X Process Research Articles

Photochemical Mass-Independent Sulfur Isotopes in Achondritic Meteorites

Sulfides from four achondrite meteorite groups are enriched in 33S (up to 0.040 per mil) as compared with primitive chondrites and terrestrial standards. Stellar nucleosynthesis and cosmic ray spallation are ruled out as causes of the anomaly, but photochemical reactions in the early solar nebula could produce the isotopic composition. The large 33S excess present in oldhamite from the Norton County aubrite (0.161 per mil) suggests that refractory sulfide minerals condensed from a nebular gas with an enhanced carbon-oxygen ratio, but otherwise solar composition is the carrier. The presence of a mass-independent sulfur effect in meteorites argues for a similar process that could account for oxygen isotopic anomalies observed in refractory inclusions in primitive chondrites.

The Search for Supernova Signatures in an Ice Core

It has been suggested that ice cores may preserve detectable enhancements of some terrestrially rare radioisotopes, 10Be, 26Al, 36Cl, resulting from a near-Earth core-collapse supernova(SN) [J. Ellis, B.D. Fields and D.N. Schramm, Astrophys. J. 470 (1996) 1227]. Both 10Be and 36Cl are also produced by atmospheric cosmic ray spallation and hence are influenced by processes that modulate the Earth's cosmic ray flux. Previous studies [G.M. Raisbeck, F. Yiou, D. Bourles, C. Lorius, J. Jouzel and N. I. Barkov, Nature 326 (1987) 273], [L.G. Thompson, T. Yao, M.E. Davis, K.A. Henderson, E. Mosley-Thompson, P.-N. Lin, J. Beer, H.-A. Synal, J. Cole-Dai and J.F. Bolzan, Science 276 (1997) 1821] have suggested that enhancements occurred in the 10Be and 36Cl fluxes at ∼35 ky and at ∼60 ky for 10Be. Thus we have searched for potential SN condensates with 26Al amongst grains filtered from the 308.6m Guliya ice core recovered from the Qinghai-Tibetan plateau in China [L.G. Thompson, T. Yao, M.E. Davis, K.A. Henderson, E. Mosley-Thompson, P.-N. Lin, J. Beer, H.-A. Synal, J. Cole-Dai and J.F. Bolzan, Science 276 (1997) 1821]. We searched for potential core-collapse SN condensate grains corundum (Al2O3), hibonite (CaAl12O19) and spinel (MgAl2O4) (see [D.S. Ebel and L. Grossman, Geochim. Cosmochim. Acta 65 (2001) 469]) in Guliya grain samples from the following time periods: ∼2−10 ky, ∼25−27 ky, ∼34−36 ky, ∼53−57 ky, ∼59−62 ky and ∼68−72 ky. These minerals are rare among terrestrial rocks and fine-grained atmospheric dust of terrestrial origin. Furthermore, they are insoluble in the acids employed in the sample preparation process and therefore separable from other minerals, such as silicates, that have high terrestrial abundances. Candidate SN condensate grains were identified among their terrestrial diluents employing a procedure developed at the University of Chicago for detecting presolar grains in meteoritic samples [S. Amari, R.S. Lewis and E. Anders, Geochim. Cosmochim. Acta 58 (1994) 459]. A set of 37 grains from the ∼34−36 ky, ∼53−57 ky and ∼59−62 ky samples were analyzed with the NanoSIMS at Washington University to measure their oxygen isotopic ratios. The preliminary results indicate that the analyzed grains, representing < 15% of those identified, do not possess the extreme O isotopic ratios expected to characterize a SN source [S. Amari and E. Zinner, Nucl. Phys. A 621 (1997) 99c], [T. Rauscher, A. Heger, R.D. Hoffman and S.E. Woosley, Astrophys. J. 576 (2002) 323].

