Evidence of fast pebble growth near condensation fronts in the HL Tau protoplanetary disk

Water and simple organic molecular ices dominate the mass of solid materials available for planetesimal and planet formation beyond the water snow line. Here we analyze ALMA long baseline 2.9, 1.3 and 0.87 mm continuum images of the young star HL Tau, and suggest that the emission dips observed are due to rapid pebble growth around the condensation fronts of abundant volatile species. Specifically, we show that the prominent innermost dip at 13 AU is spatially resolved in the 0.87 mm image, and its center radius is coincident with the expected mid-plane condensation front of water ice. In addition, two other prominent dips, at distances of 32 and 63 AU, cover the mid-plane condensation fronts of pure ammonia or ammonia hydrates and clathrate hydrates (especially with CO and N$_2$) formed from amorphous water ice. The spectral index map of HL Tau between 1.3 and 0.87 mm shows that the flux ratios inside the dips are statistically larger than those of nearby regions in the disk. This variation can be explained by a model with two dust populations, where most of solid mass resides in a component that has grown into decimeter size scales inside the dips. Such growth is in accord with recent numerical simulations of volatile condensation, dust coagulation and settling.Comment: 6 pages, 3 figures, Accepted for publication in the Astrophysical Journal Letter

Exploring the Origins of Carbon in Terrestrial Worlds

Given the central role of carbon in the chemistry of life, it is a fundamental question as to how carbon is supplied to the Earth, in what form and when. We provide an accounting of carbon found in solar system bodies, in particular a comparison between the organic content of meteorites and that in identified organics in the dense interstellar medium (ISM). Based on this accounting identified organics created by the chemistry of star formation could contain at most ~15% of the organic carbon content in primitive meteorites and significantly less for cometary organics, which represent the putative contributors to starting materials for the Earth. In the ISM ~30% of the elemental carbon is found in CO, either in the gas or ices, with a typical abundance of ~10^-4 (relative to H2). Recent observations of the TW Hya disk find that the gas phase abundance of CO is reduced by an order of magnitude compared to this value. We explore a solution where the volatile CO is destroyed via a gas phase processes, providing an additional source of carbon for organic material to be incorporated into planetesimals and cometesimals. This chemical processing mechanism requires warm grains (> 20 K), partially ionized gas, and sufficiently small <10 micron grains, i.e. a larger total grain surface area, such that freeze-out is efficient. Under these conditions static (non-turbulent) chemical models predict that a large fraction of the carbon nominally sequestered in CO can be the source of carbon for a wide variety of organics that are present as ice coatings on the surfaces of warm pre-planetesimal dust grains.Comment: 19 pages, 7 figures, to appear in Faraday Disc., vol 168, 2014, DOI: 10.1039/C4FD00003

Abstractions for designing and evaluating communication bridges for people in developing regions

This paper describes two novel abstractions that help soft- ware engineers work in developing regions to align social and technical factors when building communication systems. The abstractions extend two concepts familiar to engineers of computer networks and applications: the Open Systems Interconnect stack for design, and Quality of Service for eval- uation. The novel nature of the abstractions lies in how they help cultivate awareness of socio-cultural and technical is- sues when designing and evaluating communication bridges in the eld. Advantages of the abstractions are that they can be understood easily by software engineers, they aid communication with bene ciaries, and can therefore facili- tate collaboration. The paper makes an argument for these socially aware abstractions, describes the abstractions in de- tail, provides examples of how we used the new abstractions in the eld and then gives practical guidelines for how to use them. The simple nature of the new abstractions can help software engineers and end-users to work together to produce useful information technology based communication systems for people in developing regions.Telkom, Cisco, THRIP, NRF, SANPADWeb of Scienc

Tracing the Ingredients for a Habitable Earth from Interstellar Space through Planet Formation

We use the C/N ratio as a monitor of the delivery of key ingredients of life to nascent terrestrial worlds. Total elemental C and N contents, and their ratio, are examined for the interstellar medium, comets, chondritic meteorites and terrestrial planets; we include an updated estimate for the Bulk Silicate Earth (C/N = 49.0 +/- 9.3). Using a kinetic model of disk chemistry, and the sublimation/condensation temperatures of primitive molecules, we suggest that organic ices and macro-molecular (refractory or carbonaceous dust) organic material are the likely initial C and N carriers. Chemical reactions in the disk can produce nebular C/N ratios of ~1-12, comparable to those of comets and the low end estimated for planetesimals. An increase of the C/N ratio is traced between volatile-rich pristine bodies and larger volatile-depleted objects subjected to thermal/accretional metamorphism. The C/N ratios of the dominant materials accreted to terrestrial planets should therefore be higher than those seen in carbonaceous chondrites or comets. During planetary formation, we explore scenarios leading to further volatile loss and associated C/N variations owing to core formation and atmospheric escape. Key processes include relative enrichment of nitrogen in the atmosphere and preferential sequestration of carbon by the core. The high C/N BSE ratio therefore is best satisfied by accretion of thermally processed objects followed by large-scale atmospheric loss. These two effects must be more profound if volatile sequestration in the core is effective. The stochastic nature of these processes hints that the surface/atmospheric abundances of biosphere-essential materials will likely be variable.Comment: Accepted by PNAS per http://www.pnas.org/content/early/2015/07/01/1500954112.abstract?sid=9fd8abea-9d33-46d8-b755-217d10b1c24

