Envelope Helium Abundance Research Articles

The internal rotation of the Sun and its link to the solar Li and He surface abundances

The Sun serves as a natural reference for the modelling of the various physical processes at work in stellar interiors. Helioseismology results, which inform us on the characterization of the interior of the Sun (such as, for example, the helium abundance in its envelope), are, however, at odds with heavy element abundances. Moreover, the solar internal rotation and surface abundance of lithium have always been challenging to explain. We present results of solar models that account for transport of angular momentum and chemicals by both hydrodynamic and magnetic instabilities. We show that these transport processes reconcile the internal rotation of the Sun, its surface lithium abundance, and the helioseismic determination of the envelope helium abundance. We also show that the efficiency of the transport of chemicals required to account for the solar surface lithium abundance also predicts the correct value of helium, independently from a specific transport process.

Helium abundance in a sample of cool stars: measurements from asteroseismology

The structural stratification of a solar-type main sequence star primarily depends on its mass and chemical composition. The surface heavy element abundances of the solar-type stars are reasonably well determined using conventional spectroscopy, however the second most abundant element helium is not. This is due to the fact that the envelope temperature of such stars is not high enough to excite helium. Since the helium abundance of a star affects its structure and subsequent evolution, the uncertainty in the helium abundance of a star makes estimates of its global properties (mass, radius, age etc.) uncertain as well. The detections of the signatures of the acoustic glitches from the precisely measured stellar oscillation frequencies provide an indirect way to estimate the envelope helium content. We use the glitch signature caused by the ionization of helium to determine the envelope helium abundance of 38 stars in the {\it Kepler} seismic LEGACY sample. Our results confirm that atomic diffusion does indeed take place in solar-type stars. We use the measured surface abundances in combination with the settling predicted by the stellar models to determine the initial abundances. The initial helium and metal mass fractions have subsequently been used to get the preliminary estimates of the primordial helium abundance, $Y_p = 0.244\pm0.019$, and the galactic enrichment ratio, $\Delta Y / \Delta Z = 1.226\pm0.849$. Although the current estimates have large errorbars due to the limited sample size, this method holds great promises to determine these parameters precisely in the era of upcoming space missions.

Asteroseismic estimate of helium abundance of 16 Cyg A, B

The helium ionization zone in a star leaves a characteristic signature on its oscillation frequencies, which can be used to estimate the helium content in the envelope of the star. We use the oscillation frequencies of 16 Cyg A and B, obtained using 2.5 years of Kepler data, to estimate the envelope helium abundance of these stars. We find the envelope helium abundance to lie in the range 0.231–0.251 for 16 Cyg A and 0.218–0.266 for 16 Cyg B.

Prospects for asteroseismic inference on the envelope helium abundance in red giant stars

Regions of rapid variation in the internal structure of a star are often referred to as acoustic glitches since they create a characteristic periodic signature in the frequencies of p modes. Here we examine the localized disturbance arising from the helium second ionization zone in red giant branch and clump stars. More specifically, we determine how accurately and precisely the parameters of the ionization zone can be obtained from the oscillation frequencies of stellar models. We use models produced by three different generation codes that not only cover a wide range of stages of evolution along the red giant phase but also incorporate different initial helium abundances. We discuss the conditions under which such fits robustly and accurately determine the acoustic radius of the second ionization zone of helium. The determined radii of the ionization zones as inferred from the mode frequencies were found to be coincident with the local maximum in the first adiabatic exponent described by the models, which is associated with the outer edge of the second ionization zone of helium. Finally, we consider whether this method can be used to distinguish stars with different helium abundances. Although a definite trend in the amplitude of the signal is observed any distinction would be difficult unless the stars come from populations with vastly different helium abundances or the uncertainties associated with the fitted parameters can be reduced. However, application of our methodology could be useful for distinguishing between different populations of red giant stars in globular clusters, where distinct populations with very different helium abundances have been observed.

