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Photospheric abundances are used to determine the importance of atomic diffusion, meridional circulation, mass loss and turbulence in main sequence and horizontal branch stars. Atomic diffusion leads to the appearance of the Li gap and the AmFm and HgMn phenomena at approximately the Teff at which they are observed. It leads to a 12 to 25% reduction in the age of halo stars. The Li abundance in Halo stars is probably 50% of the original abundance. Atomic diffusion competes with meridional circulation. The V sin i at which the HgMn and AmFm phenomena disappear give an argument in favour of the meridional circulation model of Tassoul and Tassoul. Mass loss is probably present in AmFm stars and cooler F stars but only at the rate of 10-15 M⊙ yr-1. In many objects, the turbulent particle diffusion coefficient is at most 10 times larger than the atomic diffusion coefficient.
Abundance anomalies in the Sun, main sequence F, A and B stars, turnoff Pop II stars, the horizontal branch and white dwarfs are caused at least partly by particle transport processes. Detailed evolutionary models have been calculated for most of those objects taking into account the gravitational settling, thermal diffusion and radiative accelerations of 28 isotopes (24 atomic species). These will be used together with the observed abundances to put constraints on the mixing that rotationnally induced turbulence may lead to. The link between abundance anomalies and rotation on the HB is explained by the variation with log g of the ratio of the Eddington-Sweet and atomic diffusion velocities.
The various mechanisms leading to the appearance of horizontal inhomogeneities at the surface of ApBp stars are critically reviewed. The effect of magnetic fields is essential but is it more to control locally, on the surface, the appearance of anomalies that are caused by the vertical transport of particles, or is it to create directly the horizontal inhomogeneities by horizontal transport? The time scales for horizontal and vertical transport will be discussed in this context.
The processes discussed include: a) ambipolar diffusion of protons and hydrogen in the presence of magnetic fields; b) the guiding of diffusion by magnetic fields; c) the horizontal component of radiative accelerations; d) mass loss; e) light induced drift.
The cubic polytype of silicon carbide is a stimulating candidate for Micro-Electro-Mechanical-Systems (MEMS) applications due to its interesting physical and chemical properties. Recently, we demonstrated the possibility to elaborate 3C-SiC membranes on 3C-SiC pseudo-substrates, using a silicon epilayer grown by Low Pressure Chemical Vapor Deposition as a sacrificial layer. Such structures could be the starting point for the elaboration of new MEMS devices. However, the roughness still represents a major concern. Therefore, in this contribution, we investigate the influence of an excimer laser irradiation on the Si epilayer surface prior to the 3C-SiC epilayer growth. We compare these results with the 3C-SiC epilayer grown directly on the as-grown Si epilayer.
Our Commission decided to proceed as before, with a rather comprehensive report, while focusing on the subjects where most progress has been achieved during the past three years. The colleagues who kindly contributed to it are W. Dziembowski (helio- and aster-oseismology), J. Guzik (intermediate-mass stars), G. Meynet (massive stars), G. Michaud (atomic diffusion), D. VandenBerg (low mass stars), G. Vauclair (white dwarfs), J.-P. Zahn (convection, rotational mixing).
In the absence of mass loss, diffusion leads to underabundances of He in main sequence stars. Because of a very strong observational link with Ap and He weak stars, it has however been suggested that diffusion is the explanation for the He rich stars of the upper main sequence. This requires a mass loss rate of 10−12 Mo yr−1 or slightly lower. The mass loss rate must decrease as Teff increases. Magnetic fields must apparently be involved to reduce the mass loss rate. Since this model predicts that the CNO abundances should be normal in the cooler He rich stars, it leads to a clear observational test. Detailed calculations should be made to confirm the importance of this test. The effects of separation in the wind, the atmosphere and the envelope are discussed to conclude that separation in the atmosphere is likely to be most important. The importance of diffusion for He rich white dwarfs and horizontal branch stars are briefly discussed.
This work is a continuation of the Montréal stellar evolutionary model development (Richard et al., 2001 and references therein) where the effects of atomic diffusion are taken into account for 28 elements (see Turcotte, these proceedings), along with an approximate treatment of turbulent transport (Richer et al., 2000).
We used the CEFF equation of state and the Bahcall nuclear energy generation routine. The Rosseland opacity is computed at each time step, from the OPAL monochromatic opacities for 21 elements (to which we added locally computed spectrum tables for Li, Be, and B) for each mesh point in the model for the current local chemical composition. Convection and semi-convection are taken into account in every species’ diffusion velocity and in the determination of the thermal gradient (Richard et al., 2001). The models also take into account gravitational settling, thermal diffusion, radiative accelerations and turbulent transport. The radiative accelerations are from Richer et al. (1998). The turbulent transport is modeled as a diffusion process (Richer et al., 2000) with diffusion coefficient: DT = ωD(He)0 (ρ/ρ0)n where D(He)0 is the Ηe atomic diffusion coefficient at density ρ0.
