A wide binary trigger for white dwarf pollution

Metal pollution in white dwarf atmospheres is likely to be a signature of remnant planetary systems. Most explanations for this pollution predict a sharp decrease in the number of polluted systems with white dwarf cooling age. Observations do not confirm this trend, and metal pollution in old (1-5 Gyr) white dwarfs is difficult to explain. We propose an alternative, time-independent mechanism to produce the white dwarf pollution. The orbit of a wide binary companion can be perturbed by Galactic tides, approaching close to the primary star for the first time after billions of years of evolution on the white dwarf branch. We show that such a close approach perturbs a planetary system orbiting the white dwarf, scattering planetesimals onto star-grazing orbits, in a manner that could pollute the white dwarf's atmosphere. Our estimates find that this mechanism is likely to contribute to metal pollution, alongside other mechanisms, in up to a few percent of an observed sample of white dwarfs with wide binary companions, independent of white dwarf age. This age independence is the key difference between this wide binary mechanism and others mechanisms suggested in the literature to explain white dwarf pollution. Current observational samples are not large enough to assess whether this mechanism makes a significant contribution to the population of polluted white dwarfs, for which better constraints on the wide binary population are required, such as those that will be obtained in the near future with Gaia.Comment: MNRAS accepted 10 page

Vega's hot dust from icy planetesimals scattered inward by an outward-migrating planetary system

Vega has been shown to host multiple dust populations, including both hot exo-zodiacal dust at sub-AU radii and a cold debris disk extending beyond 100 AU. We use dynamical simulations to show how Vega's hot dust can be created by long-range gravitational scattering of planetesimals from its cold outer regions. Planetesimals are scattered progressively inward by a system of 5-7 planets from 30-60 AU to very close-in. In successful simulations the outermost planets are typically Neptune-mass. The back-reaction of planetesimal scattering causes these planets to migrate outward and continually interact with fresh planetesimals, replenishing the source of scattered bodies. The most favorable cases for producing Vega's exo-zodi have negative radial mass gradients, with sub-Saturn- to Jupiter-mass inner planets at 5-10 AU and outer planets of 2.5 to 20 Earth masses. The mechanism fails if a Jupiter-sized planet exists beyond ~15 AU because the planet preferentially ejects planetesimals before they can reach the inner system. Direct-imaging planet searches can therefore directly test this mechanism.Comment: Updated references. Accepted to MNRAS Letters. 5 pages, 4 figures. Blog post about the paper at http://planetplanet.net/2014/03/31/vega-a-planetary-poem

Extrasolar comets : the origin of dust in exozodiacal disks?

Comets have been invoked in numerous studies as a potentially important source of dust and gas around stars, but none has studied the thermo-physical evolution, out-gassing rate, and dust ejection of these objects in such stellar systems. We investigate the thermo-physical evolution of comets in exo-planetary systems in order to provide valuable theoretical data required to interpret observations of gas and dust. We use a quasi 3D model of cometary nucleus to study the thermo-physical evolution of comets evolving around a single star from 0.1 to 50 AU, whose homogeneous luminosity varies from 0.1 to 70 solar luminosities. This paper provides mass ejection, lifetimes, and the rate of dust and water gas mass productions for comets as a function of the distance to the star and stellar luminosity. Results show significant physical changes to comets at high stellar luminosities. The models are presented in such a manner that they can be readily applied to any planetary system. By considering the examples of the Solar System, Vega and HD 69830, we show that dust grains released from sublimating comets have the potential to create the observed (exo)zodiacal emission. We show that observations can be reproduced by 1 to 2 massive comets or by a large number of comets whose orbits approach close to the star. Our conclusions depend on the stellar luminosity and the uncertain lifetime of the dust grains. We find, as in previous studies, that exozodiacal dust disks can only survive if replenished by a population of typically sized comets renewed from a large and cold reservoir of cometary bodies beyond the water ice line. These comets could reach the inner regions of the planetary system following scattering by a (giant) planet.Comment: 21 pages, 10 figure

Scattering of small bodies by planets: a potential origin for exozodiacal dust ?

High levels of exozodiacal dust are observed around a growing number of main sequence stars. The origin of such dust is not clear, given that it has a short lifetime against both collisions and radiative forces. Even a collisional cascade with km-sized parent bodies, as suggested to explain outer debris discs, cannot survive sufficiently long. In this work we investigate whether the observed exozodiacal dust could originate from an outer planetesimal belt. We investigate the scattering processes in stable planetary systems in order to determine whether sufficient material could be scattered inwards in order to retain the exozodiacal dust at its currently observed levels. We use N-body simulations to investigate the efficiency of this scattering and its dependence on the architecture of the planetary system. The results of these simulations can be used to assess the ability of hypothetical chains of planets to produce exozodi in observed systems. We find that for older (>100Myr) stars with exozodiacal dust, a massive, large radii (>20AU) outer belt and a chain of tightly packed, low-mass planets would be required in order to retain the dust at its currently observed levels. This brings into question how many, if any, real systems possess such a contrived architecture and are therefore capable of scattering at sufficiently high rates to retain exozodi dust on long timescales

Changing relationships: Accessing Subsurface Knowledge (ASK) project UK

The need for cities to make more effective use of the subsurface on which they stand, is increasingly being recognised in the UK and further afield to be essential for future cities to be sustainable and more resilient. However, city planning worldwide remains largely 2D, with very few cities having any substantial subsurface planning – the cities of Helsinki, Montreal, Singapore being rare exceptions. The consequences of inadequate consideration and planning of the subsurface, and limited re‐use of available data, are much more far‐reaching, in economic, environmental and social terms. There are clear spatial correspondences between proximity to vacant and derelict land and areas poorest health and greatest deprivation in UK cities; and, poor understanding of ground conditions is widely recognised as the largest single cause of construction project delay and overspends across Europe

