The synthesis and destruction of elements in peculiar stars of types a and b.
Physical properties of peculiar A and B stars, especially their anomalous abundances and positions in H-R diagram
ASTROPHYSICS↗
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Difficulties with theories of nucleosynthesis in stars.
Heavy element synthesis in stellar interiors from different nuclear processes, noting solar system abundances
Suess, H. E.↗
Carbon and nitrogen abundances determined from transition layer lines
The possibility of determining relative carbon, nitrogen, and silicon abundances from the emission-line fluxes in the lower transition layers between stellar chromospheres and coronae is explored. Observations for main-sequence and luminosity class IV stars with presumably solar element abundances show that for the lower transition layers Em = BT sup -gamma. For a given carbon abundance the constants gamma and B in this relation can be determined from the C II and C IV emission-line fluxes. From the N V and S IV lines, the abundances of these elements relative to carbon can be determined from their surface emission-line fluxes. Ratios of N/C abundances determined in this way for some giants and supergiants agree within the limits of errors with those determined from molecular bands. For giants, an increase in the ratio of N/C at B-V of about 0.8 is found, as expected theoretically.
Boehm-Vitense, Erika↗
Chemical abundances in Hg-Mn stars
An abundance analysis has been conducted of 21 elements in 21 Hg-Mn, two Si-Cr, and six normal stars using model atmospheres and high-dispersion spectroscopy in the visible and UV. Manganese line strengths imply abundances that correlate well with stellar effective temperature. Within the studied sample of Hg-Mn stars there appears to be no correlation of abundances of any element with projected rotational velocity. Abundances in several Hg-Mn stars show patterns that are probably consistent with diffusion but difficult to reconcile with equilibrium nucleosynthesis. In general, no combination of gross stellar physical parameters is sufficient to characterize the patterns of line strengths observed in Hg-Mb Hg-Mn stars.
Heacox, W. D.↗
IUE observations and interpretation of the symbiotic star RW Hya
The IUE observations of the high excitation symbiotic star RW Hya (gM2 + pec) are discussed. Analysis of the intense UV continuum observed between 1100 A to 2000 A suggests this star is a binary system in which the secondary is identified as a hot subdwarf with T sub eff being approximately 100,000 K. A distance to the system of 1000 pc is deduced. The UV spectrum consists of mainly semiforbidden and allowed transition lines of which the CIV (1548 A, 1550 A) emission lines are particularly strong, and UV continuum at both shorter and longer wavelengths. Strong forbidden lines seem to be absent suggesting the presence of a nebula of high densities. Tidal interaction between the red giant primary and the hot subdwarf is suggested as a likely means to form the observed nebula. RW Hya is suggested as a possible source of soft X-ray emission from material accreting onto the surface of the hot subdwarf. Detection of such emission with HEAO-B would give information if this accretion is taking place via Roche lobe overlow or via capture from a stellar wind emitted by the primary. A general discussion of elemental and ionic abundances in the nebula is also presented.
Kafatos, M.↗
On the paragenetic modes of minerals: A mineral evolution perspective
A systematic survey of 57 different paragenetic modes distributed among 5659 mineral species reveals patterns in the diversity and distribution of minerals related to their evolving formational environments. The earliest minerals in stellar, nebular, asteroid, and primitive Earth contexts were dominated by relatively abundant chemical elements, notably H, C, O, Mg, Al, Si, S, Ca, Ti, Cr, and Fe. Significant mineral diversification subsequently occurred via two main processes, first through gradual selection and concentration of rarer elements by fluid-rock interactions (for example, in hydrothermal metal deposits, complex granite pegmatites, and agpaitic rocks), and then through near-surface biologically mediated oxidation and weathering. We find that 3349 mineral species (59.2%) are known from only one paragenetic context, whereas another 1372 species (24.2%) are associated with two paragenetic modes. Among the most genetically varied minerals are pyrite, albite, hornblende, corundum, magnetite, calcite, hematite, rutile, and baryte, each with 15 or more known modes of formation. Among the most common paragenetic modes of minerals are near-surface weathering/oxidation (1998 species), subsurface hydrothermal deposition (859 species), and condensation at volcanic fumaroles (459 species). In addition, many species are associated with compositionally extreme environments of highly differentiated igneous lithologies, including agpaitic rocks (726 species), complex granite pegmatites (564 species), and carbonatites and related carbonate-bearing magmas (291 species). Biological processes lead to at least 2707 mineral species, primarily as a consequence of oxidative weathering but also through coal-related and other taphonomic minerals (597 species), as well as anthropogenic minerals, for example as byproducts of mining (603 minerals). However, contrary to previous estimates, we find that only ~34% of mineral species form exclusively as a consequence of biological processes. By far the most significant factor in enhancing Earth’s mineral diversity has been its dynamic hydrological cycle. At least 4583 minerals—81% of all species—arise through water-rock interactions. A timeline for mineral-forming events suggests that much of Earth’s mineral diversity was established within the first 250 million years. If life is rare in the universe, then this view of a mineralogically diverse early Earth provides many more plausible reactive pathways over a longer timespan than previous models. If, however, life is a cosmic imperative that emerges on any mineral- and water-rich world, then these findings support the hypothesis that life on Earth developed rapidly in the early stages of planetary evolution
Robert M. Hazen↗
X-ray astronomy and plasma astrophysics
X-ray astronomy studies of thin, thermal plasmas in stellar coronas, supernova remnants, and clusters of galaxies are reviewed. Plasma diagnostics for density, temperature and elemental abundance, as well as for departures from ionization equilibrium are described. These were used in analyses of data from imaging and spectroscopic intruments on the Einstein satellite. Results of these diagnostics were used to study the nature of coronal loops in RS CVn stars; the masses and abundances in Type I and Type II supernova remnants with implications for stellar evolution and the enrichment of the interstellar medium; the structure of the interstellar medium; the quantity and distribution of dark matter in galaxy halos; and the existence of cooling accretion flow in clusters.
