Surface Helium Research Articles

Solar Models with Revised Abundances and Opacities

Using reconstructed opacities, we construct solar models with low heavy-element abundance. Rotational mixing and enhanced diffusion of helium and heavy elements are used to reconcile the recently observed abundances with helioseismology. The sound speed and density of models where the relative and absolute diffusion coefficients for helium and heavy elements have been increased agree with seismically inferred values at better than the 0.005 and 0.02 fractional level respectively. However, the surface helium abundance of the enhanced diffusion model is too low. The low helium problem in the enhanced diffusion model can be solved to a great extent by rotational mixing. The surface helium and the convection zone depth of rotating model M04R3, which has a surface Z of 0.0154, agree with the seismic results at the levels of 1 $\sigma$ and 3 $\sigma$ respectively. M04R3 is almost as good as the standard model M98. Some discrepancies between the models constructed in accord with the new element abundances and seismic constraints can be solved individually, but it seems difficult to resolve them as a whole scenario.

Low abundances of heavy elements in the solar outer layers: comparisons of solar models with helioseismic inversions

Recent solar photospheric abundance analyses have led to a significant reduction of the metal abundances compared to the previous determinations. The solar models computed with standard opacities and diffusion processes using these new abundances give poor agreement with helioseismic inversions for the sound-speed profile, the surface helium abundance, and the convective zone depth. We attempt to obtain a good agreement between helioseismic inversions and solar models which present the "old" mixture in the interior and new chemical composition in the convective zone. To reach this result, we assume an undermetallic accretion at the beginning of the main sequence. We compute solar models with the Toulouse-Geneva Evolution Code, in which we simulate an undermetallic accretion in the early stages of the main sequence, in order to obtain new mixture in the outer convective zone. We compare the sound-speed profile, the convective zone depth, and the surface helium abundance with those deduced from helioseismology. The model with accretion but without any mixing process inside is in better agreement with helioseismology than the solar model with the new abundances throughout. There is, however, a spike under the convective zone which reaches 3.4%. Furthermore, the convective zone depth and the surface helium abundance are too low. Introducing undershooting below the convective zone allows us to recover the good convective zone radius and the addition of rotation-induced mixing and tachocline allows us to reconcile the surface helium abundance. But in any case the agreement of the sound-speed profile with helioseismic inference is worse than obtained with the old abundances.

The Solar Heavy‐Element Abundances. I. Constraints from Stellar Interiors

The latest solar atmosphere models including non-LTE corrections and three-dimensional hydrodynamic convection simulations predict a significant reduction in the solar metal abundance. This leads to a serious conflict between helioseismic data and the predictions of solar interiors models. We demonstrate that the helioseismic constraints on the surface convection zone depth and helium abundance combined with stellar interiors models can be used to constrain chemical composition. A detailed examination of the errors in the theoretical models disfavors strongly (disagreeing at the 15 σ level with the seismic data) the proposed new low abundance, while the models constructed with the older and higher solar abundances are consistent (within 2 σ). We then use the sensitivity of the seismic properties to abundance changes to invert the problem and infer a seismic solar heavy-element abundance mix with two components: meteoritic abundances and the light metals CNONe. Seismic degeneracies between the best solutions for the elements arise for changes in the relative CNONe abundances and their effects are quantified. We obtain Fe/H = 7.50 ± 0.045 ± 0.003(CNNe) and O/H = 8.86 ± 0.041 ± 0.025(CNNe) on the logarithmic scale, where H = 12 for the relative CNNe mixtures in the Grevese & Sauval mixture; the second error term reflects the uncertainty in the overall abundance scale from errors in the C, N, and Ne abundances relative to oxygen. These are consistent within the errors with the previous standard solar mixture but in strong conflict with the low oxygen abundance inferred from the three-dimensional hydro models. Changes in the Ne abundance can mimic changes in oxygen for the purposes of scalar constraints. However, models constructed with low oxygen and high neon are inconsistent with the solar sound speed profile. Implications for the solar abundance scale are discussed.

