OH+^+ emission from cometary knots in planetary nebulae

We model the molecular emission from cometary knots in planetary nebulae (PNe) using a combination of photoionization and photodissociation region (PDR) codes, for a range of central star properties and gas densities. Without the inclusion of ionizing extreme ultraviolet (EUV) radiation, our models require central star temperatures $T_*$ to be near the upper limit of the range investigated in order to match observed H$_2$ and OH$^+$ surface brightnesses consistent with observations - with the addition of EUV flux, our models reproduce observed OH$^+$ surface brightnesses for $T_* \ge 100 \, {\rm kK}$. For $T_* < 80 \, {\rm kK}$, the predicted OH$^+$ surface brightness is much lower, consistent with the non-detection of this molecule in PNe with such central star temperatures. Our predicted level of H$_2$ emission is somewhat weaker than commonly observed in PNe, which may be resolved by the inclusion of shock heating or fluorescence due to UV photons. Some of our models also predict ArH$^+$ and HeH$^+$ rotational line emission above detection thresholds, despite neither molecule having been detected in PNe, although the inclusion of photodissociation by EUV photons, which is neglected by our models, would be expected to reduce their detectability.Comment: Accepted by MNRAS, 11 pages, 15 figures. Author accepted manuscript. Accepted on 24/04/18. Deposited on 27/04/1

Modelling the ArH+^+ emission from the Crab Nebula

We have performed combined photoionization and photodissociation region (PDR) modelling of a Crab Nebula filament subjected to the synchrotron radiation from the central pulsar wind nebula, and to a high flux of charged particles; a greatly enhanced cosmic ray ionization rate over the standard interstellar value, $\zeta_0$, is required to account for the lack of detected [C I] emission in published Herschel SPIRE FTS observations of the Crab Nebula. The observed line surface brightness ratios of the OH$^+$ and ArH$^+$ transitions seen in the SPIRE FTS frequency range can only be explained with both a high cosmic ray ionization rate and a reduced ArH$^+$ dissociative recombination rate compared to that used by previous authors, although consistent with experimental upper limits. We find that the ArH$^+$/OH$^+$ line strengths and the observed H$_2$ vibration-rotation emission can be reproduced by model filaments with $n_{\rm{H}} = 2 \times 10^4$ cm$^{-3}$, $\zeta = 10^7 \zeta_0$ and visual extinctions within the range found for dusty globules in the Crab Nebula, although far-infrared emission from [O I] and [C II] is higher than the observational constraints. Models with $n_{\rm{H}} = 1900$ cm$^{-3}$ underpredict the H$_2$ surface brightness, but agree with the ArH$^+$ and OH$^+$ surface brightnesses and predict [O I] and [C II] line ratios consistent with observations. These models predict HeH$^+$ rotational emission above detection thresholds, but consideration of the formation timescale suggests that the abundance of this molecule in the Crab Nebula should be lower than the equilibrium values obtained in our analysis.Comment: Accepted by MNRAS. Author accepted manuscript. Accepted on 05/09/2017. Deposited on 05/09/1

Internet access for disabled people: Understanding socio-relational factors in Europe

Access to the Internet has become a sine qua non-of everyday life. It also offers new routes to economic and social inclusion for disabled people. Research on the digital divide shows that social factors affect Internet access but disability status is often overlooked. This paper assesses the extent to which disability makes a difference and how it interacts with other social effects to produce distinctive forms of digital exclusion. The analysis uses survey data from 27 European countries to explore and model, statistically, the interactions between Internet access, disability status, age, gender, education, household financial situation and household composition. Multilevel analysis confirms that socio-demographic factors can explain much variance in outcomes but there is a distinctive disability effect. In particular, the adverse effects of financial constraint, aging and living alone are exacerbated among disabled people. New policies to strengthen e-accessibility, arising from the United Nations Convention on the Rights of Persons with Disabilities and from the European Union, are important but cannot ignore those who are still excluded from the online revolution. Disabled people are over-represented in this group. The evidence suggests that both accessible technologies and appropriate supportive relationships are needed to address this

The apparent anticorrelation between the mass opacity of interstellar dust and the surface density of interstellar gas