Canizares, C. R.↗
Coronal Diagnostics of Intermediate Activity Star XI Boo A
The analysis of Xi Boo A proved difficult to adapt to our line-by-line approach because of the strong wings of the RGS instrumental profile, as has been detailed in earlier reports. While progress was also delayed because of problems in using SAS v4, we succeeded in the past year or so to bring the analysis to conclusion. Abundances have been derived using both EPIC and RGS data, confirming earlier EUVE findings of a mild solar-like FIP effect, though with some evidence of a turn-up in abundances of elements with higher FIP. Plasma densities appear normal for a moderately active stellar corona. Xi Boo A nicely bridges the gap between the very active stars and stars like the Sun, and it indeed does appear that these are the stars in which the solar-like FIP effects begins to change to the "inverse FIP" type of effect seen in the very active stars. Probing this divide was the main goal of the proposal. These results are in the process of being prepared for publication, though we have not decided the target journal as yet.
Drake, Jeremy↗
Emission line nebulae in the Magellanic clouds
H II regions, planetary nebulae, and supernova remnants in the large and small Magellanic clouds have been favored targets among all galactic and extragalactic emission nebulae for study with the IUE satellite due to their known distances and low line of sight extinction. Several of the major results arising from successful IUE (International Ultraviolet Explorer) observations of these three types of emission nebulae in the clouds are presented. Carbon abundances in the H II regions from UV emission lines with implications regarding the chemical evolution of the clouds and stellar origins of these elements are discussed. The UV spectral characteristics of several evolved and young SNRs in the clouds with comparisons made to similar objects in the Galaxy and other nearby galaxies are discussed. CNO abundance variations among planetary nebulae and characteristics of their central stars with emphasis on the new insights provided regarding PN formation and C-N nucleosynthesis during the various AGB dredge-up phases are discussed.
Dufour, Reginald J.↗
Lithium in giant stars of types F0 to G5.
Lithium/calcium ratio of five F giant and six G giant stars
STELLAR SPECTRUM↗
Interstellar abundances - Gas and dust
Data on abundances of interstellar atoms, ions and molecules in front of zeta Oph are assembled and analyzed. The gas-phase abundances of at least 11 heavy elements are significantly lower, relative to hydrogen, than in the solar system. The abundance deficiencies of certain elements correlate with the temperatures derived theoretically for particle condensation in stellar atmospheres or nebulae, suggesting that these elements have condensed into dust grains near stars. There is evidence that other elements have accreted onto such grains after their arrival in interstellar space. The extinction spectrum of zeta Oph can be explained qualitatively and, to a degree, quantitatively by dust grains composed of silicates, graphite, silicon carbide, and iron, with mantles composed of complex molecules of H, C, N, and O. This composition is consistent with the observed gas-phase deficiencies.
Field, G. B.↗
Possible sources of the Population I lithium abundance and light-element evolution
One-zone numerical models of Galactic chemical evolution of the light elements (lithium, beryllium, boron, and deuterium) with a broad sample of possible stellar lithium production sites and star formation histories, including the multiple merger model of Mathews and Schramm (1992), are examined. Models with high primordial lithium are constrained by observations of lithium and potassium in the interstellar medium of the LMC to have Li abundances close to the Population I value of about 10 exp -9. Li-7 production in intermediate- or high-mass stars (greater than 4 solar masses) is found to fit observations somewhat better than production in low-mass (1-5 solar masses) stars. Since elevated levels of lithium are commonly observed in intermediate-mass stars in the LMC, it is argued that this is indeed the major source of the Population I Li-7 abundance.
