Showing papers on "Stellar nucleosynthesis published in 2005"

New light on stellar abundance analyses: Departures from LTE and homogeneity.

Abstract: ▪ Abstract The information on the chemical compositions of stars encoded in their spectra plays a central role in contemporary astrophysics. Stellar element abundances are, however, not observed: to decipher the spectral fingerprints in terms of abundances requires realistic models for the stellar atmospheres and the line-formation processes. Still today, the vast majority of abundance analyses of late-type stars rely on one-dimensional (1D), hydrostatic model atmospheres and the assumption of local thermodynamic equilibrium (LTE). In this review possible systematic errors in studies of F-, G- and K-type stars introduced by these questionable approximations are discussed. Departures from LTE are commonplace and often quite severe, in particular for low surface gravities or metallicities, with minority species and low-excitation transitions being the most vulnerable. Recently, time-dependent, 3D, hydrodynamical model atmospheres have started to be employed for stellar abundance purposes, with large differe...

A voyage from dark clouds to the early Earth

Abstract: Stellar nucleosynthesis of heavy elements, followed by their subsequent release into the interstellar medium, enables the formation of stable carbon compounds in both gas and solid phases. Spectroscopic astronomical observations provide evidence that the same chemical pathways are widespread both in the Milky Way and in external galaxies. The physical and chemical conditions—including density, temperature, ultraviolet radiation and energetic particle flux—determine reaction pathways and the complexity of organic molecules in different space environments. Most of the organic carbon in space is in the form of poorly-defined macromolecular networks. Furthermore, it is also unknown how interstellar material evolves during the collapse of molecular clouds to form stars and planets. Meteorites provide important constraints for the formation of our Solar System and the origin of life. Organic carbon, though only a trace element in these extraterrestrial rock fragments, can be investigated in great detail with sensitive laboratory methods. Such studies have revealed that many molecules which are essential in terrestrial biochemistry are present in meteorites. To understand if those compounds necessarily had any implications for the origin of life on Earth is the objective of several current and future space missions. However, to address questions such as how simple organic molecules assembled into complex structures like membranes and cells, requires interdisciplinary collaborations involving various scientific disciplines. Introduction Life in the Universe is the consequence of the increasing complexity of chemical pathways which led to stable carbon compounds assembling into cells and higher organisms.

Nucleosynthetic signatures of the first stars

Abstract: When HE010715240 was discovered in 2002 it was the most metal-deficient star known. (Astrophysicists use the term ‘metal’ for all elements bar hydrogen and helium.) It had an iron abundance 20 times lower than previously recorded, suggesting that here was a relic, a star formed soon after the Big Bang. Now a second ‘unevolved’ star has been discovered: HE132712326, with an iron abundance about half that of HE010715240. One low-metal star was a novelty; two is a new class of stellar object. The similarities (in C and N content) and contrasts (in Li and Sr) between these two stellar relics present challenges to theories of star formation and may lead to new discoveries about how the elements were synthesized in the first stars. The chemically most primitive stars provide constraints on the nature of the first stellar objects that formed in the Universe; elements other than hydrogen, helium and traces of lithium present within these objects were generated by nucleosynthesis in the very first stars. The relative abundances of elements in the surviving primitive stars reflect the masses of the first stars, because the pathways of nucleosynthesis are quite sensitive to stellar masses. Several models1,2,3,4,5 have been suggested to explain the origin of the abundance pattern of the giant star HE0107–5240, which hitherto exhibited the highest deficiency of heavy elements known1,6. Here we report the discovery of HE1327–2326, a subgiant or main-sequence star with an iron abundance about a factor of two lower than that of HE0107–5240. Both stars show extreme overabundances of carbon and nitrogen with respect to iron, suggesting a similar origin of the abundance patterns. The unexpectedly low Li and high Sr abundances of HE1327–2326, however, challenge existing theoretical understanding: no model predicts the high Sr abundance or provides a Li depletion mechanism consistent with data available for the most metal-poor stars.