The Detectability of Signatures of Rapid Variation in Low‐Degree Stellarp‐Mode Oscillation Frequencies

Regions of rapid variation in stellar interiors introduce characteristic periodic signatures into the frequencies of their p-mode oscillations. These signatures have previously been isolated in solar data and have been used to estimate the level of convective overshoot and the envelope helium abundance in the Sun. Precise asteroseismic data are now becoming available for a number of distant Sun-like stars, and the techniques developed for low-degree solar data can be used in the stellar case. In this paper, we address the observational duration required to isolate and examine these rapid-variation signatures through the use of low-degree solar data as a Sun-like stellar analog. We also outline the difficulties involved and bias introduced when using a limited number of poorly constrained mode frequencies. The signature resulting from the region of the second ionization of helium is found to be detectable in spectra of shorter duration than those required to isolate the smaller amplitude signature arising from the base of the convective envelope. Observations of just 84 days may be sufficient to extract reliable information on the acoustic depth and extent of the He II ionization zone, although longer observations are required to obtain sufficient precision to estimate the envelope helium abundance. The acoustic radius of the base of the convective envelope and the amplitude of the signature are reliably isolated in spectra of at least 182 days.

Seismic analysis of the second ionization region of helium in the Sun -- I. Sensitivity study and methodology

The region of the second ionization of helium in the Sun is a narrow layer near the surface. Ionization induces a local change in the adiabatic exponent Γ1, which produces a characteristic signature in the frequencies of p modes. By adapting the method developed by Monteiro, Christensen-Dalsgaard & Thompson, we propose a methodology for determining the properties of this region by studying such a signature in the frequencies of oscillation. Using solar data we illustrate how the signal from the helium ionization zone can be isolated. Using solar models which each use different physics — the theory of convection, equation of state and low-temperature opacities — we establish how the characteristics of the signal depend on the various physical processes contributing to the structure in the ionization layer. We further discuss how the method can be used to measure the solar helium abundance in the envelope and to constrain the physics affecting this region of the Sun. The potential usefulness of the method we propose is shown. It may complement other inversion methods developed to study the solar structure and to determine the envelope helium abundance.

Hot HB stars in globular clusters – Physical parameters and consequences for theory

We present the results of spectroscopic analyses of hot horizontal branch (HB) stars in M 13 and M 3, which form a famous “second parameter” pair. From the spectra and Strömgren photometry we derived – for the first time in M 13 – atmospheric parameters (effective temperature and surface gravity). For stars with Strömgren temperatures between 10 000 and 12 000 K we found excellent agreement between the atmospheric parameters derived from Strömgren photometry and those derived from Balmer line profile fits. However, for cooler stars there is a disagreement in the parameters derived by the two methods, for which we have no satisfactory explanation. Stars hotter than 12 000 K show evidence for helium depletion and iron enrichment, both in M 3 and M 13. Accounting for the iron enrichment substantially improves the agreement with canonical evolutionary models, although the derived gravities and masses are still somewhat too low. This remaining discrepancy may be an indication that scaled-solar metal-rich model atmospheres do not adequately represent the highly non-solar abundance ratios found in blue HB stars affected by diffusion. We discuss the effects of an enhancement in the envelope helium abundance on the atmospheric parameters of the blue HB stars, as might be caused by deep mixing on the red giant branch or primordial pollution from an earlier generation of intermediate mass asymptotic giant branch stars.

Inferences on the solar envelope with high-degree modes

We investigate the structure of the Sun by helioseismic inversion of a set of p-mode frequencies which includes new precise observations of modes with high degree ( l< 1000) obtained from the MDI instrument on the SOHO satellite (Rhodes et al. 1998). Such data have the potential to improve the resolution of the solar structure in the near-surface region, to provide detailed tests of the equation of state and constrain the envelope helium abundance. In order to suppress the uncertainties in the treatment of the surface layers in helioseismic inversion procedures, we introduce here the use of a new surface term, developed on the basis of higher-order asymptotic theory of acoustic modes and suitable for the handling of high-degree mode frequencies.

Mixing, enhanced helium and blue tails in globular clusters

We investigate the consequences of an increase in the envelope helium abundance of pre-helium flash red giants in globular clusters, an occurrence suggested by chemical peculiarities in many red giant atmospheres. Comparing predictions with the CM diagrams of a few crucial globular clusters, one finds no evidence for a substantial increase in the surface helium content of horizontal branch members of these clusters, at least for objects in the RR Lyrae region or close to it. The possibility that the most peculiar giants belong to the asymptotic giant branch is discussed. The consequences of a delay in the helium flash are briefly examined.

