Nucleosynthesis and early chemical evolution in galaxies

Nucleosynthesis and galactic chemical evolution are inter-linked topics that merge various fields in astronomy and physics. Speakers at the RAS discussion meeting of January 2003 combined theory and observation in understanding these fields. Stelios A Tsangarides, Enrico Arnone and Sean G Ryan report.     

Mercury: the enigmatic innermost planet

The planet Mercury, a difficult object for study by astronomical observation and spacecraft exploration alike, poses multiple challenges to our general understanding of the inner planets. Mercury’s anomalously high uncompressed density implies a metal fraction of 60% or more by mass, an extreme outcome of planetary formational processes common to the inner solar system. Whether that outcome was the result of chemical gradients in the early solar nebula or removal by impact or vaporization of most of the silicate shell from a differentiated protoplanet can potentially be distinguished on the basis of the chemical composition of the present crust. Our understanding of the geological evolution of Mercury and how it fits within the known histories of the other terrestrial planets is restricted by the limited coverage and resolution of imaging by the only spacecraft to have visited the planet. The role of volcanism in Mercury’s geological history remains uncertain, and the dominant tectonic structures are lobate scarps interpreted as recording an extended episode of planetary contraction, issues that require global imaging to be fully examined. That Mercury has retained a global magnetic field when larger terrestrial planets have not stretches the limits of standard hydromagnetic dynamo theory and has led to proposals for a fossil field or for exotic dynamo scenarios. Hypotheses for field generation can be distinguished on the basis of the geometry of Mercury’s internal field, and the existence and size of a fluid outer core on Mercury can be ascertained from measurements of the planet’s spin axis orientation and gravity field and the amplitude of Mercury’s forced librations. The nature of Mercury’s polar deposits, suggested to consist of volatile material cold-trapped on the permanently shadowed floors of high-latitude impact craters, can be tested by remote sensing of the composition of Mercury’s surface and polar atmosphere. The extremely dynamic exosphere, which includes a number of species derived from Mercury’s surface, offers a novel laboratory for exploring the nature of the complex and changing interactions among the solar wind, a small magnetosphere, and a solid planet. Recent ground-based astronomical measurements and several new theoretical developments set the stage for the in-depth exploration of Mercury by two spacecraft missions within the coming decade.     

Factors governing subaqueous siliceous sinter precipitation in hot springs: examples from Yellowstone National Park, USA

Abstract Siliceous sinter precipitation within hot spring systems has been attributed to a variety of mechanisms: evaporative concentration, cooling, changes in pH and cation effects. Repetitive in situ (T, pH, alkalinity, etc.) and laboratory (major, minor and trace elemental, stable isotopic) analyses of the waters plus observations of silica precipitation on natural (e.g. twigs, pine cones) as well as artificial substrates (glass slides and copper plates) in the waters substantiate that subaqueous precipitation is occurring throughout the vent to distal end of flow in both Cistern Spring (Norris Geyser Basin) and Deerbone Spring (Lower Geyser Basin), Yellowstone National Park, Wyoming, USA. Quartz and sodium–potassium geothermometers indicate that Cistern Spring is fed by a subsurface reservoir that is between 232 and 272 °C. Calculated reservoir temperatures are significantly lower at Deerbone Spring (182–197 °C). Based on a suite of measured and theoretical saturation indices, downflow changes in the system resulting from evaporative concentration (e.g. Cl increases 10%), changes in pH (e.g. 5·6–7·1) and cation effects (Al and Fe) are of negligible importance in the subaqueous precipitation of hot spring opal‐A. Similarly, at the macroenvironmental scale, potential biotic effects on opal‐A precipitation appear to be minimal. Modelling of the two active siliceous sinter precipitating systems indicates that cooling (e.g. 80–17 °C) is the predominant process governing subaqueous mineral precipitation.     