Full computation of massive AGB evolution. I. The large impact of convection on nucleosynthesis

Abstract: It is well appreciated that the description of overadiabatic convection affects the structure of the envelopes of luminous asymptotic giant branch (AGB) stars in the phase of hot bottom burning (HBB). We stress that this important uncertainty in the modeling plays a role which is much more dramatic than the role which can be ascribed, e.g., to the uncertainty in the nuclear cross-sections. Due to the role tentatively attributed today to the HBB nucleosynthesis as the site of self-enrichment of Globular Clusters stars, it is necessary to explore the difference in nucleosynthesis obtained by different prescriptions for convection. We present results of detailed evolutionary calculations of the evolution of stars of intermediate mass during the AGB phase for the metallicity typical of the Globular Clusters that show the largest spread in CNO abundances (Z ∼ 10 -3 ). We follow carefully the nucleosynthesis at the base of the external convective region, showing that very different results can be obtained according to the presciption adopted to find out the temperature gradient within the instability regions. We discuss the uncertainties in the yields of the various chemical species and the role which these sources can play as polluters of the interstellar medium.

Effects of episodic gas infall on the chemical abundances in galaxies

Abstract: The chemical evolution of galaxies that undergo an episode of massive and rapid accretion of metal-poor gas is in- vestigated with models using both simplified and detailed nucleosynthesis recipes. The rapid decrease of the oxygen abundance during infall is followed by a slower evolution which leads back to the closed-box relation, thus forming a loop in the N/O-O/H diagram. For large excursions from the closed-box relation, the mass of the infalling material needs to be substantially larger than the gas remaining in the galaxy, and the accretion rate should be larger than the star formation rate. We apply this concept to the encounter of high velocity clouds with galaxies of various masses, finding that the observed properties of these clouds are indeed able to cause substantial effects not only in low mass galaxies, but also in the partial volumes in large massive galaxies that would be affected by the collision. Numerical models with detailed nucleosynthesis prescriptions are constructed. We assume star formation timescales and scaled yields that depend on the galactic mass, and which are adjusted to reproduce the average relations of gas fraction, oxygen abundance, and effective oxygen yield observed in irregular and spiral galaxies. The resulting excursions in the N/O-O/H diagram due to a single accretion event involving a high velocity cloud are found to be appreciable, which could thus provide a contribution to the large scatter in the N/O ratio found among irregular galaxies. Nonetheless, the N/O-O/H diagram remains an important indicator for stellar nucleosynthesis.

Early star formation in the Galaxy from beryllium and oxygen abundances

Abstract: We investigate the evolution of the star formation rate in the early Galaxy using beryllium and oxygen abundances in metal poor stars. Specifically, we show that stars belonging to two previously identified kinematical classes (the so-called "accretion" and "dissipative" populations) are neatly separated in the (O/Fe) vs. log (Be/H) diagram. The dissipative population follows the predictions of our model of Galactic evolution for the thick disk component, suggesting that the formation of this stellar population occurred on a timescale significantly longer (by a factor ∼5-10) than the accretion component. The latter shows a large scatter in the (O/Fe) vs. log (Be/H) diagram, probably resulting from the inhomogeneous enrichment in oxygen and iron of the protogalactic gas. Despite the limitation of the sample, the data suggest that the combined use of products of spallation reactions (like beryllium) and elemental ratios of stellar nucleosynthesis products (like (O/Fe)) can constrain theoretical models for the formation and early evolution of our Galaxy.

