Abundance Anomalies Research Articles

The Contribution of Population III to the Enrichment and Preheating of the Intracluster Medium

Intracluster medium (ICM) abundances are higher than expected assuming enrichment by supernovae with progenitors belonging to the simple stellar population (SSP) observed in cluster galaxies if stars formed with a standard initial mass function (IMF). Moreover, new results on ICM oxygen abundances imply that nucleosynthesis occurred with nonstandard yields. The hypothesis that hypernovae in general, and hypernovae associated with Population III (Pop III) stars in particular, may significantly contribute to ICM enrichment is presented and evaluated. The observed abundance anomalies can be explained by a hypernova-producing subpopulation of the SSP, but only if the subpopulation accounts for half of all supernova explosions and if Type Ia supernova rates are very low. Also, the implied energy release may be excessive. However, an independent Pop III contribution—in the form of metal-free, very massive stars that evolve into hypernovae—can also account for all of the observed abundances while avoiding these drawbacks and accommodating a normal IMF in subsequent stellar generations. The required number of Pop III stars provides sufficient energy injection (at high redshift) to explain the ICM "entropy floor." Pop III hypernovae preenrich the intergalactic medium and can produce a significant fraction of the metals observed in the Lyα forest. Several testable predictions for ICM and intergalactic medium observations are made.

The influence of diffusion and mass loss on the chemical composition of subdwarf B stars

We investigate the influence of diusion and mass loss on the chemical composition in subdwarf B stars in the range 25 000 K Te 35 000 K, 5:5 logg 6:0. Within the outer hydrogen-rich envelope characterized by mass <10 2 M for the elements H, He, C, N and O the equations of continuity, the momentum equations and the equation of radiative transfer are solved simultaneously. For various mass loss rates the time evolution of the chemical composition is predicted within time scales of10 8 yr, which correspond to the typical lifetimes of the sdB's near the Extended Horizontal Branch. According to the results weak winds with _ M 10 13 M=yr may explain the typical helium deciencies by more than one order of magnitude in the atmospheres of these stars. Winds with 10 14 M=yr < _ M< 10 13 M=yr may lead to strong deciencies as well as to enrichments of the CNO elements. The composition in the outer envelope changes in time scales similar to the typical lifetimes of the sdB's. From estimates of the radiative acceleration in the wind region, which make the existence of weak winds plausible, and from a comparison with the case of hot white dwarfs we suggest that the abundance anomalies observed in sdB stars are related to the combined eects of diusion and mass loss.

X‐Ray Spectroscopy of II Pegasi: Coronal Temperature Structure, Abundances, and Variability

We have obtained high-resolution X-ray spectra of the coronally active binary II Pegasi (HD 224085), covering the wavelength range of 1.5-25 A. For the first half of our 44 ks observation, the source was in a quiescent state with constant X-ray flux, after which it flared, reaching twice the quiescent flux in 12 ks, then decreased. We analyze the emission-line spectrum and continuum during quiescent and flaring states. The differential emission measure derived from lines fluxes shows a hot corona with a continuous distribution in temperature. During the nonflare state, the distribution peaks near log T = 7.2, and when flaring, it peaks near 7.6. High-temperature lines are enhanced slightly during the flare, but most of the change occurs in the continuum. Coronal abundance anomalies are apparent, with iron very deficient relative to oxygen and significantly weaker than expected from photospheric measurements, while neon is enhanced relative to oxygen. We find no evidence of appreciable resonant scattering optical depth in line ratios of iron and oxygen. The flare light curve is consistent with solar two-ribbon flare models but with a very long reconnection time constant of about 65 ks. We infer loop lengths of about 0.05 to about 0.25 stellar radii in the flare, if the flare emission originated from a single, low-density loop.

A comparative analysis of chemical abundances in the atmospheres of red giants of different age groups

We analyze previously published chemical abundances in the atmospheres of red giants. Excess abundances are observed not only for Na, but also for Al and Si, with the overabundances increasing with the stars’ luminosity. The observed anomalies provide evidence that, in addition to the CNO hydrogen-burning cycle, the Mg-Al and Ne-Na cycles operate in the interiors of main-sequence stars; their products are brought to the stellar atmospheres by convection after the transition to the red-giant phase. The abundance anomalies for s-process elements, also observed in the atmospheres of field stars, testify to the presence of a substantial number of neutrons. The s-process abundance anomalies are absent from giants of the young Hyades cluster.