The etiquette of state-building and modernisation in dependent states: performing stateness and the normalisation of separate development in South Africa

The paper reflects upon the continuing allure of state-building and modernisation in peripheral, `new' states and regions. The central argument is to suggest that our understanding of the conflicts surrounding `nation-building' can be improved upon by looking at one of the most powerful symbols of the neo-colonial, economic and institutional dependency and territorial artificiality of the `nation-state'- the South African Bantustan. As enforced state-hood projects, their `newness' and illegality in the eyes of the international community suggest that the Bantustans provide a particularly striking evocation of both the limits of state-building and, above all, the significance of recognition for `newer' states. Efforts to overturn the illegal status and non-recognition of one `independent' Bantustan, namely Bophuthatswana, therefore, illustrate what are widely held to be the modern attributes and evolutionary etiquette (`growth', `building', `development') that need to be `performed' in order to legitimise or normalise a state internationally. To be given a `voice' internationally, states are seemingly required to enact state-hood and sovereignty through a range of normalising rituals associated with state-building. In this case- study, strategies promoting neo-liberal development policies and also clandestine diplomatic and economic manoeuvres were believed by the regime to deliver international recognition. Directed at locating Bophuthatswana within its place in the world, these strenuous efforts also show how crude territorial `divide and rule' tactics alone were not responsible for underpinning the apartheid project. Rather, spaces were provided for discourses and practices of state-led `national' development in the Bantustan periphery. Thought to deliver normality for the renegade Bophuthatswana state, these international and local policies exacerbated and induced further a range of crises at the heart of the Bantustan strategy and apartheid modernisation more generally. These debilitating, and in Bophuthatswana's case, disastrous, tensions between the internal and external realms of state-building point to a more general need to rethink alternatives both to neo-liberalism as well as the rituals associated with state- building.     

Regime-dependent streamflow sensitivities to Pacific climate modes cross the Georgia–Puget transboundary ecoregion

The Georgia Basin–Puget Sound Lowland region of British Columbia (Canada) and Washington State (USA) presents a crucial test in environmental management due to its combination of abundant salmonid habitat, rapid population growth and urbanization, and multiple national jurisdictions. It is also hydrologically complex and heterogeneous, containing at least three streamflow regimes: pluvial (rainfall-driven winter freshet), nival (melt-driven summer freshet), and hybrid (both winter and summer freshets), reflecting differing elevation ranges within various watersheds. We performed bootstrapped composite analyses of river discharge, air temperature, and precipitation data to assess El Niño–Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) impacts upon annual hydrometeorological cycles across the study area. Canadian and American data were employed from a total of 21 hydrometric and four meteorological stations. The surface meteorological anomalies showed strong regional coherence. In contrast, the seasonal impacts of coherent modes of Pacific circulation variability were found to be fundamentally different between streamflow regimes. Thus, ENSO and PDO effects can vary from one stream to the next within this region, albeit in a systematic way. Furthermore, watershed glacial cover appeared to complicate such relationships locally; and an additional annual streamflow regime was identified that exhibits climatically driven non-linear phase transitions. The spatial heterogeneity of seasonal flow responses to climatic variability may have substantial implications to catchment-specific management and planning of water resources and hydroelectric power generation, and it may also have ecological consequences due to the matching or phase-locking of lotic and riparian biological activity and life cycles to the seasonal cycle. The results add to a growing body of literature suggesting that assessments of the streamflow impacts of ocean–atmosphere circulation modes must accommodate local hydrological characteristics and dynamics. Copyright © 2007 John Wiley & Sons, Ltd. The copyright in Paul H. Whitfield's contribution belongs to the Crown in right of Canada and such copyright material is reproduced with the permission of Environment Canada.     