Extreme oxygen isotope ratios in the early Solar System

Abstract: The highest oxygen isotope ratios ever measured in the Galaxy have been recorded in silica-rich grains from the Murchison meteorite. The excess oxygen-18 and oxygen-17 could be the result of nucleosynthesis in a single exotic star in the early Solar System or evidence of irradiation by highly accelerated particles produced during an active early phase of the Sun. The origins of the building blocks of the Solar System can be studied using the isotopic composition of early planetary and meteoritic material. Oxygen isotopes in planetary materials show variations at the per cent level that are not related to the mass of the isotopes1,2; rather, they result from the mixture of components having different nucleosynthetic or chemical origins1,2,3. Isotopic variations reaching orders of magnitude in minute meteoritic grains are usually attributed to stellar nucleosynthesis before the birth of the Solar System, whereby different grains were contributed by different stars4,5. Here we report the discovery of abundant silica-rich grains embedded in meteoritic organic matter, having the most extreme 18O/16O and 17O/16O ratios observed (both ∼10-1) together with a solar silicon isotopic composition. Both O and Si isotopes indicate a single nucleosynthetic process. These compositions can be accounted for by one of two processes: a single exotic evolved star seeding the young Solar System6, or irradiation of the circumsolar gas by high energy particles accelerated during an active phase of the young Sun. We favour the latter interpretation, because the observed compositions are usually not expected from nucleosynthetic processes in evolved stars, whereas they are predicted by the selective trapping of irradiation products.

Interpretation of abundance ratios

Abstract: In this paper we discuss abundance ratios and their relation to stellar nucleosynthesis and other parameters of chemical evolution models, reviewing and clarifying the correct use of the observed abundance ratios in several astrophysical contexts. In particular, we start from the well-known fact that abundance ratios depend on stellar yields, initial mass function, and stellar lifetimes, and we show, by means of specific examples, that in some cases it is not correct to infer constraints on the contributions from different supernovae types (Ia, II), and particularly on different sets of yields, in the absence of a complete chemical evolution model taking into account stellar lifetimes. In spite of the fact that some of these results should be well known, we believe that it is useful to discuss the meaning of abundance ratios in the light of several recent claims based upon an incorrect interpretation of observed abundance ratios. In particular, the procedure, often used in the recent literature, of directly deriving conclusions about stellar nucleosynthesis just by relating abundance ratios to yield ratios implicitly assumes the instantaneous recycling approximation. This approximation is clearly not correct when one analyses the contributions of supernovae type Ia relative to supernovae type II as functions of cosmic time. In this paper we show that the uncertainty which arises from adopting this oversimplified procedure in a variety of astrophysical objects, such as elliptical galaxies, the intracluster medium, and high redshift objects, does not allow us to draw any firm conclusion, and that the differences between abundance ratios predicted by models with the instantaneous recycling approximation and models with detailed stellar lifetimes is of the same order as the differences between different sets of yields. On the other hand, if one is interested only in establishing the global metal production (e.g. galaxies plus intracluster medium) over the lifetime of the Universe, then the adoption of simplified arguments can be justified.

Implications of a new temperature scale for halo dwarfs on LiBeB and chemical evolution

Abstract: Big bang nucleosynthesis (BBN) and the cosmic baryon density from cosmic microwave background anisotropies together predict a primordial 7Li abundance a factor of 2-3 higher than that observed in galactic halo dwarf stars. A recent analysis of 7Li observations in halo stars, using significantly higher surface temperature for these stars, found a higher Li plateau abundance. These results go a long way toward resolving the discrepancy with BBN. Here we examine the implications of the higher surface temperatures on the abundances of Be and B that are thought to have been produced in galactic cosmic-ray nucleosynthesis by spallation of CNO together with Li (produced in α + α collisions). While the Be abundance is not overly sensitive to the surface temperature, the derived B abundances and more importantly the derived oxygen abundances are very temperature-dependent. If the new temperature scale is correct, the implied increased abundances of these elements pose a serious challenge to models of galactic cosmic-ray nucleosynthesis and galactic chemical evolution.