New Inferences on the Sun from High-degree Modes: The External Layers and the Equation of State

We investigate the structure of the Sun by helioseismic inversion of a set of p-mode frequencies which include new precise observations of modes with high degree obtained from the MDI instrument on the SOHO satellite. Such data have the potential to improve the resolution of the solar structure in the near-surface region, to test the equation of state and constrain the envelope helium abundance.

The potential of solar high-degree modes for structure inversion

It is likely that precise and reliable frequencies of high-degree modes will soon be available from the SOI/MDI experiment. Here we examine the ability of such modes (with l>300) to resolve the solar structure in the near-surface region. In particular, we investigate inversions to determine the adiabatic exponent Γ1 as a test of the solar equation of state, as well as the potential of such data to constrain the solar envelope helium abundance.

The Second-Parameter Effect in Metal-rich Globular Clusters

Recent Hubble Space Telescope observations have found that the horizontal branches (HBs) in the metal-rich globular clusters NGC 6388 and NGC 6441 slope upward with decreasing B-V. Such a slope is not predicted by canonical HB models and cannot be produced by either a greater cluster age or enhanced mass loss along the red giant branch (RGB). The peculiar HB morphology in these clusters may provide an important clue for understanding the second-parameter effect. We have carried out extensive evolutionary calculations and numerical simulations in order to explore three noncanonical scenarios for explaining the sloped HBs in NGC 6388 and NGC 6441: (1) a high cluster helium abundance scenario, in which the HB evolution is characterized by long blue loops; (2) a rotation scenario, in which internal rotation during the RGB phase increases the HB core mass; and (3) a helium-mixing scenario, in which deep mixing on the RGB enhances the envelope helium abundance. All three of these scenarios predict sloped HBs with anomalously bright RR Lyrae variables. We compare this prediction with the properties of the two known RR Lyrae variables in NGC 6388. Additional observational tests of these scenarios are suggested.

THE 'STANDARD' SUN : MODELLING AND HELIOSEISMOLOGY

The ‘standard’ solar model is based on a number of simplifying assumptions and depends on knowledge of the physical properties of matter in the Sun. Given these assumptions, the constraint that the model have the observed surface luminosity provides an estimate of the initial solar helium abundance. From helioseismic analyses further information can be obtained about the present composition, including a fairly precise measure of the envelope helium abundance and an estimate of the hydrogen profile in the radiative interior. It must be emphasized, however, that these inferences may suffer from systematic error arising from incomplete knowledge about the equation of state and opacity of the solar interior.

Effects of Helium Mixing on the Evolution of Globular Cluster Stars

New noncanonical sequences for the evolution of globular cluster stars have been computed in order to investigate the effects of mixing helium from the hydrogen shell into the envelope during the red giant branch (RGB) phase. The possible occurrence of such mixing, driven by internal rotation, is suggested by the observed abundance variations involving C, N, O, Na, and Al in globular cluster red giants. We find that helium mixing can substantially increase the envelope helium abundance. By increasing the RGB tip luminosity, helium mixing also leads to enhanced mass loss along the RGB. Both of these effects have a potentially large impact on the subsequent horizontal-branch (HB) evolution. In particular, helium mixing produces a bluer HB morphology, thereby making it easier to explain the hot HB population found in various stellar systems, as well as the difference in HB morphology between some second parameter pairs of globular clusters. Helium-mixed sequences show a larger RR Lyrae period shift and predict a smaller age for the metal-poor globular clusters. In addition, such sequences can reproduce the low gravities observed in blue HB stars.