Nitrogen in chondritic metal

We report new nitrogen isotopic data in metals of H-, L- and one LL -chondrites, with N abundances in the range of ∼0.3 to 3.3 ppm and half of these <1 ppm. Nitrogen isotopic signatures in metals with low indigenous N concentrations are modified by cosmic ray spallation components; corrections are required to determine the indigenous N signatures. The metals of type 4 and 5 show uniform indigenous nitrogen (δ 15N = −6.8 ± 0.5 ‰) and confirm a reported possible genetic association of chondritic metal with metal in IIE and IVA iron meteorites. Distinct isotopic signatures are observed in two metal samples of the Portales Valley (H6) meteorite which both are inconsistent with signatures in H4 and H5 chondrites, but possibly reveal a record of impact-induced melting and metamorphism on the parent asteroid. Anomalous nitrogen signatures in metals of type 3 chondrites, on the other hand, may reflect residues of surviving presolar isotopic signatures.

Beryllium in turnoff stars of NGC 6397: Early Galaxy spallation, cosmochronology and cluster formation

We present the first detection of beryllium in two turnoff stars of the old, metal-poor globular cluster NGC 6397. The beryllium lines are clearly detected and we determine a mean beryllium abundance of log (Be/H) = −12.35 ± 0.2. The beryllium abundance is very similar to that of field stars of similar Fe content. We interpret the beryllium abundance observed as the result of primary spallation of cosmic rays acting on a Galactic scale, showing that beryllium can be used as a powerful cosmochronometer for the first stellar generations. With this method, we estimate that the cluster formed 0.2-0.3 Gyr after the onset of star formation in the Galaxy, in excellent agreement with the age derived from main sequence fitting. From the same spectra we also find low O (noticeably different for the two stars) and high N abundances, suggesting that the original gas was enriched in CNO processed material. Our beryllium results, together with the N, O, and Li abundances, provide insights on the formation of this globular cluster, showing that any CNO processing of the gas must have occurred in the protocluster cloud before the formation of the stars we observe now. We encounter, however, difficulties in giving a fully consistent picture of the cluster formation, able to explain the complex overall abundance pattern.

Cosmogenic 7Be deposition in North Wales: 7Be concentrations in sheep faeces in relation to altitude and precipitation

Beryllium-7 ( 7Be) is a cosmogenic radionuclide with a half-life of 53.3 days produced mostly in the stratosphere by cosmic ray spallation of nitrogen and oxygen and entering the lower troposphere by atmospheric circulation processes. Most of the nuclide is removed by rainout during precipitation, so given that rainfall generally increases with altitude, it was considered probable that 7Be deposition would also be greater at higher altitudes. The aim of this study was to determine if there is any relationship between 7Be concentrations, altitude and precipitation by measuring 7Be activity within sheep faeces. An area of North Wales and Northwest England was selected for study, extending from the Dee Estuary at sea level to Snowdon at 1065 m. The results obtained showed a significant increase in 7Be activity with increasing altitude and precipitation consistent with predictions based on the existing literature.