Mass inventory of the giant-planet formation zone in a solar nebula analog

The initial mass distribution in the solar nebula is a critical input to planet formation models that seek to reproduce today's Solar System. Traditionally, constraints on the gas mass distribution are derived from observations of the dust emission from disks, but this approach suffers from large uncertainties in grain growth and gas-to-dust ratio. On the other hand, previous observations of gas tracers only probe surface layers above the bulk mass reservoir. Here we present the first partially spatially resolved observations of the $^{13}$C$^{18}$O J=3-2 line emission in the closest protoplanetary disk, TW Hya, a gas tracer that probes the bulk mass distribution. Combining it with the C$^{18}$O J=3-2 emission and the previously detected HD J=1-0 flux, we directly constrain the mid-plane temperature and optical depths of gas and dust emission. We report a gas mass distribution of 13$^{+8}_{-5}\times$(R/20.5AU)$^{-0.9^{+0.4}_{-0.3}}$ g cm$^{-2}$ in the expected formation zone of gas and ice giants (5-21AU). We find the total gas/millimeter-sized dust mass ratio is 140 in this region, suggesting that at least 2.4M_earth of dust aggregates have grown to >centimeter sizes (and perhaps much larger). The radial distribution of gas mass is consistent with a self-similar viscous disk profile but much flatter than the posterior extrapolation of mass distribution in our own and extrasolar planetary systems.Comment: Definitive version of the manuscript is published in Nature Astronomy, 10.1038/s41550-017-0130. This is the authors' versio

Towards communication and information access for deaf people

In tightly circumscribed communication situations an interactive system resident on a mobile device can assist Deaf people with their communication and information needs. The Deaf users considered here use South African Sign Language and information is conveyed by a collection of pre-recorded video clips and images. The system was developed according to our method of community-based co-design. We present several stages of the development as a series of case studies and highlight our experience. The first stage involved ethnographically inspired methods such as cultural probes. In the next stage we co-designed a medical consultation system that was ultimately dropped for technical reasons. A smaller system was developed for pharmaceutical dispensing and successfully implemented and tested. It now awaits deployment in an actual pharmacy. We also developed a preliminary authoring tool to tackle the problem of content generation for interactive computer literacy training. We are also working on another medical health information tool. We intend that a generic authoring tool be able to generate mobile applications for all of these scenarios. These mobile applications bridge communication gaps for Deaf people via accessible and affordable assistive technology

Researching for Change in a Globalising Asymmetric World

The field of research on Information and Communication Technology (ICT) in the service of social development (ICTD) is ripe for reframing. The asymmetries of the world are currently mirrored in the aims, practice and outcomes of too much ICTD research. The consequence is that people who might benefit from creative use of current and emerging technologies all over the world are excluded from the social processes and benefits of innovation and knowledge production. The ICTD research community’s widespread dissatisfaction with this situation haunts ICTD gatherings. In this workshop we want to explore critical alternatives to the current practice. We intend drafting and articulating critical alternatives for future research that is emancipatory, inclusive and oriented towards globally sustainable futures. To achieve this we first want to acknowledge and expose the vastly different knowledge interests and agendas of the various stakeholders. By examining a series of questions we shall then strive for a responsive reformulation of our approaches in a way that will not easily settle into a new orthodoxy. This will mean that we need to examine not only research and action agendas, transformation, inclusiveness, and power relations, but also our own personal growth and care for ourselves as actors in transformation. It is fortuitous that the 5th decennial Aarhus conference comes on the target date for the achievement of the United Nations Millennium Development Goals. What next? We want to ensure that after 2015, the research approaches that are adopted and promoted in ICTD are actually structured in the service of development. An ICT that is for Development cannot be realised by blindly replicating global asymmetries where aims and approaches are defined by the powerful and imposed on those at the margins.

Software Engineering for Development: A Position Statement

There is little work on targeted methodologies to develop IT applications and content in a developing world environment. This paper argues for a methodology called Socially Aware Software Engineering that we are busy formulating based on firsthand experience building Information and Communication Technology solutions. Our method is based on a classical user-centred approach from Human Computer Interaction combined with aspects of Participatory Design and cyclical software engineering practises. These approaches are wrapped into an iterative Action Research paradigm in order to directly include the community-based users of our systems. I conclude with suggestions on changing the nature of tertiary curricula in developing countries in a way that integrates this socially aware software engineering methodology

Using voice over IP to bridge the digital divide: a critical action research approach

There is a great disparity between those who have access to Information and Communication Technologies (ICTs) and those that do not. This phenomenon forms part of the Digital Divide. Many ICTs may be used to help overcome this divide if they are applied in a useful way, providing locally applicable content and services. Critical Action Research (CAR) is an approach used to develop such applications. CAR aims to empower people by involving them in the development process. Using CAR, this project investigates how Voice over IP (VoIP) may be applied in a productive way in an underserviced community. VoIP is an ICT used for sending voice over packet switched networks using Internet Protocol. It can be used to integrate data and voice to produce multimedia applications. In South Africa (SA), VoIP may only be provided by Telkom, the Second National Operator and the Under-Serviced Area Licensees. Using CAR and VoIP, an application is being developed to service a specific need of a particular rural community. The application will either provide a service to a small rural business or aid the provision of healthcare in rural areas. The project evaluates how well CAR integrates with a normal Software Development Lifecycle and makes policy recommendations for the use of VoIP in rural SA.Telkom, Siemens, THRI