The observations of AmFm, λ Booti, HgMn and He rich stars that are explained without any arbitrary parameter by diffusion are briefly reviewed, followed by those observations that are not explained by this simple model. Mass loss is then shown to explain a large fraction of the observations that are not explained in the parameter free model. It seems to play a role in the λ Booti, AmFm, He rich and the hot horizontal branch stars. It is only of about 10−15 to 10−13 Mo/yr. Abundance anomalies then help to determine stellar hydrodynamics. It is finally suggested that recent observations of Li underabundances in F stars of the Hyades represent an extension of the AmFm star phenomenon.
The evolutionary models described in the accompanying poster (Richard et al., these proceedings) have been tested for pulsations using both the adiabatic code developed by Brassard et al. (1992) and the corresponding non-adiabatic code as first used in Fontaine et al. (1994). For the stars of interest, these codes have the great advantages of being potentially able to treat atmospheric regions, as well as being numerically stable and efficient. The required derivatives were calculated by least-square fitting of spline functions to complete opacity tables constructed at each composition of the evolved model. Since there were some 1500 zones for each model, 1500 such tables were constructed and spline functions were fitted to each of them. They then gave directly the derivatives required for the pulsation calculations (see Fig. 1). Note that models equivalent to 1.70T5.3D1K-3 have already been tested for pulsations by Turcotte et al. (2000).
Richer & Michaud (1993) calculated a series of envelopes fully coupled to non-rotating, constant mass, stellar evolution models of hydrogen burning stars with masses in the range of 1.2 to 2.2 M⊙, typical of A and F main sequence stars. They included He settling. The location of the theoretically predicted gap of the Hyades agrees quite well with the observed one, a result obtained without the introduction of any free parameter. At temperatures above the gap, while the observed lithium abundances are within a factor of 2-3 of normal values, the theoretical calculated curve drops to very low values. Diffusion velocities being fairly small, any other physical process with larger or similar velocities can reduce the effect of diffusion and produce the observed results. Mass loss is one such process. Another difficulty with the present theory is the width of the gap. Observations show that the observed gap is wider than the calculated one in the Hyades. This also suggests that other physical processes play an important role.
Finding many planets in orbit around other stars has provoked new interest in the long-running debate about the existence of extraterrestrial intelligence. Now we are asked to think about what rational, practical steps we can take to prepare for discovering evidence of sapient aliens, an event whose timing and exact nature are unpredictable. Our preparations must be based on probabilities, analogies, and disciplined speculations rather than on confirmed evidence.
Many of those interested in this debate expect that what they see as the final blow to anthropocentrism – discovering extraterrestrial intelligence – is just a matter of time and effort. If that discovery occurs, it might lead to a change in the way we see our status and our position in the universe. Such a discovery might not be the kind of paradigm break that Thomas Kuhn discussed in his famous book, The Structure of Scientific Revolutions (Kuhn 1970). Kuhn was writing about breaking scientific paradigms: replacing one set of physical laws with another. The new paradigms were not just different; they were better. The anticipated discovery of extraterrestrial intelligence is part of a different kind of paradigm break. Contact with an alien civilization would involve much more than science, raising philosophical and societal questions where laws are less certain or non-existent.
Evidence, scenarios, choices
Standards of evidence
A century ago, in the wake of Percival Lowell and the controversy over the canals of Mars, many humans believed that the red planet was inhabited by an alien civilization (Crowe 1986). Subsequent observations discredited that idea; the theory failed the empirical test. Beginning in 1947, thousands of people reported seeing exotic craft in our atmosphere, even on the Earth's surface (Jacobs 1975; Peebles 1994). While large numbers of our fellow humans believed that those objects were visitors from other worlds, most analysts concluded that nearly all UFOs actually were IFOs – identifiable flying objects. As astronomer Seth Shostak pointed out, there still are no artifacts to examine (Shostak 1998, 135). Others have suggested that the sightings most difficult to explain might be unknown phenomena that are unrelated to alien visitors. Again, the empirical standard has not been met.
In the light of recent results from helioseismology, the abundance anomalies seen at the surface of Bp, Ap, Am and Fm stars appear as the measurable superficial effect of a process that has much more general impact.
The standard model of intertemporal choice assumes risk neutrality towards the length of life: under additivity of lifetime utility and expected utility assumptions, agents are not sensitive to a mean preserving spread in the length of life. Using a survey fielded in the RAND American Life Panel, this paper provides empirical evidence on possible deviation from risk neutrality with respect to longevity in the US population. The questions we ask allow to find the distribution as well as to quantify the degree of risk aversion with respect to the length of life in the population. We find evidence that roughly 75% of respondents were not neutral with respect to longevity risk. Hence, there is a little empirical support for the joint use of the expected utility and additive lifetime utility assumptions in life-cycle models. Higher income households are more likely to be risk averse towards the length of life. We do not find evidence that the degree of risk aversion varies with age or education.