10,000 Standard Solar Models: A Monte Carlo Simulation

We have evolved 10,000 solar models using 21 input parameters that are randomly drawn for each model from separate probability distributions for every parameter. We use the results of these models to determine the theoretical uncertainties in the predicted surface helium abundance, the profile of the sound speed versus radius, the profile of the density versus radius, the depth of the solar convective zone, the eight principal solar neutrino fluxes, and the fractions of nuclear reactions that occur in the CNO cycle or in the three branches of the p-p chains. We also determine the correlation coefficients of the neutrino fluxes for use in analysis of solar neutrino oscillations. Our calculations include the most accurate available input parameters, including radiative opacity, equation of state, and nuclear cross sections. We incorporate both the recently determined heavy element abundances recommended by Asplund et al. and the older (higher) heavy element abundances recommended by Grevesse & Sauval. We present best estimates of many characteristics of the standard solar model for both sets of recommended heavy element compositions.

New Solar Opacities, Abundances, Helioseismology, and Neutrino Fluxes

We construct solar models with the newly calculated radiative opacities from the Opacity Project (OP) and with recently determined (lower) heavy-element abundances. We compare the results from the new models with the predictions of a series of models that use OPAL radiative opacities, older determinations of the surface heavy-element abundances, and refinements of nuclear reaction rates. For all the variations we consider, solar models that are constructed with the newer and lower heavy-element abundances advocated by Asplund et al. disagree by much more than the estimated measuring errors with the helioseismological determinations of the depth of the solar convective zone, the surface helium composition, the internal sound speeds, and the density profile. Using the new OP radiative opacities, the ratio of the 8B neutrino flux calculated with the older and larger heavy-element abundances (or with the newer and lower heavy-element abundances) to the total neutrino flux measured by the Sudbury Neutrino Observatory is 1.09 (0.87) with a 9% experimental uncertainty and a 16% theoretical uncertainty, 1 σ errors.

Helioseismological Implications of Recent Solar Abundance Determinations

We show that standard solar models are in good agreement with the helioseismologically determined sound speed and density as a function of solar radius, the depth of the convective zone, and the surface helium abundance, as long as those models do not incorporate the most recent heavy element abundance determinations. However, sophisticated new analyses of the solar atmosphere infer lower abundances of the lighter metals (like C, N, O, Ne, and Ar) than the previously widely used surface abundances. We show that solar models that include the lower heavy element abundances disagree with the solar profiles of sound speed and density as well as the depth of the convective zone and the helium abundance. The disagreements for models with the new abundances range from factors of several to many times the quoted uncertainties in the helioseismological measurements. The disagreements are at temperatures below what is required for solar interior fusion reactions and therefore do not significantly affect solar neutrino emission. If errors in thecalculated OPAL opacities are solely responsible for the disagreements, then the corrections in the opacity must extend from 2 times 10^6 K (R = 0.7R_Sun)to 5 times 10^6 K (R = 0.4 R_Sun), with opacity increases of order 10%.

Evolution of rotating binary stars

A model for the evolution of rotating binary stars is presented. The evolution of a binary system consisting of a 9 M-circle dot and a 6 M-circle dot star is studied in the mass transfer phase Case A. Specific attention is focused on the differences between the evolutions with and without the effects of rotation. The results indicate that the effects of rotation prolong the lifetime of the primary, and causes its evolutionary track in the HR diagram to shift towards lower luminosity. The mass and the surface helium composition of the primary become lower and the orbital period of the system becomes significantly longer during the later stage of the evolution.

Thermal desorption and surface modification of He + implanted into tungsten

Tungsten divertor plates in fusion reactors will be subject to helium bombardment. Helium retention and thermal desorption is a concerned issue in controlling helium ash. In the present study, fluence dependence of thermal desorption behavior of helium in tungsten was studied at different irradiation temperatures and ion energies. Results showed that helium desorption could start at ∼400 K with increasing fluence, while no noticeable peaks were detected at low fluence. Total helium desorption reached a saturation value at high fluence range, which was not sensitive to irradiation temperature or ion energy for the conditions evaluated. Surface modifications caused by either ion irradiation or thermal desorption were observed by SEM. The relationship of surface modifications and helium desorption behavior was discussed. Some special features of elevated irradiation temperature and lower ion energy were also indicated.

Carbothermally-assisted aerosol synthesis of semiconducting materials in the system GaN/Mn

Aerosol-assisted vapor phase synthesis aimed at preparation of magnetic semiconductor nanopowders of Ga 1− x Mn x N is carried out both from aqueous and methanol solution mixtures of gallium nitrate and manganese (II) nitrate. After an additional pyrolysis at 1000 °C under an ammonia flow, the light colored products are characterized with XRD, SEM/EDX, EPR, BET surface area determinations, helium pycnometry, and oxygen analyses. The characterization data for the powders are consistent with the formation of composites of nanosized manganese-doped hexagonal GaN and regular MnO phases.

