Abundance Anomalies Research Articles

Abundance anomalies in stars: atomic physics at play

Abundance anomalies are obtained in a large fraction of both evolved and main sequence stars. They involve most chemical elements and abundances can vary by many orders of magnitude. On the main sequence they are seen in stars as cool as 6400K and as hot as 27000K. They are apparently caused by chemical separation. The competition between gravity and selective radiative acceleration leads to the appearance of either overabundances or underabundances. Calculations of radiative accelerations require large amounts of atomic data.In this paper I concentrate on abundance anomalies observed on nonmagnetic stars with 6000 < Teff < 10000K. Some 20% of the stars in that Teff range (Teff stands for effective temperature) are involved. In them the separation goes on below the visible region. It is shown that, to obtain accurate enough radiative accelerations, the f values are needed to an accuracy of 30% for the important lines of all states of ionization, up to the 18th for Fe, for instance. An analysis of the needed atomic data is done in detail.To construct models that successfully reproduce the observations will require much more atomic data than available. They are needed to properly analyze the observations produced by modern electronic detectors and by space observations. These developments lead to a better understanding of hydrodynamical processes in stellar envelopes. In particular it is shown here how mass loss modifies the effect of chemical separation and it is shown that a mass loss rate of 10−15 M0 yr−1 is implied by the anomalies observed in many objects.

The core helium flash and surface abundance anomalies

view Abstract Citations (38) References (31) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Core Helium Flash and Surface Abundance Anomalies Deupree, Robert G. ; Wallace, Richard K. Abstract Four core helium flash calculations of varying intensity have been performed. All four flash models mixed residue nuclear species of the core helium flash into and above the hydrogen shell. The peak temperatures for these four flash calculations are 7.4, 8.3, 9.2, and 10.2×108K, the last being about the highest temperature that can be expected without disrupting the entire core. Two other calculations were performed with the recently revised 12C(α,γ)16O rate (Caughlan et al.). The authors focus on the amount and types of nuclear species mixed into and above the hydrogen shell and discuss what surface abundance anomalies the core helium flash can (and cannot) be expected to explain. Publication: The Astrophysical Journal Pub Date: June 1987 DOI: 10.1086/165319 Bibcode: 1987ApJ...317..724D Keywords: B Stars; Giant Stars; Helium; Hydrogen; Nuclear Fusion; Stellar Cores; Stellar Evolution; Stellar Interiors; Stellar Temperature; Astrophysics; NUCLEOSYNTHESIS; STARS: ABUNDANCES; STARS: INTERIORS full text sources ADS | data products SIMBAD (5)

Elemental abundance analyses with coadded DAO spectrograms - I. The field horizontal-branch stars HD-64488, 109995 and 161817

It is possible to improve the quality of elemental-abundance analyses by using higher-S/N data than has been the practice at high resolution. The procedures developed at the Dominion Astrophysical Observatory to coadd high-dispersion coude spectrograms are used with a minimum of 10 6.5-A/mm IIa-O spectrograms of each of three field hoorizontal-branch (FHB) A stars to increase the S/N of the photographic data over a considerable wavelength region. Fine analyses of the sharp-lined prototype FHB stars HD 109995 and 161817 show an internal consistency which justifies this effort. Their photospheric elemental abundances are similar to those of Population II globular cluster giants. As their photometric and spectrophotometric properties are similar to blue HB stars in such clusters, they are confirmed to be the brighter analogs of such stars. HD 64488, which is photometrically and spectrophotometrically similar to the FHB stars, is found to be metal-poor (Fe/H = -1) with much broader lines (v sin i = 147 km/s). The implications of the abundance anomalies of all three stars are discussed.

THE CHEMICAL INHOMEGENEITY OF GLOBULAR CLUSTERS.

ABSTRACT This paper reviews many of the properties relating to the chemical inhomogeneity of globular clusters. Three main types of anomalous stars are found within clusters: the CH stars, weak-G-band stars, and giants with enhanced CN bands. Of these, the CN-enhanced giants are the most common, while the CH stars are the rarest. A number of characteristics of these stars, including the relationship between the carbon and nitrogen abundances; correlations between CN enhancements and atomic line strengths; and the stellar luminosity dependence of C and N abundances, are discussed. The cluster Omega Cen not only contains all three types of the above-mentioned stars but also exhibits a much wider variety of abundance anomalies, including star-to-star iron-peak element variations and the presence of a radial CN gradient. A number of the characteristics of the Omega Cen inhomogeneity are described. Possible mechanisms for establishing internal cluster-abundance variations are summarized, and arguments for and against two of the most popular of these, the mixing and primordial-enrichment hypotheses, are discussed.

