Angular momentum transport in protostellar discs

Angular momentum transport in protostellar discs can take place either radially, through turbulence induced by the magnetorotational instability (MRI), or vertically, through the torque exerted by a large-scale magnetic field that threads the disc. Using semi-analytic and numerical results, we construct a model of steady-state discs that includes vertical transport by a centrifugally driven wind as well as MRI-induced turbulence. We present approximate criteria for the occurrence of either one of these mechanisms in an ambipolar diffusion-dominated disc. We derive ``strong field'' solutions in which the angular momentum transport is purely vertical and ``weak field'' solutions that are the stratified-disc analogues of the previously studied MRI channel modes; the latter are transformed into accretion solutions with predominantly radial angular-momentum transport when we implement a turbulent-stress prescription based on published results of numerical simulations. We also analyze ``intermediate field strength'' solutions in which both modes of transport operate at the same radial location; we conclude, however, that significant spatial overlap of these two mechanisms is unlikely to occur in practice. To further advance this study, we have developed a general scheme that incorporates also the Hall and Ohm conductivity regimes in discs with a realistic ionization structure.Comment: 8 pages, 4 figures, 1 table; accepted for publication in MNRA

Enhanced OH in C-type shock waves in molecular clouds

Cosmic-ray and X-ray ionisations in molecular gas produce a weak far-ultraviolet flux through the radiative decay of H2 molecules that have been excited by collisions with energetic electrons (the Prasad-Tarafdar mechanism). I consider the effect of this dissociating flux on the oxygen chemistry in C-type shocks. Typically a few percent of the water molecules produced within the shock front are dissociated before the gas has cooled to 50K. The resulting column density of warm OH rises from 10^15 to 10^16 cm^-2 as the ionisation rate is increased from 10^-17 (typical of dark clouds) to 10^-15 s^-1 (adjacent to supernova remnants). These column densities produce substantial emission in the far-infrared rotational transitions of OH, and are consistent with the OH/H2O ratios inferred from ISO observations of emission from molecular shocks. For high ionisation rates the column of warm OH is sufficient to explain the OH(1720 MHz) masers that occur where molecular clouds are being shocked by supernova remnants. The predicted abundance of OH throughout the shock front will enable C-type shocks to be examined with high spectral resolution through radio observations of the four hyperfine ground state transitions of OH at 18cm and heterodyne measurements of emission in the FIR (e.g. from SOFIA)Comment: 5 pp incl 3 figs, LaTeX, uses emulateapj.sty; ApJ Letters in press. Revised 2nd paragraph of discussio

Recall of physical activity advice was associated with higher levels of physical activity in colorectal cancer patients.

The present study tested the hypothesis that recall of receiving physical activity (PA) advice would be associated with higher levels of PA in patients with a diagnosis of colorectal cancer (CRC)

Seismic anisotropy of Precambrian lithosphere : Insights from Rayleigh wave tomography of the eastern Superior Craton

The seismic data used in this study are freely available from the CNDC (Canadian National Data Centre for Earthquake Seismology and Nuclear Explosion Monitoring) and IRIS DMC (Data Management Center) via their data request tools. The Leverhulme Trust (grant RPG-2013-332) and National Science Foundation are acknowledged for financial support. L.P. is supported by Janet Watson Imperial College Department Scholarship and the Romanian Government Research Grant NUCLEU. F.D. is supported by NSERC through the Discovery Grants and Canada Research Chairs program. We also thank two anonymous reviewers and the Associate Editor for insightful comments that helped improve the manuscript.Peer reviewedPublisher PD

Industry Compliance Costs Under the Renewable Fuel Standard: Evidence from Compliance Credits