Supernova Neutrino Effects on r-Process Nucleosynthesis in Black Hole Formation

Abstract: Very massive stars with mass ≥8 M☉ culminate their evolution by supernova explosions, which are presumed to be the most viable candidates for the astrophysical sites of r-process nucleosynthesis. If the models for the supernova r-process are correct, then the results of nucleosynthesis could also put a significant constraint on the remnants of supernova explosions, i.e., a neutron star or black hole. In the case of very massive core collapse for a progenitor mass 20-40 M☉, a remnant stellar black hole is thought to be formed. Intense neutrino flux from the neutronized core and the neutrinosphere might suddenly cease during the Kelvin-Helmholtz cooling phase because of the black hole formation. It is important and interesting to explore the observable consequences of such a neutrino flux truncation. It has recently been argued in the literature that even the neutrino mass can be determined from the time delay of the deformed neutrino energy spectrum after the cessation of neutrino ejection (neutrino cutoff effect). Here we study the expected theoretical response of the r-process nucleosynthesis to the neutrino cutoff effect in order to look for another independent signature of this phenomenon. We found a sensitive response of the r-process yield if the neutrino cutoff occurs after the critical time when the expanding materials in the neutrino-driven wind drop out of nuclear statistical equilibrium (NSE). The r-process nucleosynthesis yields drastically change if the cutoff occurs during the r-process, having maximal effect on the change in abundance of 232Th and 235,238U. There is a large probability of finding this effect in elemental abundances of r-process-enhanced metal-deficient halo stars whose chemical composition is presumed to be affected by Population III supernovae in the early Galaxy. Using this result, connected with future detection of the time variation of the SN neutrino spectrum, we are able to identify when the black hole formation occurs in the course of SN collapse.

Origin of Short-lived Radionuclides in the Early Solar System

Abstract: Evidence for the presence of about a dozen short-lived now-extinct radionuclides in the early solar system has been found in meteorites. The half-lives of these nuclides range from 100,000 years to more than a hundred million years. Three plausible modes of origin for these nuclides have been proposed. Some of them, particularly those with half-life more than several million years, could be products of continuous galactic nucleosynthesis, while others could be freshly synthesized stellar products injected into the protosolar molecular cloud or products of energetic particle interactions taking place in a presolar or early solar environment. The inferred abundances of these short-lived nuclides have been used to delineate time scales of processes taking place during the formation and early evolution of the solar system. In particular, inferences have been made about the time interval between the last addition of stellar nucleosynthesis products to the protosolar cloud and the formation of solar system objects, the time scale for the collapse of the protosolar cloud, the time interval between formation of various early solar system objects such as the Ca-Al-rich inclusions, chondrules and differentiated meteorites and also about the energetic particle environment in the early solar system. These inferences have strongly molded our current understanding of the origin and early evolution of the solar system. However, some of these inferences depend critically on our knowledge regarding the origin of short-lived radionuclides in the early solar system. It appears that different sets of nuclides may have different origins and some of them may have contributions from more than one source. In this chapter, we summarize the present status in this field and some of the robust conclu-

Nucleosynthesis of Fe60 in massive stars

Abstract: We discuss at some extent the production of Fe60 in massive stars in the range between 11 and 120 Msun both in the hydrostatic and explosive stages. We also compare the Fe60/Al26 gamma-ray line flux ratio obtained according to the present calculations to the detected value reported by INTEGRAL/SPI.

Physics of supernovae

Abstract: The origin of cosmic rays (CR) is supposed to be closely connected with supernovae (SNe) which create the conditions favorable for various mechanisms of the CR acceleration to operate effectively. First, modern ideas about the physics of the SN explosion are briefly discussed: the explosive thermonuclear burning in degenerate white dwarfs resulting in Type Ia SNe and the gravitational collapse of stellar cores giving rise to other types of SNe (Ib, Ic, IIL, IIP). Nex t, we survey some global properties of the SNe of different types: the total explosion energy distribu tion of various components (kinetic energy of the hydrodynamic flow, electromagnetic radiation, tempo ral behavior of the neutrino emission and individual energies of different neutrino flavors). Then, w e discuss in the possibility of direct hydrodynamic acceleration by the shock wave breakout and the properties of the SN shocks in the circumstellar medium. Then the properties of the neutrino radiation from the core-collapse SNe and a possibility to incorporate both the LSD Mont Blanc neutrino event and that recorded by the K II and IMB detectors into a single scenario are described in detail. Finally, the issues of the neutrino nucleosynthesis and of the connection between supernova and gamma-ray bursts are discussed.