Neutron activation determination of iridium, gold, platinum, and silver in geologic samples

Low-level methods for the determination of iridium and other noble metals have become increasingly important in recent years due to interest in locating abundance anomalies associated with the Cretaceous and Tertiary (K-T) boundary. Typical iridium anomalies are in the range of 1 to 100 μg/kg (ppb). Thus methods with detection limits near 0.1 μg/kg should be adequate to detect K-T boundary anomalies. Radiochemical neutron activation analysis methods continue to be required although instrumental neutron activation analysis techniques employing elaborate gamma-counters are under development. In the procedure developed in this study samples irradiated in the epithermal neutron facility of the U. S. Geological Survey TRIGA Reactor (Denver, Colorado) are treated with a mini-fire assay technique. The iridium, gold, and silver are collected in a 1-gram metallic lead button. Primary contaminants at this stage are arsenic and antimony. These can be removed by heating the button with a mixture of sodium perioxide and sodium hydroxide. The resulting 0.2-gram lead bead is counted in a Compton suppression spectrometer. Carrier yields are determined by reirradiation of the lead beads. This procedure has been applied to the U.S.G.S. Standard Rock PCC-1 and samples from K-T boundary sites in the Western Interior of North America.

Mineralogy and Petrology of the Cretaceous-Tertiary Boundary Clay Bed and Adjacent Clay-Rich Rocks, Raton Basin, New Mexico and Colorado

ABSTRACT The Cretaceous-Tertiary (K-T) is preserved in a sequence of coal-bearing, fluvial rocks in the lower part of the Raton Formation (Late Cretaceous and Paleocene) at several sites in the east-central part of the Raton Basin. The K-T occurs at the top of a kaolinitic claystone commonly referred to as the boundary clay layer, in an interval of coal and carbonaceous shale. The is defined by the disappearance of certain fossil-pollen taxa. The clay layer also contains shocked quartz grains and abundance anomalies of iridium, chromium, and other elements. Each of these characteristics support the hypothesis of an asteroid impact at the end of the Cretaceous. Small goyazite spheres were also found in the clay bed; however, their origin is ncertain. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and thin-section petrography have shown that the clay bed exhibits unique mineralogic and petrologic characteristics not possessed by other clay-rich beds in coal-bearing rocks of the Raton Basin. The clay bed is composed primarily of well-crystallized kaolinite but also contains subordinate, randomly interstratified, mixed-layer illite/smectite, both of which appear to have been formed by the alteration of vitreous material in a coal-swamp environment. The bulk mineralogy of the clay bed is similar to that of other kaolinite-rich clay beds (tonsteins) that altered in coal swamps from airfall volcanic ash. The alteration process and diagenetic products of the host material in the cl y bed, however, differed from those of the tonsteins, resulting in a unique petrographic fabric. The clay bed is characterized by an overall imbricate fabric that may reflect an original vitreous material that included shards, bubbles, and spheres. The original fallout material of the clay bed first altered to cabbage-like microspherules, indicating a halloysite or allophane precursor, and then to well-crystallized kaolinite; the well-crystallized kaolinite is the diagenetic stable phase for these rocks at their maximum burial conditions. In comparison, the vitreous components of the tonsteins were altered directly to coarse vermicular and platey kaolinite. Alteration of the original fallout material in the bed was probably rapid, leading to hydrated, disordered, fine-grained clay minerals that formed into curled spherical or tubular shapes. The facto s that produced these unique fabrics in the clay bed existed at all K-T sites in the Raton Basin and were much different from those that formed the coarse platey and vermicular fabrics of ordinary tonsteins. The distinctive character of the clay may be due to the unique nature and chemistry of the fallout material and (or) physical or chemical conditions existing shortly after its deposition.

Quantitative technetium abundances in the long-period variables Chi Cygni and Omicron Ceti

We have analyzed spectra of 2.4 A mm-1 dispersion to derive abundances of V, Zr, Nb, and Tc in χ Gygni, o Ceti, and R Lyrae. For χ Cyg an excess of Zr by 0.4 dex and a Tc/Zr ratio of -1.3 dex were found. For Mira Tc/Zr is -2.0 dex. In χ Cyg Nb is not enhanced, while it is depleted in o Cet. The spectrum of R Lyr shows no Tc lines and we derive Tc/Zr < -2.6 dex. We interpret the presence of Tc, and the small enhancement of Zr, in χ Cyg as evidence for a recent conventional s-process event. The time since the event is sufficiently short (≲ 1.5 × 105 yr) that neither has the 99Tc had a chance to decay nor has any 93Zr that may have been produced had a chance to decay to 93Nb. For о Cet the excess of Tc is accompanied by a deficiency of Nb, and no other abundance anomalies. We can understand this by a very small pulse of neutrons in which no more than one neutron is absorbed by any isotope.