3He and 7Li in Low- and Intermediate Mass Stars

We discuss the problem of 3He and 7Li production and destruction by low and intermediate mass stars, in connection with abundance anomalies in RGB and AGB stars. We focus on the 7Li rich episodes which are expected to occur at the RGB bump and on the early-AGB and account for the so-called Li-rich giants.

Digital Spectroscopy of O3–O5 and ON/OC Supergiants in Cygnus

ABSTRACTHigh signal‐to‐noise, optical digital classification spectrograms of three northern very early O supergiants and four late O supergiants with CNO anomalies are illustrated and discussed. Several of these exceptional objects either are the prototypes of their classes, originally discovered photographically, or are unique representatives of them in the northern hemisphere. The full blue‐violet and yellow‐red spectrum of the extreme ON supergiant BD +36°4063, discovered by G. Mathys with limited wavelength coverage, is shown for the first time. Extensive line identifications are provided, and the high quality of these data reveals numerous new absorption and emission features corresponding either to the high ionization in the case of the very early objects, or to the chemical abundance anomalies in the later ones. These data provide standards for digital classification of related objects in the north and guidance for subsequent astrophysical analyses with higher spectral resolution.

Ir anomalies and other elemental markers near the Palaeocene-Eocene boundary in a flysch sequence from the Western Tethys (Slovenia)

Ir abundance anomalies, platinum-group elements (PGE) enrichment and increased concentrations of meteoritic (Ni, Fe, Co) and nonmeteoritic (Sb, As, Zn and Cu) elements were found in the Palaeocene–Eocene (P/E) boundary interval in a flysch sequence from the Western Tethys (Goriska Brda section, W. Slovenia). This records one of the most important calcareous deep benthic extinctions in the history of the Earth. Although the observed geochemical patterns could indicate complex sources for these metals, such as weathering of the continental and oceanic crust, volcanic processes, as well as diagenetic mobilization and redistribution, we cannot preclude the highly speculative possibility that the observed Ir abundances may also indicate extraterrestrial contamination.

Electron density and temperature measurements, and abundance anomalies in the solar atmosphere

Using spectra obtained from the SUMER (Solar Ultraviolet Measurements of Emitted Radiation) spectrograph on the spacecraft SOHO (Solar and Heliospheric Observatory), we investigate the height dependence of electron density, temperature and abundance anomalies in the solar atmosphere. In particular, we present the behaviour of the solar FIP effect (the abundance enhancement of elements with first ionization potential l 10 eV in the corona with respect to photospheric values) with height above an active region observed at the solar limb, with emphasis on the so-called transition region lines.

Mixing through shear instabilities

In this paper we present the results of numerical simulations of the Kelvin–Helmholtz instability in a stratified shear layer. This shear instability is believed to be responsible for extra mixing in differentially rotating stellar interiors and is the prime candidate to explain the abundance anomalies observed in many rotating stars. All mixing prescriptions currently in use are based on phenomenological and heuristic estimates whose validity is often unclear. Using three-dimensional numerical simulations, we study the mixing efficiency as a function of the Richardson number and compare our results with some semi-analytical formalisms of mixing.

K, Mg, Ti and Ca isotopic compositions and refractory trace element abundances in hibonites from CM and CV meteorites: implications for early solar system processes

Potassium and magnesium isotopic compositions of hibonites from the Murchison (CM) and Allende (CV) meteorites are determined by an ion microprobe to look for possible presence of the short-lived nuclides 41Ca and 26Al at the time of their formation. Abundance anomalies in the neutron-rich isotopes 50Ti and 48Ca as well as REE and additional refractory trace element abundances have also been determined to infer a plausible formation environment of the hibonites in the early solar system. The results obtained in this study suggest a widespread distribution of the short-lived nuclide 41Ca in the early solar system. Observation of correlated presence of the two short-lived nuclides 41Ca and 26Al on a microscopic scale in refractory hibonites and silicates from CM and CV meteorites suggests these nuclides to be cogenetic and support a stellar source for these and several other short-lived nuclides in the early solar system. Moderate to high enrichments in the neutron-rich isotopes 50Ti and 48Ca are seen in some of the analyzed hibonites. Our data for radiogenic and stable isotopic anomalies as well as refractory trace element abundance patterns in isolated CM hibonites with platelet morphology show trends similar to those reported earlier. They are devoid of 41Ca and 26Al, show large enrichment in 50Ti and 48Ca and have typical Group III REE patterns. However, hibonites present within hibonite-spinel inclusions show variations in their REE patterns. One of them shows the normally expected Group II REE pattern, while another has an ultrarefractory REE pattern and a third has affinities towards platelet hibonites that appear to suggest a link between these two groups of hibonites with distinct morphological characteristics. We propose a new model to explain the absence of the short-lived nuclides 41Ca and 26Al in some of the hibonites, whose REE abundance patterns and stable isotopic anomalies suggest that they are some of the first solar system solids, assuming a stellar origin for these nuclides. We attribute this absence to the very early formation of these hibonites near the central region of the collapsing protosolar cloud prior to the arrival of the short-lived nuclides injected into the cloud from a stellar source.