Stellar models including atomic diffusion processes (gravitational settling, radiative accelerations, thermal diffusion, in addition to the purely diffusive term) have now been evolved throughout most of stellar evolution. We review some of the major roles of diffusion processes in stellar evolution. The emphasis is on interior properties. Examples include Populations I and II stars. It is emphasized that competing advective processes such as mass loss or meridional circulation modify internal concentrations differently from turbulent diffusion even when they lead to the same surface abundances. Radiative accelerations play a major role in horizontal branch and sdB stars.
We followed lifetime trajectories of reproductive allocation in Coleomegilla maculata females of three different size classes produced by rearing beetles on three different daily larval feeding regimes (30 min, 6 h or ad libitum access to eggs of Ephestia kuehniella). We hypothesized that small females would produce fewer and smaller eggs than larger females and that reproductive effort would decline with female age. Females were mated with a male from the same treatment and then isolated with ad libitum food for their entire adult lives. Egg size increased over time in all treatments; small females started off laying the smallest eggs, but increased egg size more rapidly than larger females, until all treatments converged on a similar egg size around the 20th day of oviposition. Large females realized a larger proportion of their fecundity early in life, but smaller females increased daily fecundity over time. Reproductive effort (egg mass/body mass) did not decline over 30 oviposition days; it remained constant in large females, but increased among small and medium females, suggesting gradual compensation for larval food deprivation. An increase in egg size with maternal age may be an adaptive strategy to maximize fitness on ephemeral patches of aphid prey, assuming females reproduce in a single aphid outbreak and that offspring produced later in the aphid cycle experience greater competition and risk of mortality compared to those produced earlier. We demonstrate for the first time in Coleoptera that dynamic changes in both egg size and number occur as a function of female age and illustrate that such changes are constrained by larval feeding histories via their effects on maternal body size.
Life table studies of sunn pest were carried out in Varamin, Iran, from 1998–2001 in order to determine stage-specific mortalities and the impact of specific natural enemies on population dynamics. Populations were sampled 2–3 times weekly in agricultural fields during the growing season and monthly during the period of dormancy at resting sites in nearby mountains some 30 km away from cereal fields. Adults spend a period of 9–10 months in diapause and suffered overcompensatory, density-dependent mortality during this period. Variation in adult overwintering survival was inferred to be largely a function of the physiological condition of bugs that is reduced in a density-dependent manner by intraspecific competition for food among newly molted adults prior to migration to resting sites. Adult mortality emerged as the primary factor in key factor analysis, contributing 73% of the total variance in mortality. Other important factors were egg parasitism by Trissolcus vassilievi Mayr and adult parasitism by several species of Tachinidae. Although T. vassilievi made only a minor contribution to overall variance in total mortality, it had a significant effect on the number of newly molted adults, the life stage that is most damaging to cereal crops. The equilibrium level of the pest population in wheat fields was inferred to be ca. 72.6 adults m−2, a number that substantially exceeds the economic threshold that ranges from 3–5 adults m−2.
We review the interaction in intermediate and high mass stars between their evolution and magnetic and chemical properties. We describe the theory of Ap-star ‘fossil’ fields, before touching on the expected secular diffusive processes which give rise to evolution of the field. We then present recent results from a spectropolarimetric survey of Herbig Ae/Be stars, showing that magnetic fields of the kind seen on the main-sequence already exist during the pre-main sequence phase, in agreement with fossil field theory, and that the origin of the slow rotation of Ap/Bp stars also lies early in the pre-main sequence evolution; we also present results confirming a lack of stars with fields below a few hundred gauss. We then seek which macroscopic motions compete with atomic diffusion in determining the surface abundances of AmFm stars. While turbulent transport and mass loss, in competition with atomic diffusion, are both able to explain observed surface abundances, the interior abundance distribution is different enough to potentially lead to a test using asterosismology. Finally we review progress on the turbulence-driving and mixing processes in stellar radiative zones.
Although chemical separation is generally accepted as the main physical process responsible for the anomalous surface abundances of AmFm stars, its exact behavior within the interior of these stars is still uncertain. We will explore two hydrodynamical processes which could compete with atomic diffusion: mass loss and turbulence. We will also discuss the extent to which separation occurs immediately below the surface convection zone as well as the extent to which separation occurs below 200,000 K. To do so, self-consistent stellar models with mass loss and turbulence where calculated using the Montreal stellar evolution code and compared to observations of A and F stars. It is shown that to the precision of observations available for F stars, a mass loss rate of 2×10−14M⊙ · yr−1, is compatible with observations and that no turbulence is then required.