Multiple sources for basaltic arc rocks from the southern volcanic zone of the Andes (34°–41°S): Trace element and isotopic evidence for contributions from subducted oceanic crust, mantle, and continental crust

Potential sources for arc magmas include subducted midocean ridge basalt and sediment, subarc mantle peridotite, and overlying crust. Geochemical data for arc rocks in general, including those for the southern volcanic zone (SVZ) of the Andes (34°–41°S in Chile), cannot be explained by one‐stage partial melting of subducted oceanic crust or overlying mantle peridotite. Sr and Nd isotope systematics in arc rocks are similar to those in oceanic island basalts (OIB's). However, plots of 207Pb/204Pb versus 208Pb/204Pb show clearly that there is an input from subducted sediment in most arcs. In arc rocks, radiogenic isotope ratios are decoupled from abundance anomalies of Th, U, Pb, alkali metals, Sr, and Ba. This decoupling is expected if the trace element abundance anomalies are caused by a process such as partial melting of or volatile release from the subducted oceanic crust. In basalts from different volcanoes in the SVZ, anomalous trace element abundance ratios, such as Ba/Nb, are coupled with incompatible element abundances. This feature is consistent with mixing of a component derived from the subducted oceanic crust (i.e., having high Ba/Nb, Cs/Rb, and 207Pb204Pb) with enriched asthenosphere or lithosphere. Although crustal contamination may have had minor effects in all SVZ volcanoes, basaltic lavas from volcanoes 34°–38°S, which are sited on thicker crust, exhibit more evidence for continental crustal contamination, specifically, lower 143Nd/144Nd and higher Al2O3/CaO, than lavas from volcanoes 38°–41°S, which are sited on thinner crust.

Palynological and Iridium Anomalies at Cretaceous-Tertiary Boundary, South-Central Saskatchewan

The Cretaceous-Tertiary boundary in south-central Saskatchewan is marked by coincident anomalies in abundance of iridium and fern spores at the extinction level of a suite of Cretaceous pollen taxa. Evidence of disruption of the terrestrial flora includes the fern-spore abundance anomaly and local extinction of as much as 30 percent of angiosperm species. The reorganized earliest Tertiary flora is made up largely of surviving species that assumed new roles of dominance. Persistence of climatically sensitive taxa across the boundary indicates that if paleoclimate was altered by the terminal Cretaceous event, it returned quickly to the pre-event condition.

Particle Transport in Non-Magnetic Stars

AbstractThe 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.

Isotope abundance anomalies and the early solar system (some) facts and (some) implications

Variations have been found in the isotopic composition of a number of chemical elements which appear to reflect primordial variations in space and/or time of the composition of the proto-solar nebula. The agreement with theoretical predictions is the first proof that (some of) the nucleosynthetic processes invoked by astrophysicists to explain the abundance pattern of nuclides in the solar system may indeed have happened. Data for xenon, oxygen, titanium and evidence for the former existence of now extinct 244Pu and 26Al are discussed in some detail as well as implications from the presence of 26Al as an effective heat source in the early history of the solar system.

Hα and OI Photometry of Upper Main Sequence Stars with Anomalous Abundances

AbstractAdditional α(16)Λ(9)-photometry of upper main sequence stars with anomalous abundances confirm our previous results, namely, Ap stars are neatly separated from normal main sequence stars. Furthermore, Ap stars are located on three different zones of the α(16)Λ(9)-plane, according to their abundance anomalies to form three photometric groups, (i) Si stars, (ii) Hg,Mn stars, and (iii) Cr,Eu,Sr stars.

Model Atmospheres and Radiative Transfer in Chemically Peculiar Stars: Interpretational Significance of Non-LTE

One of the main problems in studying chemically peculiar (CP) stars is the question of the extent to which the conceptual framework of the contemporary spectroscopic diagnostics is reliable. As the first step, it should be clarified whether the traditional assumption of local thermodynamic equilibrium (LTE) provides an adequate approximation to reality, or whether a more general non-LTE approach should be employed.The period of rapid development of computational methods and extensive calculations of NLTE model atmospheres is now past its culmination point. The importance of a relevant NLTE description is viewed as unquestionable for hot stars (B and earlier). Consequently, the hot stars attract the attention of most “NLTE theoreticians,” while considerably less attention is being devoted to later types (late B and A). Moreover, the abundance anomalies found in the CP stars are no longer expected to be a spurious result of an inadequate (LTE) analysis (Cowley 1981). It is not commonly accepted that other phenomena such as diffusion, magnetic field, inhomogeneous abundance distribution, etc., are quite essential ingredients of the general atmospheric pattern of CP stars. Therefore, a large part of the astronomers investigating the CP stars now consider the NLTE effects to be rather minor perturbations of the LTE predictions.