The Renewable Fuel Standard requires US oil refineries to blend biofuels into domestic transportation fuels. To ensure that compliance costs under this mandate don’t disproportionately affect any subset of refiners, the regulation includes a compliance credit program, whereby refiners blending excess biofuels can sell their excess compliance to refiners that do not blend enough. The price of these credits can be interpreted as the marginal cost of compliance with the mandate. I measure how changes in the prices of these compliance credits affect the stock prices of oil refining firms. There are a number of ways one might expect these compliance credits to affect firms. Much economic research finds that oil refiners are able to pass the costs of RFS compliance to consumers quite easily, suggesting that changes in the compliance cost should not affect firms’ value at all. Large refiners tend to claim that the RFS imposes a large cost and drags down their profits. Perhaps the most interesting claim is that of the “merchant refiners”—generally small refiners who do not own the infrastructure to blend biofuels on their own and are thus forced to comply with the mandate completely with compliance credits. They claim that larger refiners are able to hoard the credits and sell them for windfall profits at the expense of the merchant refiners. My results indicate that positive shocks in compliance credit prices are associated with stock losses only among large, non-merchant refiners, and that even this association is small. This discredits the claims of merchant refiners, but opens a new puzzle: why are large, integrated refiners the only ones affected? I conclude my paper with a number of potential explanations, though I am not able to test between them using my data

MOJAVE: Monitoring of Jets in AGN with VLBA Experiments. IV. The Parent Luminosity Function of Radio-Loud Blazars

(Abridged) We use a complete sample of active galactic nuclei (AGN) selected on the basis of relativistically beamed 15 GHz radio flux density to derive the parent radio luminosity function (RLF) of bright radio-selected blazar cores. We use a maximum likelihood method to fit a beamed RLF to the observed data and thereby recover the parameters of the intrinsic (unbeamed) RLF. We analyze two subsamples of the MOJAVE sample: the first contains only objects of known FR II class, with a total of 103 sources, and the second subsample adds 24 objects of uncertain FR class for a total of 127 sources. Both subsamples exclude four known FR I radio galaxies and two gigahertz-peaked spectrum sources. We obtain good fits to both subsamples using a single power law intrinsic RLF with pure density evolution function. We find that a previously reported break in the observed MOJAVE RLF actually arises from using incomplete bins (because of the luminosity cutoff) across a steep and strongly evolving RLF, and does not reflect a break in the intrinsic RLF. The derived space density of the parent population of the FR II sources from the MOJAVE sample (with L>1.3e25 W/Hz) is approximately 1600/Gpc^3.Comment: 31 pages, 7 figures and 2 tables. Accepted for publication in ApJ. Changes: classification of sources based on radio morphology instead of optical classes; added the parameters of the RLF of the FR II sources; added more explanations; added a table listing the sample sources; added 2 extra figures related to the observed break in the RLF; updated reference

The use of chronosequences in studies of ecological succession and soil development

1. Chronosequences and associated space-for-time substitutions are an important and often necessary tool for studying temporal dynamics of plant communities and soil development across multiple time-scales. However, they are often used inappropriately, leading to false conclusions about ecological patterns and processes, which has prompted recent strong criticism of the approach. Here, we evaluate when chronosequences may or may not be appropriate for studying community and ecosystem development. 2. Chronosequences are appropriate to study plant succession at decadal to millennial time-scales when there is evidence that sites of different ages are following the same trajectory. They can also be reliably used to study aspects of soil development that occur between temporally linked sites over time-scales of centuries to millennia, sometimes independently of their application to shorter-term plant and soil biological communities. 3. Some characteristics of changing plant and soil biological communities (e.g. species richness, plant cover, vegetation structure, soil organic matter accumulation) are more likely to be related in a predictable and temporally linear manner than are other characteristics (e.g. species composition and abundance) and are therefore more reliably studied using a chronosequence approach. 4. Chronosequences are most appropriate for studying communities that are following convergent successional trajectories and have low biodiversity, rapid species turnover and low frequency and severity of disturbance. Chronosequences are least suitable for studying successional trajectories that are divergent, species-rich, highly disturbed or arrested in time because then there are often major difficulties in determining temporal linkages between stages. 5. Synthesis. We conclude that, when successional trajectories exceed the life span of investigators and the experimental and observational studies that they perform, temporal change can be successfully explored through the judicious use of chronosequences

The Effect of Co-Combusted Biomass-Coal Fly Ash on the Behaviour Portland Cement

This project has investigated the hydration behaviour of pfa-OPC blended cements, comparing conventional pfa with that obtained from co-firing of coal with biomass (palm kernel expeller). Calorimetry, thermal analysis and electron microscopy have been used to investigate the compositions and microstructures of the hydrated pastes. These have been used to explain the materials’ engineering properties (strength development and workability). The results showed that, in the short term, the behaviour of the co-fired material is comparable with that of conventional pfa, there being no discernable differences between the two systems