Income volatility in New Zealand

‘Economic risk is a lot like a hurricane. Hurricanes strike powerfully and suddenly. They rip apart what they touch; property, landscape and lives … And although they can be prepared for, they cannot be prevented.’ These sentiments, from Yale political scientist Jacob Hacker, explain why economic risk is a concern for households, and why the extent of that concern depends a great deal on how well households are protected against risk. The potential for individual bad luck to lead to hardship has meant that society has, in many instances, determined that individual risk should be borne collectively through systems of social welfare or social insurance (Hacker, 2008, p.5)

The Gas Properties of the W3 GMC: A HARP study

We present 12CO, 13CO and C18O J=3-2 maps of the W3 GMC made at the James Clerk Maxwell Telescope. We combine these observations with Five Colleges Radio Astronomy Observatory CO J=1-0 data to produce the first map of molecular-gas temperatures across a GMC and the most accurate determination of the mass distribution in W3 yet obtained. We measure excitation temperatures in the part of the cloud dominated by triggered star formation (the High Density Layer, HDL) of 15-30 K, while in the rest of the cloud, which is relatively unaffected by triggering (Low Density Layer, LDL), the excitation temperature is generally less than 12 K. We identify a temperature gradient in the HDL which we associate with an age sequence in the embedded massive star-forming regions. We measure the mass of the cloud to be 4.4+/-0.4 x 10^5 solar masses, in agreement with previous estimates. Existing sub-mm continuum data are used to derive the fraction of gas mass in dense clumps as a function of position in the cloud. This fraction, which we interpret as a Clump Formation Efficiency (CFE), is significantly enhanced across the HDL, probably due to the triggering. Finally, we measure the 3D rms Mach Number as a function of position and find a correlation between the Mach number and the CFE within the HDL only. This correlation is interpreted as due to feedback from the newly-formed stars and a change in its slope between the three main star-forming regions is construed as another evolutionary effect. We conclude that triggering has affected the star-formation process in the W3 GMC primarily by creating additional dense structures that can collapse into stars. Any traces of changes in CFE due to additional turbulence have since been overruled by the feedback effects of the star-forming process itself.Comment: 14 pages, 11 figures, 1 table, accepted for publication in MNRA

The Lesser Role of Shear in Star Formation: Insight from the Galactic Ring Survey

We analyse the role played by shear in regulating star formation in the Galaxy on the scale of individual molecular clouds. The clouds are selected from the 13^CO J=1-0 line of the Galactic Ring Survey. For each cloud, we estimate the shear parameter which describes the ability of density perturbations to grow within the cloud. We find that for almost all molecular clouds considered, there is no evidence that shear is playing a significant role in opposing the effects of self-gravity. We also find that the shear parameter of the clouds does not depend on their position in the Galaxy. Furthermore, we find no correlations between the shear parameter of the clouds with several indicators of their star formation activity. No significant correlation is found between the shear parameter and the star formation efficiency of the clouds which is measured using the ratio of the massive young stellar objects luminosities, measured in the Red MSX survey, to the cloud mass. There are also no significant correlations between the shear parameter and the fraction of their mass that is found in denser clumps which is a proxy for their clump formation efficiency, nor with their level of fragmentation expressed in the number of clumps per unit mass. Our results strongly suggest that shear is playing only a minor role in affecting the rates and efficiencies at which molecular clouds convert their gas into dense cores and thereafter into stars.Comment: Accepted to ApJ. 30 pages, 11 figures. Content substantially enlarged and includes quantitative correlations between the SFE of molecular clouds and their shear parameters. Some references correcte

An Unbiased Survey for Outflows in the W3 and W5 Star-Formation Regions

During their birth all stars undergo periods of copious mass loss, frequently characterized by the occurrence of bipolar outflows. These outflows are believed to play a fundamental role in the star formation process. However the exact outflow generating method is obscure at present. To elucidate this problem we are investigating whether the flow properties are correlated over the entire protostellar mass spectrum. Progress in this area requires that we assemble a statistically valid sample of high-mass outflow systems. This is necessary since existing catalogues of such objects are heterogeneous and statistically incomplete.Comment: 2 pages, 1 figure, uses newpasp.sty. To appear in "Hot Star Workshop III: The Earliest Phases of Massive Star Birth" (ed. P.A. Crowther

The structure and dynamics of an environment forming high-mass stars

A detailed investigation is presented of the physical environment associated with high-mass star formation. This is carried out by means of an in-depth multi-wavelength study of one such region in W75N. An analysis is made of the nature of the major em­bedded luminosity sources and the cloud core from which they have recently formed,from the hot dust very close to each object to the cold dust and molecular gas sur­rounding them. The dynamical relationship between the large molecular outflow and the ambient cloud is studied with regard to limitations that might be placed on plausi­ble flow generation mechanisms. The details of the reflection nebulae associated with the mass outflows are interpreted in terms of the scattering properties of dust.A recently-formed stellar cluster has been found, associated with the known signs of higli-mass star formation in W75N (compact HII regions, O H and H2O masers,CO outflow etc.). The main heating source and origin of the molecular outflow is identified and found to be very deeply embedded (Av > 90 mag). Imaging near-infrared polarimetry, millimetre-wave molecular line spectroscopy and submillimetre continuum observations at scales of 0.01 to 0.2 pc have shown no disk or torus structure in the dense obscuring material around this source. However, circumstantial evidence exists in the near-infrared colours for significant amounts of dusty material close to the major luminosity sources. Therefore very small-scale (few X100 A.U.) disk-like formations may exist and play an important dynamical role.The large molecular outflow in W75N is found to be unable to overcome the gravitational binding force of the large surrounding core unless the flow is initially highly collimated. The lobes of high-velocity gas possess a large degree of collimation,probably induced through confinement by the ambient material, but the outflow as a whole is irregular and possibly multipolar and appears to be intrinsically largely isotropic. The flow lobes are not wind-blown bubbles but are filled with high velocity molecular material. The driving mechanism is most likely to be a massive, semi-isotropic stellar wind but there could still be a cylindrically symmetrical, rotation-driven mechanism acting close to the star.Of the two reflection nebulae in the region the largest and brightest is associ­ated with the blue-shifted molecular outflow lobe. Features in the nebula, similar to those in other such objects, are consistent with scattering from large grains which produce diffraction-affected and strongly forward-biased scattering patterns. It is shown that large grains should dominate the scattering in the near-infrared, regardless of the detailed grain size distribution. The ubiquitous suppression of backward-directed re­flection lobes in bipolar sources may be caused by forward scattering and not by large obscuring disks. A smaller reflection nebula surrounds the less luminous source IRS-2,indicating a limited-scale outflow from this object also. Hydrogen recombination line ratios in IRS-2 are consistent with current models of massive, partly-ionised stellar winds