Trace-element anomalies at the Mississippian/Pennsylvanian boundary in Oklahoma and Texas

Trace-element abundance anomalies have been found at the Mississippian/Pennsylvania boundary at sites in Oklahoma and Texas where the boundary has been precisely located on the basis of an abrupt change in conodont diversity and species composition. Enriched elements include osmium, indium, platinum, chromium, most chalcophiles, rare earths, and uranium. The anomalies are more intense (e.g., Os = 4 ppb, Ir = 0.38 ppb, Pt = 6 ppb, Cr = 12000 ppm, U = 380 ppm) and peisist through a thicker interval at the south-central Texas locality than in Oklahoma, and in bolh locations the anomalies are associated with an increase in phosphate content of the rocks. There is no tangible evidence of an asteroid or comet impact source for the excess Pt-group elements and fauna! crisis. The cause of the elemental enrichments and the biological disturbance may possibly be related to a change in the ocean chemistry of the Paleozoic seaway, such as increased upwelling, stagnation, or nearby submarine volcanism.

Studies of hot B subdwarfs. IV - Radiative forces, mass loss, and metal abundances in sdB stars

The abundance anomalies generated by diffusion in the envelopes of hot, hydrogen-rich subdwarfs are studied. It is shown that unimpeded diffusion cannot lead to the large silicon underabundance observed in those stars at effective temperatures above 30,000 K. Calculations of diffusion of heavy elements in the presence of mass loss are also performed. For a mass-loss rate of 2.5 x 10 to the -15th solar masses/year, the observed abundance patterns of C, N, and Si are reproduced on a time scale of about 100,000 yr. Lower mass-loss rates would necessitate longer time scales. The pattern of abundance anomalies may eventually be used to constrain both the mass-loss rate and the stellar lifetime in the sdB evolutionary phase. 12 references.

THE PECULIAR A STAR HD 43819 : A PHOTOGRAPHIC REGION LINE-IDENTIFICATION STUDY.

ABSTRACT A line identification study of the sharp-lined silicon star HD 43819 has been performed for the photographic region λλ3759 - 4924. Comparison of this star's spectrum with those of other silicon stars shows that it shares many of their apparent abundance anomalies. The Ti II, Cr II, Fe I, and Fe II spectra are well represented while the singly-ionized rare earths are represented by at best a few lines per species.

HD 160538 and HD 185510 - Two active-chromosphere stars with hot companions

Short-wavelength ultraviolet spectra of two active-chromosphere giants, HD 160538 and HD 185510, show hot stellar continua and Lα absorption features. The hot companion of HD 160538 is a white dwarf whose ultraviolet spectrum is matched best to the energy distribution of a T<SUB>eff</SUB> = 30000K and log g = 8 model atmosphere. The ultraviolet spectrum of HD 185510 shows numerous absorption features and is identified as a B subdwarf. Since the giant star of each of these binary systems has no known abundance anomalies, these systems appear to be consistent with the mass-transfer scenario of barium-star formation.

Abundance Analysis of Alpha Ursae Minoris

AbstractWe have derived atmospheric abundances of the bright Cepheid a UMi in order to study the abundance anomalies in different elements. The atmospheric abundance of C, 0, Fe-peak elements Ca, Sc, Ti, Cr, Fe and heavier s-process elements Y, Ba, Ce and Sm have been derived using the method of spectrum synthesis. The abundance of carbon is derived using the C I lines in 4700A region, whereas for oxygen, the forbidden line at 6300.311A is employed. Abundances of the Fe-peak elements and s-process elements are obtained by synthesizing selected portions in the wavelength range 4330A - 4650A. The estimates of C/0 derived from the present investigation are compared with other Cepheids of similar period. The evolutionary status of a UMi is discussed in the light of these derived abundances.

THE BARIUM STARS.

ABSTRACT The barium stars are Population I G-K giants that have enhanced abundances of carbon and s-process elements, and are probably related in their peculiarities to several other carbon enhanced red-giant types such as CH, R, N, and S stars. Since the abundance anomalies in the barium stars are likely the result of mixing of processed material from deep within a stellar interior, and since they are numerous with many bright examples suitable for detailed observations, these stars provide very valuable information on nucleosynthesis, and the advanced stages of stellar evolution. A clue to the origin of the anomalous abundances in the barium stars is the recent discovery that they are likely all members of binary systems.

A Search for Iridium Abundance Anomalies at Two Late Cambrian Biomere Boundaries in Western Utah

Iridium concentrations have been measured in samples taken across two Late Cambrian biomere boundaries (crisis zones) in search of evidence for possible elemental abundance anomalies similar to the one observed at the Cretaceous-Tertiary boundary. Sampling was performed in uplifted marine limestone deposits in the House Range of western Utah. Although the two trilobite-brachiopod extinction boundaries could be assigned to +/-4 millimeters of vertical section by laboratory examination of the rocks, only background amounts of iridium (2 x 10(-12) to 17 x 10(-12) gram per gram of whole rock) were observed.