detected in all the main sequence and turn-off sample stars, both slow- and fast-rotating stars, including the Li-dip stars, but was not detected in the red giants. Confirming previous results, we find that the Li dip is also a Be dip, although the depletion of Be is more modest than that of Li in the corresponding effective temperature range. For post-main-sequence stars, the Be dilution starts earlier within the Hertzsprung gap than expected from classical predictions as does the Li dilution. A clear dispersion in the Be abundances is also observed. Theoretical stellar models including the hydrodynamical transport processes mentioned above are able to reproduce well all the observed features.Comment: 12 pages, accepted for publication in A&A, revised final versio

XANES Study of Structural Disorder in Amorphous Silicon

An investigation of the structure of several amorphous silicon (a-Si) films is presented. Samples were prepared by using the ion beam sputtering technique at different substrate deposition temperatures. X-ray absorption spectroscopy and multiple scattering formalism have been used to detect structural variations of the a-Si films. The analysis of the XANES (X-ray absorption near-edge structure) spectra shows that increasing the substrate deposition temperature leads to a structural change toward a higher-level short-range order.

The long-term effects of free care on birth outcomes: Evidence from a national policy reform in Zambia

As women in many countries still fail to give birth in facilities due to financial barriers, many see the abolition of user fees as a key step on the path towards universal coverage. We exploited the staggered removal of user charges in Zambia from 2006 to estimate the effect of user fee removal up to five years after the policy change. We used data from the birth histories of two nationally representative Demographic and Health Surveys to implement a difference-in-differences analysis and identify the causal impact of removing user charges on institutional and assisted deliveries, caesarean sections and neonatal deaths. We also explored heterogeneous effects of the policy. Removing fees had little effect in the short term but large positive effects appeared about two years after the policy change. Institutional deliveries in treated areas increased by 10 and 15 percentage points in peri-urban and rural districts respectively (corresponding to a 25 and 35 percent change), driven entirely by a reduction in home births. However, there was no evidence that the reform changed the behaviours of women with lower education, the proportion of caesarean sections or reduced neonatal mortality. Institutional deliveries increased where care quality was high, but not where it was low. While abolishing user charges may reduce financial hardship from healthcare payments, it does not necessarily improve equitable access to care or health outcomes. Shifting away from user fees is a necessary but insufficient step towards universal health coverage, and concurrent reforms are needed to target vulnerable populations and improve quality of care

Prevention of HIV and other STIs in rural Senegal: a study of prevention-related events collected by sentinel observers

We conducted a study of AIDS/STI prevention-related events based on a network of sentinel observers over a 35-month period in three rural communities of Senegal (May 2000 - March 2003). In one of these communities we also conducted a population-based study on self-reported prevention events and attitudes towards AIDS. Sentinel observers recorded 1 590 AIDS/STI prevention-related events. More than half of the reported events were radio programmes.The proportion of events on the occasion of which therapies was addressed increased significantly between 2000 and 2001, from less than 3% to about 10%, while sharp decreases in the proportion of events dealing with condoms and STIs, both from around 25% to less than 15%, were observed at the same time. The population-based survey indicated that men were more influenced by individual events like informal discussions or radio programmes while women seemed to be more influenced by collective events such as public meetings and school education. Keywords: rural Africa,AIDS/STI prevention-related events, sentinel observers, population-based survey.RÉSUMÉNous avons mené une étude des événements relatifs à la prévention du SIDA et des IST à l\'aide d\'un réseau d\'observateurs-sentinelles sur une durée de 35 mois (mai 2000 - mars 2003) dans trois communautés rurales du Sénégal. Dans l\'une d\'elle, nous avons également conduit en population générale une étude des événements autodéclarés relatifs à la prévention et des attitudes vis-à-vis du SIDA. Les observateurs-sentinelles ont enregistré 1 590 événements. Plus de la moitié concernaient des émissions de radio. La part des événements faisant état des traitements du SIDA a augmenté de 3% à 10% entre 2000 et 2001 alors que celles des événements qui faisaient état du préservatif d\'une part et des IST d\'autre part, ont chuté toutes deux de 25% à 15% sur la même période. L\'enquête en population générale a révélé que les hommes étaient plus influencés par les événements individuels comme les discussion informelles ou les programmes radiodiffusés alors que les femmes semblaient avoir été plus sensibles aux événements collectifs du type réunion publique ou cours donnés à l\'école. Mots clés: Afrique, zone rurale, SIDA/STI, événements relatifs à la prévention, observateurs-sentinelles, enquête en population générale.Journal of Social Aspects of HIV/AIDS, Vol 2(2) 2005: 251-25

Beryllium abundances along the evolutionary sequence of the open cluster IC 4651

The simultaneous investigation of Li and Be in stars is a powerful tool in the study of the evolutionary mixing processes. Here, we present beryllium abundances in stars along the whole evolutionary sequence of the open cluster IC 4651. This cluster has a metallicity of [Fe/H] = +0.11 and an age of 1.2 or 1.7 Gyr. Abundances have been determined from high-resolution, high signal-to-noise UVES spectra using spectrum synthesis and model atmospheres. Lithium abundances for the same stars were determined in a previous work. Confirming previous results, we find that the Li dip is also a Be dip. For post-main-sequence stars, the Be dilution starts earlier within the Hertzsprung gap than expected from classical predictions, as does the Li dilution. Theoretical hydrodynamical models are able to reproduce well all the observed feature

Carrier and polarization dynamics in monolayer MoS2

In monolayer MoS2 optical transitions across the direct bandgap are governed by chiral selection rules, allowing optical valley initialization. In time resolved photoluminescence (PL) experiments we find that both the polarization and emission dynamics do not change from 4K to 300K within our time resolution. We measure a high polarization and show that under pulsed excitation the emission polarization significantly decreases with increasing laser power. We find a fast exciton emission decay time on the order of 4ps. The absence of a clear PL polarization decay within our time resolution suggests that the initially injected polarization dominates the steady state PL polarization. The observed decrease of the initial polarization with increasing pump photon energy hints at a possible ultrafast intervalley relaxation beyond the experimental ps time resolution. By compensating the temperature induced change in bandgap energy with the excitation laser energy an emission polarization of 40% is recovered at 300K, close to the maximum emission polarization for this sample at 4K.Comment: 7 pages, 7 figures including supplementary materia