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Register a new userTime-dependent atomic diffusion in the atmospheres of CP stars. A big step forward: introducing numerical models including a stellar mass loss
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Calculating abundance stratifications in ApBp/HgMn star atmospheres, we are considering mass-loss in addition to atomic diffusion in our numerical code in order to achieve more realistic models. These numerical simulations with mass-loss solve the time dependent continuity equation for plane-parallel atmospheres; the procedure is iterated until stationary concentrations of the diffusing elements are obtained throughout a large part of the stellar atmosphere. We find that Mg stratifications in HgMn star atmospheres are particularly sensitive to the presence of a mass-loss. For main-sequence stars with $T_{rm{eff}}approx 12000$ K, the observed systematic mild underabundances of this element can be explained only if a mass-loss rate of around $4.2,10^{-14}$ solar mass per year is assumed in our models. Numerical simulations also reveal that the abundance stratification of P observed in the HgMn star HD53929 may be understood if a weak horizontal magnetic field of about 75G is present in this star. However, for a better comparison of our results with observations, it will be necessary to carry out 3D modelling, especially when magnetic fields and stellar winds -- which render the atmosphere anisotropic -- are considered together.
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A series of recent theoretical atomic diffusion studies has address the challenging problem of predicting inhomogeneous vertical and horizontal chemical element distributions in the atmospheres of magnetic ApBp stars. Here we critically assess the most sophisticated of such diffusion models - based on a time-dependent treatment of the atomic diffusion in a magnetised stellar atmosphere - by direct comparison with observations as well by testing the widely used surface mapping tools with the spectral line profiles predicted by this theory. We show that the mean abundances of Fe and Cr are grossly underestimated by the time-dependent theoretical diffusion model, with discrepancies reaching a factor of 1000 for Cr. We also demonstrate that Doppler imaging inversion codes, based either on modelling of individual metal lines or line-averaged profiles simulated according to theoretical three-dimensional abundance distribution, are able to reconstruct correct horizontal chemical spot maps despite ignoring the vertical abundance variation. These numerical experiments justify a direct comparison of the empirical two-dimensional Doppler maps with theoretical diffusion calculations. This comparison is generally unfavourable for the current diffusion theory, as very few chemical elements are observed to form overabundance rings in the horizontal field regions as predicted by the theory and there are numerous examples of element accumulations in the vicinity of radial field zones, which cannot be explained by diffusion calculations.
Subdwarf B stars show chemical peculiarities that cannot be explained by diffusion theory alone. Both mass loss and turbulence have been invoked to slow down atomic diffusion in order to match observed abundances. The fact that some sdB stars show pulsations gives upper limits on the amount of mass loss and turbulent mixing allowed. Consequently, non-adiabatic asteroseismology has the potential to decide which process is responsible for the abundance anomalies. We compute for the first time seismic properties of sdB models with atomic diffusion included consistently during the stellar evolution. The diffusion equations with radiative forces are solved for H, He, C, N, O, Ne, Mg, Fe and Ni. We examine the effects of various mass-loss rates and mixed surface masses on the abundances and mode stability. It is shown that the mass-loss rates needed to simulate the observed He abundances (10^{-14}<=Mdot [Msun/yr]<=10^{-13}) are not consistent with observed pulsations. We find that for pulsations to be driven the rates should be Mdot<=10^{-15} Msun/yr. On the other hand, weak turbulent mixing of the outer 10^{-6} Msun can explain the He abundance anomalies while still allowing pulsations to be driven. The origin of the turbulence remains unknown but the presence of pulsations gives tight constraints on the underlying turbulence model.
The evolution of helium stars with initial masses in the range 1.6 to 120 Msun is studied, including the effects of mass loss by winds. These stars are assumed to form in binary systems when their expanding hydrogenic envelopes are promptly lost just after helium ignition. Significant differences are found with single star evolution, chiefly because the helium core loses mass during helium burning rather than gaining it from hydrogen shell burning. Consequently presupernova stars for a given initial mass function have considerably smaller mass when they die and will be easier to explode. Even accounting for this difference, the helium stars with mass loss develop more centrally condensed cores that should explode more easily than their single-star counterparts. The production of low mass black holes may be diminished. Helium stars with initial masses below 3.2 Msun experience significant radius expansion after helium depletion, reaching blue supergiant proportions. This could trigger additional mass exchange or affect the light curve of the supernova. The most common black hole masses produced in binaries is estimated to be about 9 Msun. A new maximum mass for black holes derived from pulsational pair-instability supernovae is derived - 46 Msun, and a new potential gap at 10 - 12 Msun is noted. Models pertinent to SN 2014ft are presented and a library of presupernova models is generated.
When modelling stars with masses larger than 1.2Msun with no observed chemical peculiarity, atomic diffusion is often neglected because, on its own, it causes unrealistic surface abundances compared with those observed. The reality is that atomic diffusion is in competition with other transport processes. The purpose of this study is to quantify the opposite or conjugated effects of atomic diffusion and rotationally induced mixing in stellar models of low mass stars. Our second goal is to estimate the impact of neglecting both rotational mixing and atomic diffusion in stellar parameter inferences for stars with masses larger than 1.3Msun. Using the AIMS code, we infer the masses and ages of a set of representative artificial stars for which models were computed with the CESTAM evolution code, taking into account rotationally induced mixing and atomic diffusion, including radiative accelerations. We show that for masses lower than 1.3Msun, rotation dominates the transport of chemical elements, and strongly reduces the effect of atomic diffusion, with net surface abundance modifications similar to solar ones. At larger mass, atomic diffusion and rotation are competing equally. Above 1.44Msun, atomic diffusion dominates in stellar models with initial rotation smaller than 80km.s-1 producing a chemical peculiarity which is not observed in Kepler-legacy stars. This indicates that a transport process of chemical elements is missing. Importantly, neglecting rotation and atomic diffusion (including radiative accelerations) in the models, when inferring the parameters of F-type stars, may lead to errors of 5%, 2.5% and 25% respectively for stellar masses, radii and ages. Atomic diffusion (including radiative accelerations) and rotational mixing should be taken into account in stellar models in order to determine accurate stellar parameters.
A large number of massive stars are known to rotate, resulting in significant distortion and variation in surface temperature from the pole to the equator. Radiatively driven mass loss is temperature dependent, so rapid rotation produces variation in mass loss and angular momentum loss rates across the surface of the star, which is expected to affect the evolution of rapidly rotating massive stars. In this work, we investigate the two dimensional effects of rotation on radiatively driven mass loss and the associated loss of angular momentum in ZAMS models with solar metallicity. Using 2D stellar models, which give the variation in surface parameters as a function of co-latitude, we implement two different mass loss prescriptions describing radiatively driven mass loss. We find a significant variation in mass loss rates and angular momentum loss as a function of co-latitude. We find that the mass loss rate decreases as the rotation rate increases for models at constant initial mass, and derive scaling relations based on these models. When comparing 2D to 1D mass loss rates, we find that although the total angle integrated mass loss does not differ significantly, the 2D models predict less mass loss from the equator and more mass loss from the pole than the 1D predictions using von Zeipels law. As a result, rotating models lose less angular momentum in 2D than in 1D, which will change the subsequent evolution of the star. The evolution of these models will be investigated in future work.
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