Theoretical Models for Producing Circularly Polarized Radiation in Extragalactic Radio Sources

We discuss the production of circular polarization in compact radio sources both by the intrinsic mechanism and by Faraday conversion. We pay particular attention to the magnetic field structure, considering partially ordered fields and Laing sheets, and distinguishing between uniform and unidirectional fields. (The latter can be constrained by flux conservation arguments.) In most cases, Faraday conversion is the more important mechanism. Conversion operates on Stokes U, which can be generated by internal Faraday rotation, or by magnetic field fluctuations, which can therefore produce circular polarization even in a pure pair plasma. We also show that the spectrum of circular polarization in an inhomogeneous jet can be quite different from that in a uniform source, being flat or even inverted.     

Occurrence of the symbiotic rhabdocoele flatwormParavortex gemellipara in Chesapeake Bay and gulf of Mexico molluscs,with notes on its biology and geographic range

Paravortex gemellipara is reported from its type hostGeukensia (=Modiolus) demissa and from two new hostsIschadium (=Brachidontes) recurvum andMytilopsis (=Congeria) leucopheata. Its range on the Atlantic seaboard is extended from Woods Hole, Massachusetts south to the lower Chesapeake Bay. The worm is also reported from the Gulf of Mexico at Galveston Bay, Texas. The presence of dermal protuberances and posterior adhesive areas bearing mucous strands is recorded. Mature worms were found living in the digestive tract of their host, contrary to the findings of previous investigators who reported them from the gill and kidney.     

The Balbus–Hawley instability in weakly ionized discs

MHD in protostellar discs is modified by the Hall current when the ambipolar diffusion approximation breaks down. Here I examine the Balbus–Hawley (magnetorotational) instability of a weak, vertical magnetic field within a weakly ionized disc. Vertical stratification is neglected, and a linear analysis is undertaken for the case in which the wavevector of the perturbation is parallel to the magnetic field.
The growth rate depends on whether the initial magnetic field is parallel or antiparallel to the angular momentum of the disc. The parallel case is less (more) unstable than the antiparallel case if the Hall current is dominated by negative (positive) species. The less-unstable orientation is stable for χ ≲0.5, where χ is the ratio of a generalized neutral–ion collision frequency to the Keplerian frequency. The other orientation has a formal growth rate of the order of the Keplerian angular frequency even in the limit χ →0! In this limit the wavelength of the fastest-growing mode tends to infinity, so the minimum level of ionization for instability is determined by the requirement that a wavelength fit within a disc scaleheight. In the ambipolar diffusion case, this requires χ > v _A c _s; in the Hall case this imposes a potentially much weaker limit,         

Cyanopolyynes in hot cores: modelling G305.2+0.2

We present results from a time-dependent gas-phase chemical model of a hot core based on the physical conditions of G305.2+0.2. While the cyanopolyyne HC₃N has been observed in hot cores, the longer chained species, HC₅N, HC₇N and HC₉N, have not been considered as the typical hot-core species. We present results which show that these species can be formed under hot core conditions. We discuss the important chemical reactions in this process and, in particular, show that their abundances are linked to the parent species acetylene which is evaporated from icy grain mantles. The cyanopolyynes show promise as 'chemical clocks' which may aid future observations in determining the age of hot core sources. The abundance of the larger cyanopolyynes increases and decreases over relatively short time-scales,  ∼10^2.5 yr  . We present results from a non-local thermodynamic equilibrium statistical equilibrium excitation model as a series of density, temperature and column density dependent contour plots which show both the line intensities and several line ratios. These aid in the interpretation of spectral-line data, even when there is limited line information available. In particular, non-detections of HC₅N and HC₇N in Walsh et al. are analysed and discussed.     

Australia Telescope Compact Array 1.2-cm observations of the massive star-forming region G305.2+0.2

We report on Australia Telescope Compact Array observations of the massive star-forming region G305.2+0.2 at 1.2 cm. We detected emission in five molecules towards G305A, confirming its hot core nature. We determined a rotational temperature of 26 K for methanol. A non-local thermodynamic equilibrium excitation calculation suggests a kinematic temperature of the order of 200 K. A time-dependent chemical model is also used to model the gas-phase chemistry of the hot core associated with G305A. A comparison with the observations suggest an age of between  2 × 10⁴  and  1.5 × 10⁵ yr  . We also report on a feature to the south-east of G305A which may show weak Class I methanol maser emission in the line at 24.933 GHz. The more evolved source G305B does not show emission in any of the line tracers, but strong Class I methanol maser emission at 24.933 GHz is found 3 arcsec to the east. Radio continuum emission at 18.496 GHz is detected towards two H  ii regions. The implications of the non-detection of radio continuum emission towards G305A and G305B are also discussed.     

