Lateral Expansion of the Bridges of Cygnus A and other Powerful Radio Sources

Measurements of the width of the radio bridge at several locations along the bridge for each of four powerful extended radio sources are presented. Adopting a few simple assumptions, these measurements may be used to predict the radio surface brightness as a function of position across the radio bridge. The predicted and observed surface brightnesses across the bridges are compared and found to agree fairly well. The results are consistent with a simple picture in which the radio power and size of the radio lobe at the forward edge of the radio bridge are roughly time-independent for a given source, and the expansion of the bridge in the lateral direction is adiabatic. There is no indication that reacceleration or energy transport is important in the bridges of these sources. The rate of lateral expansion of the bridge just behind the radio lobe and hotspot in terms of the rate of forward propagation is compared with that predicted, and found to be in good agreement with the predicted value.Comment: 7 pages, uuencoded compressed postscript. To appear in the Proceding of the Cygnus A workshop, May 1-4, Green Bank, W

Nano and Micro indentation studies of bulk zirconia and EB PVD TBCs

In order to model the erosion of a material it is necessary to know the material properties of both the impacting particles as well as the target. In the case of electron beam (EB) physical vapour deposited(PVD) thermal barrier coatings (TBCs) the properties of the columns as opposed to the coating as a whole are important. This is due to the fact that discrete erosion events are on a similar scale as the size of the individual columns. Thus nano* and micro* indentation were used to determine the hardness and the Young"s modulus of the columns. However, care had to be taken to ensure that it was the hardness of the columns that was being measured and not the coating as a whole. This paper discusses the differences in the results obtained when using the two different tests and relates them to the interactions between the indent and the columns of the EB PVD TBC microstructure. It was found that individual columns had a hardness of 14 GPa measured using nano indentation, while the hardness of the coating, using micro indentation decreased from 13 to 2.4 GPa as the indentation load increased from 0.1 to 3N. This decrease in hardness was attributed to the interaction between the indenter and a number of adjacent columns and the ability of the columns to move laterally under indentation

Mindblind eyes: an absence of spontaneous theory of mind in Asperger syndrome

Adults with Asperger syndrome can understand mental states such as desires and beliefs (mentalizing) when explicitly prompted to do so, despite having impairments in social communication. We directly tested the hypothesis that such individuals nevertheless fail to mentalize spontaneously. To this end, we used an eye-tracking task that has revealed the spontaneous ability to mentalize in typically developing infants. We showed that, like infants, neurotypical adults’ (n = 17 participants) eye movements anticipated an actor’s behavior on the basis of her false belief. This was not the case for individuals with Asperger syndrome (n = 19). Thus, these individuals do not attribute mental states spontaneously, but they may be able to do so in explicit tasks through compensatory learning

The use of radio observations to probe ambient gas densities

The radio properties of powerful extended radio sources may be used to estimate the ambient gas density in the vicinity of radio lobes. A sample of 27 radio lobes from 14 radio galaxies and of 14 radio lobes from 8 radio loud quasars was constructed using sources from the published literature with sufficient radio information to allow an estimate of the ambient gas density. The ambient gas density as a function of separation of the lobe from the center of the parent galaxy indicates a composite density profile, where Cygnus A plays the key role of determining the normalization for the ambient gas density. The composite density profile of the galaxies and quasars studied here is similar to the density profile of gas in low-redshift clusters of galaxies, which confirms the result obtained by Daly [1] using a somewhat smaller sample of radio sources. The data presented here allow an estimate of the core gas density, core radius, and slope of the density profile assuming that the gas density can be fit by a King model. The data suggest that the core gas density decreases as the redshift of the source increases, and is consistent with either a roughly constant core radius, or a core radius that increases with increasing source redshift. Thus, our results are completely consistent with observations indicating negative evolution of the cluster X-ray luminosity function. (truncated to <24 lines

An introduction to spacecraft thermal control

External and internal passive and active methods for spacecraft thermal control - material insulation and structural propertie

Information Flow in Social Groups

We present a study of information flow that takes into account the observation that an item relevant to one person is more likely to be of interest to individuals in the same social circle than those outside of it. This is due to the fact that the similarity of node attributes in social networks decreases as a function of the graph distance. An epidemic model on a scale-free network with this property has a finite threshold, implying that the spread of information is limited. We tested our predictions by measuring the spread of messages in an organization and also by numerical experiments that take into consideration the organizational distance among individuals

An exploration of concepts of community through a case study of UK university web production

The paper explores the inter-relation and differences between the concepts of occupational community, community of practice, online community and social network. It uses as a case study illustration the domain of UK university web site production and specifically a listserv for those involved in it. Different latent occupational communities are explored, and the potential for the listserv to help realize these as an active sense of community is considered. The listserv is not (for most participants) a tight knit community of practice, indeed it fails many criteria for an online community. It is perhaps best conceived as a loose knit network of practice, valued for information, implicit support and for the maintenance of weak ties. Through the analysis the case for using strict definitions of the theoretical concepts is made

The Dynamics of a Mobile Phone Network

The empirical study of network dynamics has been limited by the lack of longitudinal data. Here we introduce a quantitative indicator of link persistence to explore the correlations between the structure of a mobile phone network and the persistence of its links. We show that persistent links tend to be reciprocal and are more common for people with low degree and high clustering. We study the redundancy of the associations between persistence, degree, clustering and reciprocity and show that reciprocity is the strongest predictor of tie persistence. The method presented can be easily adapted to characterize the dynamics of other networks and can be used to identify the links that are most likely to survive in the future