The Helium-Core Mass at the Helium Flash in Low-Mass Red Giant Stars: Observations and Theory

The method developed by Raffelt (1990a,b,c) to estimate a possible increase in the standard values of the helium-core mass at the tip of the red giant branch, $\Mc$, from properties of the color-magnitude diagrams of Galactic globular clusters is employed. In the present study, we revise and update Raffelt's database, including also constraints from RR Lyrae pulsation, and find that a small increase, by $\Delta\Mc \approx 0.01\pm 0.015 \Msun$, cannot be ruled out with the present data and evolutionary models. Our new upper limits on $\Delta\Mc$ are less restrictive than those previously obtained by Raffelt, as are the corresponding constraints on novel astroparticle phenomena which may affect the evolution of low-mass red giants. Within the estimated uncertainties, however, the standard values of $\Mc$ may also be acceptable. Raffelt's method does not rule out a low envelope helium abundance in globular cluster giants, though again the standard values are compatible with the available constraints. The influence of a non-solar ratio for the $\alpha$-capture elements upon these results is also investigated. In addition, we review several aspects of the input physics employed in red giant stellar evolutionary calculations, with the purpose of evaluating possible sources of uncertainty in the value of the helium-core mass at the helium flash that is obtained from evolutionary computations, such as: heat conduction by electrons in the degenerate core: Coulomb effects upon the Equation of State: triple-$\alpha$ reaction rates and screening factors: neutrino emission rates, both standard and enhanced by a possible non-zero magnetic moment: stellar rotation: microscopic element diffusion: and energy losses by axions and Weakly Interacting Massive Particles.

The structure of horizontal-branch models. I - The zero-age horizontal branch

view Abstract Citations (40) References (45) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Structure of Horizontal-Branch Models. I. The Zero-Age Horizontal Branch Dorman, Ben Abstract A detailed study of the structure of zero-age horizontal-branch (ZAHB) models is studied in order to show how the hydrostatic structure of these models changes with the input parameters and determines the H-R diagram location of a given model. The properties of composite polytropes on the homology-invariant (U,V)-plane are demonstrated. A variety of test models and sequences were constructed to elucidate the underlying factors that give rise to the wide variation in HB model properties with composition. The roles of the CNO elements as nuclear catalysts and of the envelope sources, as well as the envelope helium abundance are reexamined. It is found that, for stars of a fixed range of mass arriving on the HB, the stellar distribution is determined mainly by CNO for low metallicities (Fe/H of less than about -1), but mainly by opacity sources for high metallicities. The value of Fe/H where CNO ceases to dominate depends significantly on the adopted opacity and will decrease if and when opacity estimates are revised upward. Publication: The Astrophysical Journal Supplement Series Pub Date: June 1992 DOI: 10.1086/191678 Bibcode: 1992ApJS...80..701D Keywords: Hertzsprung-Russell Diagram; Horizontal Branch Stars; Stellar Models; Stellar Structure; Globular Clusters; Polytropic Processes; Stellar Evolution; Stellar Interiors; Stellar Mass Ejection; Astrophysics; STARS: ABUNDANCES; STARS: EVOLUTION; STARS: HORIZONTAL-BRANCH; STARS: INTERIORS; STARS: VARIABLES: OTHER MISCELLANEOUS full text sources ADS |

Application of oxygen-enhanced evolutionary sequences to the horizontal branch of M15

Results are presented on an investigation of the horizontal branch (HB) of M15, using theoretical sequences calculated on the assumption of various values in the envelope helium and oxygen abundances. Comparisons of the theoretical horizontal-branch sequences, for a range in envelope helium abundance and O/Fe, to data available for M15, showed that the calculations can reproduce (at least in part) the observed morphology. A major difference was found between the luminosity-to-mass ratios determined by the present calculations and the values found by Bingham et al. (1984) and Sandage (1990), due to adoption of different temperature-color relations, as well as differing reddening assumptions.

The Evolutionary Status of Population II Cepheids

Schwarzschild and Härm (1970, hereafter abbreviated SH) have suggested that the presence of Cepheids in globular clusters can be explained by the occurrence of ‘flashes’ in the thermally unstable helium-burning shells of post-horizontal-branch, or ‘asymptotic-branch’ stars. These flashes in the deep interior of the star produce large changes in the energy flux at the base of the envelope, which in turn cause the position of the model in the Mbol — logTe diagram to move. For most values of the total stellar mass and envelope composition studied by SH, the models simply moved up and down the asymptotic branch in response to a helium shell flash. However in a few cases, characterized by low total mass and a relatively high value of the envelope helium abundance, the models left the asymptotic branch and described loops extending into the region of pulsational instability. The Cepheids of Population II were thus tentatively identified by SH as asymptotic-branch stars that had recently undergone a helium shell flash and whose tracks were as a result describing a loop extending into the instability strip.