The s-process Nucleosynthesis

Abstract: Theoretical as well as observational aspects of the s-process nucleosynthesis are reviewed. The classical site-independent s-process model as well as the s-process in massive stars are shortly described. A special attention is paid to the nucleosynthesis taking place in AGB stars and the extra-mixing invoked to explain the production of neutrons in the C-rich layers during the interpulse. We also discuss the nucleosynthesis found in hot AGB stars for which the s-process during the interpulse phase is inhibited, but the one resulting from the large temperatures in the thermal pulse is boosted. We comment on the uncertainties affecting our understanding of the physical mechanisms responsible for a successful s-process. Finally, various types of spectroscopic observations of s-process elements are discussed. © 2005 International Astronomical Union.

Astrophysics: the process of carbon creation.

Abstract: In the Universe, the element carbon is created only in stars, in a remarkable reaction called the triple-α process. Fresh insights into the reaction now come from the latest experiments carried out on Earth. In 1953 Fred Hoyle predicted that in the extreme conditions prevailing in the centre of stars, three α-particles (helium nuclei) can combine to form an excited form of carbon-12. This type of stellar nucleosynthesis is now recognized as the source of heavy elements in the Universe. Surprisingly there is still considerable uncertainty about the nature of the reaction that produces carbon-12. Fynbo et al. have now obtained a much more accurate measure of the reaction rate for carbon-12 synthesis than previously available, by measuring the reverse reaction rate. At temperatures found in young stars the new reaction rate is roughly twice the previous value, but at the 109 K found in supernovae, nucleosynthesis is much slower than was thought.

Lithium and Big-Bang Nucleosynthesis

Abstract: Since the discovery of the “Spite plateau” in 1982, lithium observations in halo stars have been used to deduce the primordial discrepancy.

Neon and Oxygen Abundances in M33

Abstract: We present new spectroscopic observations of 13 H II regions in the Local Group spiral galaxy M33. The regions observed range from 1 to 7 kpc in distance from the nucleus. Of the 13 H II regions observed, the [O III] 4363 Angstrom line was detected in six regions. Electron temperatures were thus able to be determined directly from the spectra using the [O III] 4959,5007 A/4363 A line ratio. Based on these temperature measurements, oxygen and neon abundances and their radial gradients were calculated. For neon, a gradient of -0.016 +/- 0.017 dex/kpc was computed, which agrees with the Ne/H gradient derived previously from ISO spectra. A gradient of -0.012 +/- 0.011 dex/kpc was computed for O/H, much shallower than was derived in previous studies. The newly calculated O/H and Ne/H gradients are in much better agreement with each other, as expected from predictions of stellar nucleosynthesis. We examine the correlation between the WC/WN ratio and metallicity, and find that the new M33 abundances do not impact the observed correlation significantly. We also identify two new He II-emitting H II regions in M33, the first to be discovered in a spiral galaxy other than the Milky Way. In both cases the nebular He II emission is not associated with Wolf-Rayet stars. Therefore, caution is warranted in interpreting the relationship between nebular He II emission and Wolf-Rayet stars when both are observed in the integrated spectrum of an H II region.