Boron Benchmarks for the Galactic Disk

Sixteen Population I solar-type dwarfs have been selected to ascertain the baseline B abundance in the Galactic disk for a range of a factor of 4 in metallicity: from [Fe/H] of -0.5 to +0.1. All the stars selected are undepleted in Be, which ensures that they have also retained their full initial abundance of B. Evaluation of the trend of B with Fe provides a means to study the evolution of B in the Galactic disk. We observed 16 bright stars around the B I 2497 A line, using the STIS echelle spectrograph on HST. New observations of Li and Be in some stars were made, and previous abundance studies of Li and Be in these stars were reevaluated using revised parameters and a modified spectral synthesis code for consistency with the B measurements. Abundances of B were calculated by spectrum synthesis with the revised MOOG code, which accounts for the increased opacity in the UV due to metals; the LTE B abundances were then corrected for non-LTE effects. Four additional stars with undepleted Be have HST B observations, which increase our sample to 20. For these disk stars there is a shallow slope for B versus Fe and Be versus Fe, such that as Fe increases by a factor of 4, B and Be increase by 1.7 times. The slope for BLTE versus Fe is 0.31 ± 0.09, for BNLTE versus Fe 0.40 ± 0.12, and for Be versus Fe 0.38 ± 0.14. We have estimated the effect of additional UV opacity from Mg and find that an increase of 0.3 dex in Mg results in a higher B abundances by 0.1 dex for all the disk stars. Individual stars are not consistently above (or below) the mean in both B and Be, implying that the star-to-star differences are not due to variations in the elemental content of the natal clouds. We find that the trend of B abundance with [Fe/H] is consistent with the general trend observed in halo stars. If we connect the halo and disk stars, then an increase in the Fe abundance by 103 is accompanied by increases of 100 times in B and 550 times in Be. However, fitting two separate relations for the disk and the halo stars results in a somewhat steeper slope for Be for the halo stars (1.08 ± 0.07) relative to the disk stars (0.38 ± 0.14). This is the case for B also in LTE, with Bhalo (0.90 ± 0.07) versus Bdisk (0.32 ± 0.12). However, the NLTE B abundance increases more slowly for halo stars than the Be abundance does; since this is not predicted by light-element synthesis or depletion, we suggest that a full NLTE analysis would be preferable to making the (small) corrections to the LTE abundances. Some of the lowest metallicity stars are thought to have only upper limits on the B abundance; if that is the case, the NLTE B slope is steeper, nearing 1.0. The abundance of B in the disk stars is observed to be a factor of ~15 more than the abundance of Be in these stars, a result consistent with the predictions of Galactic cosmic-ray (GCR) spallation, B/Be = 15 ± 5. The upper envelope for Li versus Fe yields Li/B and Li/Be ratios that, when coupled with models and predictions, indicate that 20%-45% of Li might be produced by GCRs. While there is no evidence to support the production of B by neutrino spallation, we cannot rule it out.

Application of129I as an environmental tracer

129I is produced naturally by cosmic-ray spallation of Xe and by spontaneous fission of 238U. In the environment 129I is mainly due to nuclear weapon tests and reprocessing plants. The high water solubility of iodine makes 129I a good oceanographic tracer. Long residence time and large air releases of 129I from reprocessing plants allow 129I to be used as a geochemical and metrological tracer. The same chemical and physical properties of 129I and 131I enable one to use 129I as a tool for the reconstruction of 131I doses after a nuclear accident. Some studies using 129I as a tracer which were carried out in the author's laboratory, are summarized.

The solar boron, stellar lithium and deuterium, interstellar deuterium, and extragalactic deuterium abundances

D, Li, Be, and B are not by formed by ordinary stellar nucleosynthesis. D is primarily formed in the big bang. Because each generation of stars replenished the ISM with material depleted in D, the D abundance decreases with time and is larger in low-metallicity regions. Because Li, Be and B are primarily formed via cosmic-ray spallation reactions, their abundances will increase with time. Some Li and B is produced in supernovae via ν-spallation reactions and some Li is produced via mass loss from Li-rich AGB stars. To determine if the B abundance has increased during past 4.5 Gyr, an accurate Solar B abundance will be determined from ongoing observations of the 1.6 μ lines of B. Initial negative results yield B/H < 3.5 × 10 −10. The Balmer D α line was not detected in the high-metallicity star HD 82943; the low-metallicity Pop II halo star HD 140283 (both with detected 6Li); or in the slowly rotating B stars τ Her and γ Peg with D/H < 1.0 × 10 −5. Observations of Li in super-Li-rich AGB C and S stars with strong Li lines confirms that these stars have the largest Li abundance in the Galaxy (Li/H = 10 −7 and that mass loss from these stars may contribute to the ISM Li abundance. Observations of the DCN/HCN ratio in the Galaxy yield D/H < 1.4 × 10 −6 in the Galactic Center molecular clouds (10 pc from the center) and a positive D/H gradient in the Galaxy implying that there are no Galactic sources of D and D is cosmological. Extragalactic D (in DCN) in currently being searched for in two gravitational lenses against quasars at z = 0.7 and 0.9 and in the Seyfert Galaxy NGC 1068.