Hierarchical probabilistic regionalization of volcanism for Sengan region,Japan

A 1 km square regular grid system created on the Universal Transverse Mercator zone 54 projected coordinate system is used to work with volcanism related data for Sengan region. The following geologic variables were determined as the most important for identifying volcanism: geothermal gradient, groundwater temperature, heat discharge, groundwater pH value, presence of volcanic rocks and presence of hydrothermal alteration. Data available for each of these important geologic variables were used to perform directional variogram modeling and kriging to estimate geologic variable vectors at each of the 23949 centers of the chosen 1 km cell grid system. Cluster analysis was performed on the 23949 complete variable vectors to classify each center of 1 km cell into one of five different statistically homogeneous groups with respect to potential volcanism spanning from lowest possible volcanism to highest possible volcanism with increasing group number. A discriminant analysis incorporating Bayes’ theorem was performed to construct maps showing the probability of group membership for each of the volcanism groups. The said maps showed good comparisons with the recorded locations of volcanism within the Sengan region. No volcanic data were found to exist in the group 1 region. The high probability areas within group 1 have the chance of being the no volcanism region. Entropy of classification is calculated to assess the uncertainty of the allocation process of each 1 km cell center location based on the calculated probabilities. The recorded volcanism data are also plotted on the entropy map to examine the uncertainty level of the estimations at the locations where volcanism exists. The volcanic data cell locations that are in the high volcanism regions (groups 4 and 5) showed relatively low mapping estimation uncertainty. On the other hand, the volcanic data cell locations that are in the low volcanism region (group 2) showed relatively high mapping estimation uncertainty. The volcanic data cell locations that are in the medium volcanism region (group 3) showed relatively moderate mapping estimation uncertainty. Areas of high uncertainty provide locations where additional site characterization resources can be spent most effectively. The new data collected can be added to the existing database to perform future regionalized mapping and reduce the uncertainty level of the existing estimations.     

Quantifying urbanization-associated changes in terrestrial hydrologic system memory

A common feature of watershed urbanization is increased hydrograph ‘flashiness,’ whereby river discharge fluctuations grow more erratic. Such changes might be intuitively interpreted as a decrease in watershed-scale hydrologic system memory. Here, I investigate this hypothesis through a paired-catchment experiment. The serial correlation coefficient, a common metric of short-term time series memory, is applied to daily winter streamflow data from urbanizing and rural watersheds in the Puget Sound lowland of Washington State, USA. Statistical comparisons confirm that this metric shows highly significant decreases over time in the catchment undergoing land use change, but not in the control watershed, which remains rural over the hydrometric record. Moreover, the mean serial correlation coefficients are statistically indistinguishable between the two catchments over the early period of record, when both watersheds are largely rural, whereas the system memory is far weaker in the urbanized stream relative to the rural stream over the late period, following land use change in the former. The results appear readily interpretable in terms of the physical hydrologic changes typically associated with urbanization. The serial correlation coefficient thus appears to be an instructive measure of urbanization impacts for small streams in this region.     

Reprocessed emission line profiles from dense clouds in geometrically thick accretion engines

The central engines of active galactic nuclei (AGN) contain cold, dense material as well as hot X-ray-emitting gas. The standard paradigm for the engine geometry is a cold thin disc sandwiched between hot X-ray coronae. Strong support for this geometry in Seyferts comes from the study of fluorescent iron line profiles, although the evidence is not ubiquitously airtight. The thin disc model of line profiles in AGN and in X-ray binaries should still be benchmarked against other plausible possibilities. One proposed alternative is an engine consisting of dense clouds embedded in an optically thin, geometrically thick X-ray-emitting engine. This model is also motivated by studies of geometrically thick engines such as advection-dominated accretion flows (ADAFs). Here we compute the reprocessed iron line profiles from dense clouds embedded in geometrically thick, optically thin X-ray-emitting discs near a Schwarzschild black hole. We consider a range of cloud distributions and disc solutions, including ADAFs, pure radial infall and bipolar outflows. We find that such models can reproduce line profiles similar to those from geometrically thin, optically thick discs and might help alleviate some of the problems encountered from the latter. Thus, independent of thin discs, thick disc engines can also exhibit iron line profiles if embedded dense clouds can survive long enough to reprocess radiation.