Surface morphology and helium retention on tungsten exposed to low energy and high flux helium plasma

Surface modification and helium retention on powder metallurgy tungsten and plasma sprayed tungsten coated CFC exposed by low energy (100 eV) high flux (3.83×10 21–1.20×10 22 m −2 s −1) helium plasma have been investigated. Fluence is about 10 26 He m −2. It is found that fine surface morphology change occurs by the helium exposure. There is little difference of the surface modification between the powder metallurgy tungsten and the plasma sprayed tungsten. In the case that the surface morphology do not changed, two helium desorption peak appeared. The amount of desorption of helium is larger than that of deuterium from tungsten under the same exposure condition by heating up to 1273 K by TDS. In the case that fine surface morphology change occurs, desorption peak at high temperature newly appears. Surface modification such as blister is not formed on the surface exposed to helium under the same exposure condition which the blister is formed by deuterium exposure.

Stellar structure modeling using a parallel genetic algorithm for objective global optimization

Genetic algorithms are a class of heuristic search techniques that apply basic evolutionary operators in a computational setting. We have designed a fully parallel and distributed hardware/software implementation of the generalized optimization subroutine PIKAIA, which utilizes a genetic algorithm to provide an objective determination of the globally optimal parameters for a given model against an observational data set. We have used this modeling tool in the context of white dwarf asteroseismology, i.e., the art and science of extracting physical and structural information about these stars from observations of their oscillation frequencies. The efficient, parallel exploration of parameter-space made possible by genetic-algorithm-based numerical optimization led us to a number of interesting physical results: (1) resolution of a hitherto puzzling discrepancy between stellar evolution models and prior asteroseismic inferences of the surface helium layer mass for a DBV white dwarf; (2) precise determination of the central oxygen mass fraction in a white dwarf star; and (3) a preliminary estimate of the astrophysically important but experimentally uncertain rate for the 12 C( α, γ) 16 O nuclear reaction. These successes suggest that a broad class of computationally intensive modeling applications could also benefit from this approach.

Hydrogen and helium entrapment in flowing liquid metal plasma-facing surfaces

The ability to use liquids as plasma-facing component (PFCs) depends on their interaction with the plasma and the magnetic field. One important issue for the moving liquid is the ability to entrain particles that strike the PFC surface (helium and hydrogen isotopes) while accommodating high heat loads. To study this problem, a numerical model has been developed using the HEIGHTS computer simulation package. The model was used to investigate pumping of He particles by the flowing liquid rather than requiring a standard vacuum system. Hydrogen isotope (DT) particles are likely be trapped in the liquid metal surface (e.g., lithium) due to the high chemical solubility of hydrogen. The incident He particles in the established low-recycling regime at PFCs could be harder to pump using standard vacuum pumping techniques. The analysis results indicate, however, a reasonable chance of adequate helium self-trapping in flowing lithium as PFC without active pumping.

Orbital periods of 22 subdwarf B stars

Subdwarf B (sdB) stars are thought to be core helium burning stars with low mass hydrogen envelopes. In recent years it has become clear that many sdB stars lose their hydrogen through interaction with a binary companion and continue to reside in binary systems today. In this paper we present the results of a programme to measure orbital parameters of binary sdB stars. We determine the orbits of 22 binary sdB stars from 424 radial velocity measurements, raising the sample of sdBs with known orbital parameters to 38. We calculate lower limits for the masses of the companions of the sdB stars which, when combined with the orbital periods of the systems, allow us to discuss approximate evolutionary constraints. We find that a formation path for sdB stars consisting of mass transfer at the tip of the red giant branch (RGB) followed by a common envelope phase explains most, but not all of the observed systems. It is particularly difficult to explain both long period systems and short period, massive systems. We present new measurements of the effective temperature, surface density and surface helium abundance for some of the sdB stars by fitting their blue spectra. We find that two of them (PG 0839 + 399 and KPD 1946 + 4340) do not lie in the extreme horizontal branch (EHB) band, indicating that they are post-EHB stars.