An Ultra–high‐Resolution Survey of the Interstellar7Li/6Li Isotope Ratio in the Solar Neighborhood

In an effort to probe the extent of variations in the interstellar 7Li/6Li ratio seen previously, ultra-high-resolution (R ~ 360,000), high signal-to-noise spectra of stars in the Perseus OB2 and Scorpius OB2 associations were obtained. These measurements confirm our earlier findings of an interstellar 7Li/6Li ratio of about 2 toward o Per, the value predicted from models of Galactic cosmic-ray spallation reactions. Observations of other nearby stars yield limits consistent with the isotopic ratio of ~12 seen in carbonaceous chondrite meteorites. If this ratio originally represented the gas toward o Per, then to decrease the original isotope ratio to its current value an order of magnitude increase in the Li abundance is expected, but it is not seen. The elemental K/Li ratio is not unusual, although Li and K are formed via different nucleosynthetic pathways. Several proposals to account for the low 7Li/6Li ratio were considered, but none seems satisfactory. Analysis of the Li and K abundances from our survey highlighted two sight lines where depletion effects are prevalent. There is evidence for enhanced depletion toward X Per, since both abundances are lower by a factor of 4 when compared to other sight lines. Moreover, a smaller Li/H abundance is observed toward 20 Aql, but the K/H abundance is normal, suggesting enhanced Li depletion (relative to K) in this direction. Our results suggest that the 7Li/6Li ratio has not changed significantly during the last 4.5 billion years and that a ratio of ~12 represents most gas in the solar neighborhood. In addition, there appears to be a constant stellar contribution of 7Li, indicating that one or two processes dominate its production in the Galaxy.

Antiprotons from spallation of cosmic rays on ISM

Antiprotons from spallation of cosmic rays on ISM

Cosmic‐Ray Production of6Li by Structure Formation Shocks in the Early Milky Way: A Fossil Record of Dissipative Processes during Galaxy Formation

While the abundances of Be and B observed in metal-poor halo stars are well explained as resulting from spallation of CNO-enriched cosmic rays (CRs) accelerated by supernova shocks, accounting for the observed 6Li in such stars with supernova CRs is more problematic. Here we propose that gravitational shocks induced by infalling and merging subgalactic clumps during hierarchical structure formation of the Galaxy should dissipate enough energy at early epochs so that the CRs accelerated by such shocks can provide a natural explanation of the observed 6Li. In clear contrast to supernovae, structure formation shocks do not eject freshly synthesized CNO nor Fe, so that ?-? fusion, which is the only effective production channel at low metallicity, is capable of generating sufficient 6Li with no accompanying Be or B and no direct correspondence with Fe. Correlations between the 6Li abundance and the kinematic properties of the halo stars may also be expected in this scenario. Further, more extensive observations of 6Li in metal-poor halo stars, e.g., by the Subaru High Dispersion Spectrograph or the Very Large Telescope UV-Visual Echelle Spectrograph, may offer us an invaluable fossil record of dissipative dynamical processes that occurred during the formation of our Galaxy.

Antiprotons from spallation of cosmic rays on ISM

We provide the first evaluation of the secondary interstellar cosmic antiproton flux that is fully consistent with cosmic ray nuclei in the framework of a two-zone diffusion model. We also study and conservatively quantify all possible sources of uncertainty that may affect that antiproton flux. Uncertainties related to propagation are shown to range between 10% and 25%, depending on which part of the spectrum is considered, while the ones related to nuclear physics stand around 22–25 % over all the energy spectrum.

Light Elements Abundances: New Insights on Stellar Mixing and Galactic Production

The remarkable finding that B follows Fe in almost direct proportion from the earliest times to the present, with little change of slope (if any) between halo and disk metallicities (cf Figure 1, open circles, Duncan et al. 1997), has stimulated the need for a revision of the classical cosmic-ray (CR) spallation scenario. A straightforward interpretation of Duncan et al. results is that the net rate of production of B (and Be too, since it shows a similar trend) does not depend on the CNO abundances in the ISM. It has been suggested that the CR spallation most important for light element production is not primarily protons and alpha particles colliding with CNO nuclei in the ISM (Reeves et al. 1970; Meneguzzi et al. 1971). Rather, it is C and O nuclei colliding with ambient protons and alpha particles, probably in regions of massive star formation (Cassé et al. 1995). This decouples light element production from the metallicity of the ISM and results in the approximately linear relationships observed, but some aspects are difficult to justify (see Prantzos, this volume).