Diffusion in the Atmospheres of Blue Horizontal-Branch Stars.

We investigate the effects of diffusion in the atmospheres of hot horizontal-branch stars using a model atmosphere code including diffusion self-consistently. Equilibrium stratifications (i.e., for which the diffusion velocity equals zero in each layer) are computed for models of effective temperatures between 10,000 and 25,000 K. The stratified models provide much better agreement with many observational features [jump in the (u, u-y) color-magnitude diagram, gaps, lower spectroscopic gravities] in comparison with classical horizontal-branch models. The observed abundance anomalies are also consistent with the amounts that can be supported in the atmospheres.

Presupernova Evolution of Rotating Massive Stars. II. Evolution of the Surface Properties

We investigate the evolution of the surface properties of models for rotating massive stars, i.e., their luminosities, effective temperatures, surface rotational velocities, and surface abundances of all isotopes, from the zero-age main sequence to the supernova stage. Our results are based on the grid of stellar models by Heger, Langer, & Woosley, which covers solar metallicity stars in the initial-mass range 8-25 M☉. Results are parameterized by initial mass, initial rotational velocity and major uncertainties in the treatment of the rotational mixing inside massive stars. Rotationally induced mixing processes widen the main sequence and increase the core hydrogen-burning lifetime, similar to the effects of convective overshooting. It can also significantly increase the luminosity during and after core hydrogen burning, and strongly affects the evolution of the effective temperature. Our models predict surface rotational velocities for various evolutionary stages, in particular for blue supergiants, red supergiants, and for the immediate presupernova stage. We discuss the changes of the surface abundances due to rotationally induced mixing for main sequence and post-main-sequence stars. We single out two characteristics by which the effect of rotational mixing can be distinguished from that of massive close binary mass transfer, the only alternative process leading to nonstandard chemical surface abundances in massive stars. A comparison with observed abundance anomalies in various types of massive stars supports the concept of rotational mixing in massive stars and indicates that it is responsible for most of the observed abundance anomalies.

Doppler-Zeeman mapping of the rapidly rotating magnetic CP star HD 37776

We present the results of our analysis of magnetic-field configuration and abundance anomalies on the surface of the rapidly rotating, chemically peculiar helium-strong variable B2 V star HD 37776 with unresolved Zeeman components of spectral lines. Simultaneous inversion of the observed Stokes I and V profiles, which realizes the method of Doppler-Zeeman mapping [1], has been applied for the first time. Spectroscopic observations were carried out with the Main stellar spectrograph of the 6-m Special Astrophysical Observatory telescope equipped with a Zeeman analyzer and a CCD array, which allowed spectra in right-and left-hand circularly polarized light to be taken simultaneously at a signal-to-noise ratio S/N≥200 [2]. The profile width of winged spectral lines (reaching 5 A) is determined by Zeeman line splitting; however, the observed Zeeman components are blurred and unresolved because of the rapid stellar rotation. When solving the inverse problem, we sought for the magnetic-field configuration in the form of a combination of arbitrarily oriented dipole, quadrupole, and octupole placed at the stellar center. The observed Stokes I and V profiles for eight spectral lines of He, O II, Al III, Si III, and Fe III averaged over the visible stellar surface were used as input data. We constructed a model of the magnetic field from the condition of coincidence of magnetic maps obtained from different lines of different chemical elements and from the condition of a minimum profile residual. This model is a combination of centered coaxial dipole and quadrupole with the dominant quadrupole component at 30°<i<50°, β=40°, and a maximum surface field strength Hs=60 kG. A comparison of our abundance maps with the field configuration shows that the He concentration is at a maximum in the regions of maximum radial field, while the maximum concentrations of O, Al, Si, and Fe coincide with the regions of maximum tangential field.