Nucleosynthesis of pair-instability supernovae

Abstract: The first generation of stars to form in the universe may have been very massive, and, due to the absence of initial metals, they could have retained most of their mass until their death and thus explode as pair instability supernovae. These supernovae encounter the late burning phases beyond carbon burning in an implosive/explosive way, leading to very powerful thermonuclear-powered explosions, up to a hundred times more powerful than ordinary supernovae. For primordial stars, these explosions also produce a peculiar abundance pattern, showing a strong odd-even pattern in the elemental abundances, a sharp drop-off of nucleosynthetic production beyond the iron group, and no r-process contribution. These results are greatly altered if only a small mass of 14N is dredged down into the helium burning core before the star becomes unstable. Such mixing could be a consequence of differential rotation or convective overshooting.

Nucleosynthesis and Galactic Chemical Evolution of the Isotopes of Oxygen

Abstract: Introduction: The stable isotopes of oxygen are important diagnostics of stellar nucleosynthesis and Galactic chemical evolution. This is primarily due to the fact that O is a principal product of stellar evolution and is therefore very abundant in the Galaxy. It is also due to the fact that O is a primary isotope while O and O are secondary isotopes. Nucleosynthesis: O is primarily produced at the end of helium burning in stars. C produced by the triple-alpha reaction captures another He to produce O. This means that the interplay of the triple-alpha reaction and C(alpha,gamma)O determines the ratio of C to O in the star after helium burning, which, in turn, governs the subsequent stellar evolution (e.g., [1]). Experimental determination of the C(alpha,gamma)O reaction rate is difficult and is the subject of intense study (e.g., [2]). O abundance is increased further during neon burning. Because O can be produced by stars initially composed only of hydrogen, it is a primary isotope. It is worth noting that O is, in fact, one of the dominant products of massive stars. For example, one may consider a model of a star 25 times the mass of the Sun [3]. This model began with about 0.23 solar masses of O but ejected 3.24 solar masses of that isotope. By contrast, O and O are secondary isotopes, which means their production requires pre-existing seed nuclei. O is dominantly produced by CNO burning of hydrogen into helium and is thus a prevalent isotope in hydrogen burning shells in stars. O is primarily made when abundant N, left over from CNO burning, captures He. This means O is abundant in helium-rich zones in stars. Because O and O production requires helium burning while O only requires hydrogen burning, low-mass stars may contribute more significantly to the synthesis of O than to O or O. Galactic Chemical Evolution: Since O is a primary isotope, it was produced in the first generation of stars. Observations of very metal-poor stars show the rise of oxygen with metallicity in the early Galaxy (e.g., [4]). The primary nature of the nucleosynthesis of O means that this rise is roughly linear in time. By contrast, the secondary nucleosynthesis of O and O means that the abundance of these isotopes in the Galaxy will rise roughly quadratically with time in a chemical evolution model (e.g., [5]). Such evolution is evident in Figure 1, which shows the evolution of the mass fraction of the oxygen isotopes in the interstellar medium from a standard Galactic chemical evolution model. The figure was generated with the Clemson University online Galactic Chemical Evolution code available at the web site

Post‐AGB Stars and PNe: Crucial Probes of Stellar Nucleosynthesis

Abstract: Stellar nucleosynthesis is the corner‐stone of many astrophysical problems. Its understanding leads to insights into the stellar structure and evolution and provides crucial clues to the physics of galaxies and of the Universe. Precise answers can be given to the questions ‘When, where and how are the chemical elements synthesized in stars?’ However, in spite of the observational confirmation of many predictions, important weaknesses remain in many crucial details of the global view. We discuss here some of the theoretical developments which are required in order to improve the nucleosynthesis predictions for low and intermediate mass stars. We conclude on the necessity to get new observational constraints from detailed abundance analysis of post‐AGB stars and of planetary nebulae.

Nucleosynthesis of popIII core collapse supernovae and the abundances in extremely metal poor stars

Abstract: We present a new analysis of the abundances observed in extremely metal poor stars based on both a new generation of theoretical presupernova models and explosions of zero metallicity massive stars and a new abundance analysis of an homogeneous sample of stars having $\rm [Fe/H]\leq -2.5$ (Cayrel et al. 2004).