NERC Briefing note: integrating NERC(BGS) subsurface environmental research and data to city development processes and policy: key learning outcomes

This report summarises the midpoint findings of a three year NERC Knowledge Exchange (KE) Fellowship examining how NERC(BGS) subsurface environmental data could have higher impact to city development processes, planning and policy. The NERC Fellowship is the first to see a NERC(BGS) researcher to be embedded within local government (Glasgow City Council) over a significant time period (three years) working with multiple service teams and levels of local government (LG), from senior management, to development policy teams, to geotechnical, engineering and project design groups within Development and Regeneration and Land and Environmental services. At present, there is a key gap in the use of NERC(BGS) research within early strategic decisions in LG development planning and policy – Figure (i). This is despite the relevance of the research and data to these decisions, the significant historical investment by NERC(BGS) in LG data acquisition, and the availability of regional datasets of subsurface ground conditions. Strategic knowledge of likely ground conditions and resource opportunities is essential for LG to inform Local Development Plan (LDP) policies, to be able to ‘screen’ and utilise land assets to greatest effect, to stimulate most appropriate city development and investment, and to deliver required housing and infrastructure. Key upfront LDP decisions are made largely in the absence of any strategic subsurface knowledge or screening data of subsurface ground conditions, (e.g. likely construction and remediation costs) with the exception of mining, or subsurface opportunities (e.g. building space, geothermal energy). This is in stark contrast to the high level of utilisation of NERC(BGS) data by the insurance industry to inform decisions of risk and land value/costs, and by both LG and engineering and geotechnical consultancies in the later-stages of the development process to inform project-scale decisions of design and construction – Figure (i). There are, therefore, key disconnects in the current use and impact of NERC(BGS) data within early stages of city development planning and policy. There is now a significant opportunity to bridge the knowledge gaps within both NERC(BGS) and LG, and to understand: what are the most relevant knowledge of subsurface conditions and opportunities for LG development planning and policy; and, what is the most accessible and relevant mechanism for delivery of the optimal knowledg

Delivery and impact of subsurface data in the absence of a comprehensive legislative framework : shared issues and difficulties in Odense and Glasgow

Rationale: BGS and VCS Odense proposed this STSM based on ongoing work in both cities to improve the delivery and impact of subsurface data within the cities. Shared difficulties in improving the delivery and impact of subsurface data were highlighted between the cities at the NAG-CITY workshop discussions in Odense (January 2013). In both cities, the lack of standardised data reporting formats, and the selective legislation requirements for reporting subsurface data to national or city authorities, strongly limits the amount and accessibility of subsurface data which is available to inform decision making. Purpose: The purpose of the STSM was to facilitate knowledge exchange between specialists in VCS Odense, GEUS, Odense City Municipality and BGS to compare key issues of the subsurface data management within the cities of Glasgow and Odense, in the absence of a comprehensive legislative framework in the UK or Denmark which ensures all borehole data are submitted to national or local public authorities. The STSM also aimed to discuss methods which could be used to improve the delivery and impact of subsurface data within the cities, and which could be transferred as a best practice to other cities within the COST Action with similar legislation. Improving access to standardised subsurface data, which can be readily available to inform policy and decision making within the public and private sectors, is increasingly required in all COST cities to meet key current urban redevelopment and groundwater management demands. The lessons learnt from the STSM are, therefore, of benefit to all COST-participants, particularly those within countries of similar data legislation

Potential impact of climate change on improved and unimproved water supplies in Africa

With significant climate change predicted in Africa over the next century, this chapter explores a key question: how will rural water supplies in Africa be affected? Approximately 550 million people in Africa live in rural communities and are reliant on water resources within walking distance of their community for drinking water. Less than half have access to improved sources (generally large diameter wells, springs, or boreholes equipped with handpumps); the majority rely on unimproved sources, such as open water and shallow wells. Major climate modelling uncertainties, combined with rapid socio-economic change, make predicting the future state of African water resources difficult; an appropriate response to climate change is to assume much greater uncertainty in climate and intensification of past climate variability. Based on this assumption the following should be considered: 1. Those relying on unimproved water sources (300 million in rural Africa) are likely to be most affected by climate change because unimproved sources often use highly vulnerable water resources. 2. Improved rural water supplies in Africa are overwhelmingly dependent on groundwater, due to the unreliability of other sources. 3. Climate change is unlikely to lead to continent-wide failure of improved rural water sources that access deeper groundwater (generally over 20 metres below ground surface) through boreholes or deep wells. This is because groundwater-based domestic supply requires little recharge, and the groundwater resources at depth will generally be of sufficient storage capacity to remain a secure water resource. However, a significant minority of people could be affected if the frequency and length of drought increases – particularly those in areas with limited groundwater storage. 4. In most areas, the key determinants of water security will continue to be driven by access to water rather than absolute water availability. Extending access, and ensuring that targeting and technology decisions are informed by an understanding of groundwater conditions, will become increasingly important. 5. Accelerating groundwater development for irrigation could increase food production, raise farm incomes and reduce overall vulnerability. However, ad hoc development could threaten domestic supplies and, in some areas, lead to groundwater depletion. Although climate change will undoubtedly be important in determining future water security, other drivers (such as population growth and rising food demands) are likely to provide greater pressure on rural water supplies