Shocked molecular gas towards the supernova remnant G359.1–0.5 and the Snake

We have found a bar of shocked molecular hydrogen (H₂) towards the OH(1720 MHz) maser located at the projected intersection of supernova remnant (SNR)  G359.1–0.5  and the non-thermal radio filament known as the Snake. The H₂ bar is well aligned with the SNR shell and almost perpendicular to the Snake. The OH(1720 MHz) maser is located inside the sharp western edge of the H₂ emission, which is consistent with the scenario in which the SNR drives a shock into a molecular cloud at that location. The spectral line profiles of ¹²CO, HCO⁺ and CS towards the maser show broad-line absorption, which is absent in the ¹³CO spectra and most probably originates from the pre-shock gas. A density gradient is present across the region and is consistent with the passage of the SNR shock, while the H₂ filament is located at the boundary between the pre-shock and post-shock regions.     

Intercomparison of NO,NO2 and HNO3 measurements with photochemical theory

The simultaneous measurements of NO, NO₂ and HNO_A mixing‐ratio profiles carried out on the Stratoprobe balloon flight of 22 July 1974 have been simulated with a time‐dependent model using the measured temperature and ozone profiles. The calculated ratios of NO/NO₂, HNO₃/NO₂ using currently accepted photochemistry are consistent with the measured ratios within the experimental errors of the measurements. The measured NO₂/NO ratio is almost a factor of two smaller than predicted, although the discrepancy is still within the experimental errors. A remarkable proportionality in the NO₂ and O₃ profiles has been noted and is unexplained. A time‐dependent simulation has been employed to convert the measurements into diurnally‐averaged profiles suitable for intercomparison with two‐dimensional stratospheric models and a comparison with constituent profiles from Prinn et al. (1975) is carried out as an example. The NO_V mixing ratio, formed from the sum of the NO, NO₂ and HNO₂ measurements is similar to the NO_V mixing ratio from several one‐ and two‐dimensional models used to predict the effects of SST's on the ozone layer. The odd nitrogen mixing ratio is roughly constant from 20 to 35 km at 11 ppbv.     

A fresh framework for the ecology of arid Australia

A revised set of propositions about ecology in arid Australia is presented, based on research literature since publication of Stafford Smith and Morton (1990). Fourteen propositions distil our argument that most features of the Australian deserts are explicable in terms of two dominant physical and climatic elements: rainfall variability, leading to extended droughts and occasional flooding rains; and widespread nutrient poverty. Different landscapes within the arid zone show these features to varying degrees, and so it is important to think about different places separately when considering our propositions. Plant life-histories strongly reflect temporal patterns of soil moisture; because Australian deserts receive more variable rainfall than most others, there is a distinctive spectrum of life-histories. Low levels of phosphorus (together with abundant soil moisture on irregular occasions) favour plants producing a relative excess of carbohydrate (C). In turn, C-rich plant products sometimes lead to fire-prone ecosystems, assemblages dominated by consumers of sap and other C-based products, and abundant detritivores (particularly termites). Fluctuations in production due to variable rainfall provide openings for consumers with opportunistic life-histories, including inhabitants of extensive but ephemeral rivers and lakes. Most consumer species exhibit some dietary flexibility or utilise more dependable resources; these strategies give rise to greater stability in species dynamics and composition of assemblages than might first be imagined under the variable rainfall regime. Aboriginal people have had long-standing ecological influence as they accessed resources. For each proposition we suggest the extent to which it is ‘different’, ‘accentuated’ or ‘universal’ in comparison with other deserts of the world, recognising that this categorisation is in need of critical testing. Further tests of each proposition are also suggested to fill the many gaps that still exist in our knowledge of the structure and functioning of Australia’s deserts.