Abundance Analyses of Field RV Tauri Stars. V. DS Aquarii, UY Arae, TW Camelopardalis, BT Librae, U Monocerotis, TT Ophiuchi, R Scuti, and RV Tauri

Abundance analyses are presented and discussed for eight RV Tauri variables. The RVB star UY Ara shows the abundance anomalies seen in other RVB stars, namely, elements that condense into grains at high temperature are underabundant, but elements of low condensation temperature are much less underabundant. This pattern is ascribed to a separation of dust from gas with accretion of gas but not dust by the atmosphere. Abundances for two RVC stars with earlier results for other RVC stars show that these intrinsically metal-poor stars do not show effects of dust-gas separation. Analyses of five RVA stars show that these cooler stars are very largely unaffected by dust-gas separation. It is proposed that the deeper convective envelope of cooler stars dilutes anomalies resulting from dust-gas separation. Possible sites for dust formation and dust-gas separation—the dusty wind off the RV Tauri variable or a dusty circumbinary disk—are reviewed and observational tests suggested.

Element Abundances in the Upper Atmospheres of the Sun and Stars: Update of Observational Results

We review observational progress in the determination of element abundances in the solar corona, largely due to the new capabilities offered by the instrumentation on the SOHO satellite. Many new facets to coronal abundance anomalies with respect to the photosphere are revealed. This includes new results on the FIP (First Ionization Potential) Effect, whereby elements with FIP < 10 eV are enriched in the corona by a factor ~4 with respect to the photosphere, and the first evidence for gravitational settling of heavy elements in the corona. Advances in EUV and x-ray astronomy instrumentation have also yielded the first spectra of stellar coronae of sufficient quality to allow element abundance measurements. We survey these new results and compare the various stellar cases to the solar corona.

Globular Cluster Giant Branch Luminosity Functions: “Extra Stars” and “Deep Mixing”

We explore the possibility that is the reason that there are red giants in the Galactic globular clusters M5 and M30. Deep mixing has often been invoked to account for the abundance anomalies observed in many bright Galactic globular cluster giants. Here we assume that it also adds fuel to the hydrogen-burning shell of a typical giant in some clusters and returns helium to the envelope above. We use a simple mixing model to estimate how much fuel has been added to the hydrogen-burning shell of a typical red giant in M5 and M30 from the number of stars in the red giant branch (RGB) luminosity functions for these two clusters. Presumably, red giants are common in Galactic globular clusters. M5 and M30 were chosen solely because the available luminosity functions for both were derived from an exceptionally large sample of stars in two colors: the RGB luminosity functions are especially well defined and well normalized to unevolved main-sequence stars. If deep mixing is the reason that there are too many M5 giants, the number of giants suggests that the envelope of a typical M5 star should be significantly enriched in helium (ΔY ≈ 0.07) by the time it reaches the tip of the giant branch. Hence, deep mixing may be (at least part of) the reason that M5 has a surprisingly blue horizontal branch (HB) for a moderately metal-rich cluster. If deep mixing occurs and ΔY ≈ 0.07, we can account for part, but only a small part, of the very low value of R—and the surprisingly low estimate for Y—that Sandquist et al. found from their B-band luminosity function. Sandquist et al.'s value of the helium indicator delta suggests that the helium abundance is enhanced (ΔY = 0.04 ± 0.02) in HB stars at the blue edge of the instability strip. If deep mixing accounts for the extra giants that have been observed in M30, Y will be about 0.05 higher in the envelope of a typical M30 HB star than it was when that star left the main sequence. In addition to accounting for the M30 giants, deep mixing can give a good account of (1) the lack of a on the RGB or the presence of a small bump in the wrong place, (2) the surprisingly bright horizontal branch, and (3) the anomalous values of both Δ and R. The abundances of the light elements suggest that deep mixing is much more common in some clusters than in others and more extensive in some stars in a cluster than in others. If deep mixing does add fuel to evolving giants in some clusters, there are many interesting ramifications. For example, all horizontal branches may not have been created equal; some might well be brighter than others. Sweigart and Sweigart & Catelan have recently described how mixing helium out into the envelope of a red giant may solve a number of cluster blue star mysteries. We point out a number of additional mysteries that might be solved by looking at the fuel that is mixed down into the hydrogen-burning shell: e.g., the large range in the values that have been observed for R (the ratio of bright giants to HB stars), the observed range in the helium estimator Δ at a constant metallicity, and the variations observed in the RGB bumps among otherwise similar clusters.