Brown, Lawrence E.↗
Heavy-metal Abundances in DO-type White Dwarfs
Spectral lines of a variety of trans-iron elements were identified in high-resolution ultraviolet spectra of DO-type white dwarfs. Abundance determinations by means of non-local thermodynamic equilibrium stellar-atmosphere techniques have shown that, without exception, their abundances are unexpectedly strong supersolar, much higher than predicted by asymptotic-giant-branch nucleosynthesis calculations. We present our analyses and discuss the photospheric trans-iron element prominence.
Thomas Rauch↗
Abundances in the metal poor dwarf Ross 451
High dispersion echelle spectra of the high velocity subdwarf Ross 451 (= G236-080) were obtained using the 4-m telescope at Kitt Peak. Initial abundance determinations for six elements are presented, using absolute oscillator strengths and metal-poor stellar-atmosphere models.
Spiesman, William J.↗
Volatiles in interplanetary dust particles - A review
The paper presents a review of the volatiles found within interplanetary dust particles. These particles have been shown to represent primitive material from early in the solar system's formation and also may contain records of stellar processes. The organogenic elements (i.e., H, C, N, O, and S) are among the most abundant elements in our solar system, and their abundances, distributions, and isotopic compositions in early solar system materials permit workers to better understand the processes operating early in the evolutionary history of solar system materials. Interplanetary dust particles have a range of elemental compositions, but generally they have been shown to be similar to carbonaceous chondrites, the solar photosphere, Comet Halley's chondritic cores, and matrix materials of chondritic chondrites. Recovery and analysis of interplanetary dust particles have opened new opportunities for analysis of primitive materials, although interplanetary dust particles represent major challenges to the analyst because of their small size.
Gibson, Everett K., Jr.↗
The development of the red giant branch. I - Theoretical evolutionary sequences
A grid of 100 evolutionary sequences extending from the zero-age main sequence to the onset of helium burning has been computed for stellar masses between 1.4 and 3.4 solar masses, helium abundances of 0.20 and 0.30, and heavy-element abundances of 0.004, 0.01, and 0.04. Using these computations the transition in the morphology of the red giant branch (RGB) between low-mass stars, which have an extended and luminous first RGB phase prior to helium ignition, and intermediate-mass stars, which do not, is investigated. Extensive tabulations of the numerical results are provided to aid in applying these sequences. The effects of the first dredge-up on the surface helium and CNO abundances of the sequences is discussed.
Sweigart, Allen V.↗
Nucleosynthesis Predictions for Intermediate-Mass AGB Stars: Comparison to Observations of Type I Planetary Nebulae
Type I planetary nebulae (PNe) have high He/H and N/O ratios and are thought to be descendants of stars with initial masses of approx. 3-8 Stellar Mass. These characteristics indicate that the progenitor stars experienced proton-capture nucleosynthesis at the base of the convective envelope, in addition to the slow neutron capture process operating in the He-shell (the s-process). We compare the predicted abundances of elements up to Sr from models of intermediate-mass asymptotic giant branch (AGB) stars to measured abundances in Type I PNe. In particular, we compare predictions and observations for the light trans-iron elements Se and Kr, in order to constrain convective mixing and the s-process in these stars. A partial mixing zone is included in selected models to explore the effect of a C-13 pocket on the s-process yields. The solar-metallicity models produce enrichments of [(Se, Kr)/Fe] less than or approx. 0.6, consistent with Galactic Type I PNe where the observed enhancements are typically less than or approx. 0.3 dex, while lower metallicity models predict larger enrichments of C, N, Se, and Kr. O destruction occurs in the most massive models but it is not efficient enough to account for the greater than or approx. 0.3 dex O depletions observed in some Type I PNe. It is not possible to reach firm conclusions regarding the neutron source operating in massive AGB stars from Se and Kr abundances in Type I PNe; abundances for more s-process elements may help to distinguish between the two neutron sources. We predict that only the most massive (M grester than or approx.5 Stellar Mass) models would evolve into Type I PNe, indicating that extra-mixing processes are active in lower-mass stars (3-4 Stellar Mass), if these stars are to evolve into Type I PNe.
Karakas, Amanda I.↗
Light-element abundances in Population II dwarfs
This paper reviews the abundances of the light elements lithium, beryllium, and boron in main-sequence and subgiant, Population II stars. Li is important to cosmology because it is synthesized in the Big Bang, but is also used to study stellar structure. Beryllium is useful for studying galactic chemical evolution because its formation in the interstellar medium involves different physics to stellar nucleosynthesis, and it thus provides independent data on the evolution of the halo. Some (though not all) inhomogeneous Big Bang nucleosynthesis codes predict a significant primordial component to this element, so its observed abundance may constrain such models. Boron, observations of which became feasible with the operation of the Hubble Space Telescope, provides complementary data to Be, helping check the element ratios predicted by calculations of spallation reactions in the interstellar medium, and will indicate whether the observed Be abundance has an excess over the expected spallation component, indicating a possible primordial component.
Ryan, Sean G.↗