Environmental Chemistry of Beryllium-7

In addition to the stable isotope 9Be, Be is also formed as two cosmogenic isotopes of interest to earth scientists. Cosmogenic Be is formed primarily in the stratosphere from cosmic-ray spallation of oxygen and nitrogen, but some is produced in the troposphere and in situ on the surface of the earth. Production of cosmogenic Be was hypothesized by Peters (1955). Soon after, naturally occurring 7Be was identified in precipitation by Arnold and Al-Salih (1955) and essentially concurrently and independently by Goel et al. (1956). 10Be (T1/2 = 1.5 × 106 yr) was first detected by Arnold (1956) in marine sediment cores. The 10Be/7Be production ratio is approximately 0.5 (Lal and Peters 1967). After its formation in the atmosphere, cosmogenic Be adsorbs electrostatically to aerosols that may be washed out by precipitation and delivered to ecosystems. The amount of cosmogenic Be that reaches the surface of the earth is a function of production rate (cosmic-ray intensity), stratosphere-troposphere mixing, circulation and advection within the troposphere, and efficiency of removal from the troposphere (wet and dry deposition) (Feely et al. 1989). Since the discovery of cosmic-ray produced Be, numerous researchers have gathered data on its (1) production, (2) fluxes to the oceans and terrestrial ecosystems, (3) distribution and inventory in soils, snow, sediments, and vegetation, and (4) geochemical behavior. In 1955, Peters suggested that the relatively long half-life of 10Be could make it useful for quantifying Tertiary sedimentation rates and other surficial processes. The focus of this chapter is the short-lived radionuclide 7Be (T1/2 = 53.12±0.07 days; Jaeger et al. 1996). Due to its short half-life, relative ease of measurement, and well-defined source term, 7Be serves as a useful tool for tracing and quantifying environmental processes on the …

Evolution of Li, Be, and B in the Galaxy

In this paper we study the production of Li, Be, and B nuclei by Galactic cosmic-ray spallation processes. We include three kinds of processes: (1) spallation by light cosmic rays impinging on interstellar CNO nuclei (direct processes), (2) spallation by CNO cosmic-ray nuclei impinging on interstellar p and 4He (inverse processes), and (3) α-α fusion reactions. The latter dominate the production of 6,7Li. We calculate production rates for a closed box Galactic model, verifying the quadratic dependence of the Be and B abundances for low values of Z. These are quite general results and are known to disagree with observations. We then show that the multizone, multipopulation model that we used previously for other aspects of Galactic evolution produces quite good agreement with the linear trend observed at low metallicities without fine-tuning. We argue that reported discrepancies between theory and observations do not represent a nucleosynthetic problem but instead are the consequences of inaccurate treatments of Galactic evolution.

Beryllium-10 from the Sun.

Beryllium-10 (10Be) in excess of that expected from in situ cosmic ray spallation reactions is present in lunar surface soil 78481; its presence was revealed with a sequential leaching technique. This excess 10Be, representing only 0.7 to 1.1% of the total 10Be inventory, is associated with surface layers (<1 micrometer) of the mineral grains composing 78481. This excess 10Be and its association with surficial layers corresponds to (1.9 +/- 0.8) x 10(8) atoms per square centimeter, requiring a 10Be implantation rate of (2.9 +/- 1.2) x 10(-6) atoms per square centimeter per second on the surface of the Moon. The most likely site for the production of this excess (10)Be is the Sun's atmosphere. The 10Be is entrained into the solar wind and transported to the lunar surface.