Paper folding, Riemann surfaces, and convergence of pseudo-Anosov sequences

A method is presented for constructing closed surfaces out of Euclidean polygons with infinitely many segment identifications along the boundary. The metric on the quotient is identified. A sufficient condition is presented which guarantees that the Euclidean structure on the polygons induces a unique conformal structure on the quotient surface, making it into a closed Riemann surface. In this case, a modulus of continuity for uniformizing coordinates is found which depends only on the geometry of the polygons and on the identifications. An application is presented in which a uniform modulus of continuity is obtained for a family of pseudo-Anosov homeomorphisms, making it possible to prove that they converge to a Teichm\"uller mapping on the Riemann sphere.Comment: 75 pages, 18 figure

Unimodal generalized pseudo-Anosov maps

An infinite family of generalized pseudo-Anosov homeomorphisms of the sphere S is constructed, and their invariant foliations and singular orbits are described explicitly by means of generalized train tracks. The complex strucure induced by the invariant foliations is described, and is shown to make S into a complex sphere. The generalized pseudo-Anosovs thus become quasiconformal automorphisms of the Riemann sphere, providing a complexification of the unimodal family which differs from that of the Fatou/Julia theory.Comment: Published by Geometry and Topology at http://www.maths.warwick.ac.uk/gt/GTVol8/paper31.abs.htm

Solenoidal turbulent modes and star formation efficiency in Galactic-plane molecular clouds

It is speculated that the high star-formation efficiency observed in spiral-arm molecular clouds is linked to the prevalence of compressive (curl-free) turbulent modes, while the shear-driven solenoidal (divergence-free) modes appear to be the main cause of the low star-formation efficiency that characterises clouds in the Central Molecular Zone. Similarly, analysis of the Orion B molecular cloud has confirmed that, although turbulent modes vary locally and at different scales within the cloud, the dominant solenoidal turbulence is compatible with its low star formation rate. This evidence points to inter-and intra-cloud fluctuations of the solenoidal modes being an agent for the variability of star formation efficiency. We present a quantitative estimation of the relative fractions of momentum density in the solenoidal modes of turbulence in a large sample of plane molecular clouds in the \ce{^{13}CO}/\ce{C^{18}O} ($J=3\rightarrow 2$) Heterodyne Inner Milky Way Plane Survey (CHIMPS). We find a negative correlation between the solenoidal fraction and star-formation efficiency. This feature is consistent with the hypothesis that solenoidal modes prevent or slow down the collapse of dense cores. In addition, the relative power in the solenoidal modes of turbulence (solenoidal fraction) appears to be higher in the Inner Galaxy declining with a shallow gradient with increasing Galactocentric distance. Outside the Inner Galaxy, the slowly, monotonically declining values suggest that the solenoidal fraction is unaffected by the spiral arms.Comment: 16 pages, 14 figure

Spatial variations in silicate-to-nitrate ratios in Southern Ocean surface waters are controlled in the short term by physics rather than biology

The nutrient composition (high in nitrate but low in silicate) of Subantarctic Mode Water (SAMW) forces diatom scarcity across much of the global surface ocean. This is because diatoms cannot grow without silicate. After formation and downwelling at the Southern Ocean's northern edge, SAMW re-emerges into the surface layers of the mid- and low-latitude oceans, providing a major nutrient source to primary producers in those regions. The distinctive nutrient composition of SAMW originates in the surface waters of the Southern Ocean, from which SAMW is formed. These waters are observed to transition from being rich in both silicate and nitrate in high-latitude areas of the Southern Ocean to being nitrate-rich but silicate-depleted at SAMW formation sites further north. Here we investigate the key controls of this change in nutrient composition with an idealised model, consisting of a chain of boxes linked by a residual (Ekman- and eddy-induced) overturning circulation. Biological processes are modelled on the basis of seasonal plankton bloom dynamics, and physical processes are modelled using a synthesis of outputs from the data-assimilative Southern Ocean State Estimate. Thus, as surface water flows northward across the Southern Ocean toward sites of SAMW formation, it is exposed in the model (as in reality) to seasonal cycles of both biology and physics. Our results challenge previous characterisations of the abrupt northward reduction in silicate-to-nitrate ratios in Southern Ocean surface waters as being predominantly driven by biological processes. Instead, our model indicates that, over shorter timescales (years to decades), physical processes connecting the deep and surface waters of the Southern Ocean (i.e. upwelling and entrainment) exert the primary control on the spatial distribution of surface nutrient ratios