Transport in porous graphite: gas permeation and ion diffusion experiments

Permeabilities k and formation factors F of a number of monoliths made of compressed expanded graphite (CEG) having different porosities are measured from gas permeation and ion diffusion experiments, respectively. The results are analyzed and discussed in the framework of a model, according to which the pores are cylinders of lengths l and diameters δ such that l= δ. Formation factors are calculated using this model and from simple assumptions concerning the packing of expanded graphite particles. For each porosity of the monoliths, a characteristic pore size Λ is derived from the latter hypotheses. All these results are shown to be accurate basing on three statements. First, the expected and well-known relationship k∼ F −2 is recovered. Secondly, the calculated values are in good agreement with the experimental ones. Finally, the characteristic pore sizes Λ are obtained from both surface area and helium pycnometry measurements, and shown to support the former calculations of Λ.

Modeling hydrogen and helium entrapment in flowing liquid metal surfaces as plasma-facing components in fusion devices

In a fusion reactor, the ability to use liquids as plasma-facing components (PFCs) depends on their interaction with the plasma and the magnetic field. One important issue for the moving liquid is the ability to entrain particles that strike the PFC surface (helium and hydrogen isotopes) while accommodating high heat loads. To study this problem, an analytical model and a two-dimensional comprehensive numerical model have been developed and implemented in the HEIGHTS computer simulation package. The models take into account the kinetics of particle injection, motion and interactions with the liquid lattice, and the ultimate release from the surface. The models were used to investigate an important issue, whether He particles can be pumped by the PFC liquid rather than requiring a standard vacuum system. Hydrogen isotope (DT) particles that strike the surface will likely be trapped in the liquid–metal surface (e.g., lithium) due to the high chemical solubility of hydrogen. The impinging He particles in the established low-recycling regime at PFCs could be harder to pump using the standard vacuum pumping techniques. The analysis results indicate a reasonable chance of adequate helium self-trapping in flowing lithium as PFC without active pumping.

Non-Linear Radial Pulsation Models for the Early-Type Helium Stars V652 Her and BX Cir

AbstractHydrogen-deficient early-type stars have an extremely high surface helium abundance (> 98% by number) and have represented a considerable challenge to stellar evolution theory. Recent work supports the view that they are the product of a merger between two white dwarfs.All very luminous stars (log L/M > 4) are believed to pulsate, but at effective temperatures around 20 000 K, helium stars with lower luminosities (log-L/M < 3) are also known to pulsate due to iron-group bump instability. The two known cases are V652 Her and BX Cir. Their observed periods are in good agreement with linear theory. Recently high-resolution Spectroscopic observations were used to measure the stellar dimensions and radial velocity curves with high precision.A hydrodynamic code including recent OPAL opacity data has been used to construct non-linear models of the pulsations of these two stars. The results impose additional constraints on those stellar dimensions, including mass, which remain poorly determined by observation.

Rotational mixing in early-type main-sequence stars

We present quantitative observational investigations into the importance of rotationally induced mixing in late-O stars. First, we conduct non-LTE, hydrostatic, plane-parallel H/He model-atmosphere analyses of the optical spectra of three of the most rapidly rotating late-O near-main-sequence stars known: HD 93521 (O9.5 V), HD 149757 (ζ Ophiuchi; O9.5 V), and HD 191423 (ON9 III: n), all of which have equatorial rotation velocities of ∼ 430 km s−1 and . The analysis allows for the expected (von Zeipel) variation of Teff and log g with latitude. These three stars are found to share very similar characteristics, including substantially enhanced surface-helium abundances . Secondly, we compare the distribution of projected rotational velocities for ON and morphologically normal dwarf O stars, and demonstrate that the ON stars are drawn from a population with more rapid rotation. The results provide qualitative support for rotationally induced mixing, although there remain discrepancies between atmospheric and evolutionary models (which we show employ inappropriate mass-loss rates for late-O main-sequence stars). We show that the most rapid rotator known, HD 191423, is an ON star, and note the implied disparity between O/ON morphology and surface helium abundance; we discuss consequences for the interpretation of spectral morphology in O-type main-sequence stars. We demonstrate a new, purely spectroscopic, method of distance determination for rapid rotators, and thereby confirm that HD 93521 lies at ∼ 2 kpc, and is not, as previously suggested, a low-mass Population II star. Finally, our models contradict earlier claims of strongly differential surface rotation, and are consistent with uniform angular velocity at the surface.

Computational Asteroseismology11The full text of the dissertation is available on the World Wide Web at http://whitedwarf.org/metcalfe.