A 33S enrichment in ureilite meteorites: evidence for a nebular sulfur component

Acid volatile sulfur extracted from ureilite meteorites carries a small 33S enrichment relative to carbonaceous chondrites, enstatite chondrites, ordinary chondrites, and troilite from iron meteorites: Δ 33S (=δ 33S − 1,000 × (1 δ 34S/1,000) 0.515− 1) = 0.042‰ ± 0.007‰ (standard error of 22 analyses). In situ production of sulfur by cosmic-ray spallation reactions involving Fe is unlikely to cause the enrichment because the ureilites have short cosmic-ray exposure ages, low Fe/S relative to the only documented phases that contain spallogenic sulfur (the metal phase in iron meteorites), and no corresponding 36S enrichment. Sulfur derived from cosmic-ray spallation has been documented in the metal phase in iron meteorites, and it is characterized by Δ 36S/Δ 33S ∼ 8, inconsistent with present observations. We argue that this enrichment derives from heterogeneity in the presolar nebula. A 33S enrichment in the presolar reservoir may derive from mixing among diverse nucleosynthetic sources or from mass-independent fractionations caused by gas-phase chemistry. In addition, several gas-phase reactions have been shown to produce mass-independent compositions for sulfur isotopes. One that both matches fractionations for all sulfur isotopes and is relevant to the presolar nebula has yet to be identified. An appropriate additive nucleosynthetic component has also not been identified.

The Galactic Evolution of Boron

Boron, together with lithium and beryllium, belongs to the group of the so-called light elements, the importance of which ranges from providing important tests to Big Bang nucleosynthesis scenarios to being useful probes of stellar interiors and useful tools to further constrain the chemical evolution of the Galaxy.Since it became operative in the late eighties, the Hubble Space Telescope (HST) and its high- and medium-resolution spectrographs have played a key role in analyzing boron. Boron has now been observed in several stars and in the interstellar medium (ISM), providing important information in different fields of astrophysical research (nucleosynthesis, cosmic-ray spallation, stellar structure). In particular, determinations of boron in unevolved stars of different metallicity have allowed to study how boron evolves with iron.After a general review of the current status of boron observations and of the major uncertainties affecting the measurements of its abundance, I will mainly concentrate on unevolved stars and discuss the ‘evolutionary’ picture emerging from the most recent analyses and how its interpretation compares with theoretical expectations. A brief discussion on future prospects will conclude this contribution, showing how the field may evolve and improve.

7Li in Metal-Poor Stars: The Spread of the Li Plateau

A highly homogeneous study of 23 halo field dwarf stars has achieved a Li abundance accuracy of 0.033 dex per star. The work shows that the intrinsic spread of the Li abundances of these stars at a given metallicity is &lt; 0.02 dex, and consistent with zero. That is, the Spite Li plateau for halo field dwarfs is incredibly thin. The thinness rules out depletion by more than 0.1 dex by a rotational-induced extra-mixing mechanism. Despite the thinness of the plateau, an increase of Li with [Fe/H] is seen, interpreted as evidence of Galactic chemical evolution (GCE) of Li, primarily due to Galactic cosmic ray (GCR) spallation reactions in the era of halo formation. The rate of Li evolution is concordant with: (1) observations of spallative 6Li in halo dwarfs; (2) GCE models; and (3) data on Li in higher metallicity halo stars. New data have also revealed four new ultra-Li-deficient halo dwarfs, doubling the number known. Based on their propensity to cluster at the halo main sequence turnoff and also to exist redward of the turnoff, we hypothesise that they are the products of binary mergers that ultimately will become blue stragglers. We explain their low Li abundances by normal pre-main-sequence (and possibly main-sequence) destruction in the low mass stars prior to their merging. If this explanation is correct, then such stars need no longer be considered an embarrassment to the existence of negligible Li destruction in the majority of field halo dwarfs.

Concluding Remarks II

The symposium has shown that our subject is well and very alive. It is progressing rapidly, thanks to a large amount of new observational data, obtained in particular by a young generation of competent astronomers. It is encouraging, incidentally, to note the large fraction of women in this generation. When I started in this field, some forty years ago, the feminine contribution was much smaller. I plan to review the state of the situation for each of the three nucleosynthesis processes responsible for the light elements: Big Bang Nucleosynthesis (BBN), galactic cosmic ray spallation of interstellar nuclei (GCR) and stellar nucleosynthesis. I will point out their successes, their riddles and possible avenues by which these riddles could be solved. I plan first to review the observations.