The Evolution of AmFm Stars, Abundance Anomalies, and Turbulent Transport

Stellar evolution models of stars of 1.45-3.0 M☉ have been calculated, including the atomic diffusion of metals and radiative accelerations for all species in the OPAL opacities. As the abundances change, the opacities and radiative accelerations are continuously recalculated during evolution. These models develop iron-peak convection zones centered at a temperature of approximately 200,000 K. If one then assumes that there is sufficient overshoot to homogenize the surface regions between the hydrogen, helium, and iron-peak convection zones, it is shown here that the surface abundance variations that are produced, without any arbitrary parameter, closely resemble the abundance anomalies of AmFm stars, except that they are larger by a factor of about 3. Detailed evolutionary model calculations have been carried out, varying the turbulence in the outer stellar regions in order to improve the agreement with the observed anomalies in AmFm stars. The outer mass mixed by turbulence has been varied, as well as the density dependence of the turbulent diffusion coefficient. It is shown that the anomalies depend on only one parameter characterizing turbulence, namely, the depth of the zone mixed by turbulence. The calculated surface abundances are compared to observations of a number of recently observed AmFm stars. For Sirius A, 16 abundances (including four upper limits) are available for comparison. Of these, 12 are well reproduced by the model, while three are not so well reproduced, and one is a very uncertain observation. In cluster AmFm stars, the age and initial abundances are known. There is then less arbitrariness in the calculations, but fewer chemical species have been observed than in Sirius. The available observations (Hyades, Pleiades, and Praesepe stars are compared) agree reasonably well with the calculated models for the five stars that are compared. The zone mixed by turbulence is deeper than the iron convection zone, reducing the abundance anomalies to values that are too small for iron-peak convection zones to develop in many of the models. The origin of the mixing process then remains uncertain. There is considerable scatter in the observations between different observers, so it is premature to conclude that hydrodynamical processes other than turbulence are needed to explain the observations. We do not rule out the possibility that this might be the case, but the observations do not appear to us to be good enough to establish it.

Electron Density and Temperature Measurements, and Abundance Anomalies in the Solar Atmosphere

AbstractUsing spectra obtained from the SUMER (Solar Ultraviolet Measurements of Emitted Radiation) spectrograph on the spacecraft SOHO (Solar and Heliospheric Observatory), we investigate the height dependence of electron density, temperature and abundance anomalies in the solar atmosphere. In particular, we present the behaviour of the solar FIP effect (the abundance enhancement of elements with first ionization potential &amp;lt; 10 eV in the corona with respect to photospheric values) with height above an active region observed at the solar limb, with emphasis on the so-called transition region lines.

The Chemical Composition and Orbital Parameters of Barium Stars

The observational relation between s-process abundance anomalies and orbital periods for barium stars is discussed and compared with mass-transfer simulations. Recent detailed abundance analyses of a large sample of single-lined long-period binaries provide evidence that all giants with white dwarf companions are likely to have abundance anomalies.

Constraints on Stellar Hydrodynamics from Abundance Anomalies of LiBeB and Metals

The availability of large atomic data bases has made it possible to calculate stellar evolution models taking into detailed account the atomic diffusion of all important contributors to opacity. The radiative accelerations and the opacity are continuously calculated during evolution taking the abundance changes of 28 species into account. This leads to the first self-consistent stellar evolution models for A and F stars. In A and F stars an iron-peak convection zone appears.The calculated abundance anomalies are very similar to those observed in AmFm stars in open clusters except that they are larger by a factor of about 3. To reduce the calculated anomalies to the observed ones, an additional source of turbulence (or some other hydrodynamical process) must be introduced. The mixed zone must extend about 5 times deeper than the iron convection zone. Detailed comparisons to a few AmFm stars have been carried out.The LiBeB abundances observed in clusters give additional information. The abundances of the 28 species offer considerable constraints on the models. Various potential turbulence models have been introduced in a stellar evolution code and results of evolutionary calculations for Li gap stars are discussed in the light of the constraints offered by the abundances of LiBeB and metals. The radiative accelerations of LiBeB have also been recalculated taking the effect of changing metal abundances into account. This modifies the expected Li gap in the absence of turbulence.