White dwarf asteroseismology offers the opportunity to probe the structure and composition of stellar objects governed by relatively simple physical principles. The observational requirements of asteroseismology have been addressed by the development of the Whole Earth Telescope, but the analysis procedures still need to be refined before this technique can yield the complete physical insight that the data can provide. We have applied an optimization method utilizing a genetic algorithm to the problem of fitting white dwarf pulsation models to the observed frequencies of the most thoroughly characterized helium-atmosphere pulsator, GD 358. The free parameters in this initial study included the stellar mass, the effective temperature, the surface helium layer mass, the core composition, and the internal chemical profile. For many years, astronomers have promised that the study of pulsating white dwarfs would ultimately lead to useful information about the physics of matter under extreme conditions of temperature and pressure. The optimization approach developed in this dissertation has allowed us to finally make good on that promise by exploiting the sensitivity of our models to the core composition. We empirically determine that the central oxygen abundance in GD 358 is 84 +/- 3 percent. We use this value to place a preliminary constraint on the C12(alpha,gamma)O16 nuclear reaction cross-section of S_300 = 295 +/- 15 keV barns. We find a thick helium layer solution for GD 358 that provides a better match to the data than previous fits, and helps to resolve a problem with the evolutionary connection between PG 1159 stars and DBVs. [abridged]

Structure and electronic properties of CH 3- and CF 3-terminated alkanethiol monolayers on Au( [formula omitted]): a scanning tunneling microscopy, surface X-ray and helium scattering study

The structure and the electronic properties of a series of CH 3- and CF 3-terminated alkanethiol monolayers on Au(1 1 1) have been studied by scanning tunneling microscopy (STM) and surface X-ray and helium scattering. At full coverage, the CH 3-terminated monolayers form long-range ordered domains of a (√3×√3) R30° and a (2√3×3) R30° standing-up phase. By thermal desorption, distinct lying-down phases of intermediate density as well as the ( p×√3) lying-down phase were generated. In contrast, the CF 3-terminated monolayers at full coverage form a standing-up phase of hexagonal symmetry that exhibits no long-range order at room temperature. Even after annealing, the domain sizes are smaller by more than one order of magnitude compared to the CH 3-terminated thiol monolayers. A comparison of the low-density lying-down phases suggests no measurable influence of the CF 3-group on the ordering. The current–voltage dependence ( I– V-curves) measured by scanning tunneling spectroscopy (STS) shows no voltage gap for CH 3-terminated decanethiols. In contrast, in the I– V-curves for CF 3-terminated decanethiol monolayers, an asymmetric voltage gap of about 2 V can be clearly observed. The latter results are discussed in terms of a microscopic model that includes the formation of an interfacial Coulomb barrier at the CF 3/vacuum boundary. In addition, the effects of the tunneling conditions on the STM image contrast were examined. These studies demonstrate that the nature of the STM images and thus, the respective apparent lateral order of the films, strongly depends on the choice of the tunneling parameters.

The binary fraction of extreme horizontal branch stars

We have used precise radial velocity measurements of subdwarf-B stars from the Palomar–Green catalogue to look for binary extreme horizontal branch (EHB) stars. We have determined the effective temperature, surface gravity and surface helium abundance for 20 of the targets from new or existing blue spectra and have compiled published values for these quantities for all but one other. We identify 36 EHB stars in our sample and find that at least 21 of these stars are binaries. All but one or two of these are new identifications. The minimum binary fraction for EHB stars implied by our survey is 60±8 per cent. Our survey is sensitive to binaries with orbital periods P≲10 d. For reasonable assumptions concerning the period distribution and the mass ratio distribution of the binaries, we find that the mean detection efficiency of our survey over this range of orbital periods is 87 per cent. Allowing for this estimated detection efficiency, the fraction of EHB stars that are short-period binaries (0.03 d≲P≲10 d) is 69±9 per cent. The value is not strongly dependent on the period distribution below P≈10 d or the mean companion mass for these short-period binaries. The orbital separation of the stars in these binaries is much less than the size of the red giant from which the EHB star has formed. This is strong evidence that binary star evolution is fundamental to the formation of the majority of EHB stars. If there are also binary EHB stars, the orbital periods of which are ≳10 d, the fraction of EHB stars for which evolution was affected by the presence of a companion may be much higher, e.g. if one third of EHB stars are binaries with orbital periods 10 d≲P≲100 d, then our observations are consistent with all EHB stars being formed through some type of binary star evolution. We find that five of the other stars we observed are likely to be post-EHB stars, one of which is also a binary.