Recent analyses of Herschel observations suggest that in nearby disc galaxies the dust mass opacity at 500μm⁠, κ500, decreases with increasing gas surface density, ΣISM. This apparent anticorrelation between κ500 and ΣISM is opposite to the behaviour expected from theoretical dust evolution models; in such models, dust in denser, cooler regions (i.e. regions of increased ΣISM) tends to grow and therefore to have increased κ500. We show, using a toy model, that the presence of a range of dust temperatures along the line of sight can lead to spuriously low estimated values of κ500. If in regions of higher ΣISM the range of dust temperatures extends to lower values (as seems likely), the magnitude of this effect may be sufficient to explain the apparent anticorrelation between κ500 and ΣISM. Therefore there may not be any need for spatial variation in the intrinsic dust properties that run counter to theoretical expectations

Molecular line signatures of cloud-cloud collisions

Collisions between interstellar gas clouds are potentially an important mechanism for triggering star formation. This is because they are able to rapidly generate large masses of dense gas. Observationally, cloud collisions are often identified in position-velocity (PV) space through bridging features between intensity peaks, usually of CO emission. Using a combination of hydrodynamical simulations, time-dependent chemistry, and radiative transfer, we produce synthetic molecular line observations of overlapping clouds that are genuinely colliding, and overlapping clouds that are just chance superpositions. Molecules tracing denser material than CO, such as NH$_3$ and HCN, reach peak intensity ratios of $0.5$ and $0.2$ with respect to CO in the `bridging feature' region of PV space for genuinely colliding clouds. For overlapping clouds that are just chance superpositions, the peak NH$_3$ and HCN intensities are co-located with the CO intensity peaks. This represents a way of confirming cloud collisions observationally, and distinguishing them from chance alignments of unrelated material.Comment: 7 pages, 4 figures, MNRAS accepte

Cogeneration Technology Alternatives Study (CTAS). Volume 5: Cogeneration systems results

The use of various advanced energy conversion systems is examined and compared with each other and with current technology systems for savings in fuel energy, costs, and emissions in individual plants and on a national level. About fifty industrial processes from the largest energy consuming sectors were used as a basis for matching a similar number of energy conversion systems that are considered as candidate which can be made available by the 1985 to 2000 time period. The sectors considered included food, textiles, lumber, paper, chemicals, petroleum, glass, and primary metals. The energy conversion systems included steam and gas turbines, diesels, thermionics, stirling, closed cycle and steam injected gas turbines, and fuel cells. Fuels considered were coal, both coal and petroleum based residual and distillate liquid fuels, and low Btu gas obtained through the on site gasification of coal. The methodology and results of matching the cogeneration energy conversion systems to approximately 50 industrial processes are described. Results include fuel energy saved, levelized annual energy cost saved, return on investment, and operational factors relative to the noncogeneration base cases

Cogeneration Technology Alternatives Study (CTAS). Volume 2: Analytical approach

The use of various advanced energy conversion systems were compared with each other and with current technology systems for their savings in fuel energy, costs, and emissions in individual plants and on a national level. The ground rules established by NASA and assumptions made by the General Electric Company in performing this cogeneration technology alternatives study are presented. The analytical methodology employed is described in detail and is illustrated with numerical examples together with a description of the computer program used in calculating over 7000 energy conversion system-industrial process applications. For Vol. 1, see 80N24797

Line emission from filaments in molecular clouds

Filamentary structures are often identified in column density maps of molecular clouds, and appear to be important for both low- and high-mass star formation. Theoretically, these structures are expected to form in regions where the supersonic cloud-scale turbulent velocity field converges. While this model of filament formation successfully reproduces several of their properties derived from column densities, it is unclear whether it can also reproduce their kinematic features. We use a combination of hydrodynamical, chemical and radiative transfer modelling to predict the emission properties of these dynamically-forming filaments in the $^{13}$CO, HCN and N$_2$H$^+$ $J=1-0$ rotational lines. The results are largely in agreement with observations; in particular, line widths are typically subsonic to transonic, even for filaments which have formed from highly supersonic inflows. If the observed filaments are formed dynamically, as our results suggest, no equilibrium analysis is possible, and simulations which presuppose the existence of a filament are likely to produce unrealistic results.Comment: 9 pages, 9 figures. MNRAS accepte

The state of the upper mantle beneath Southern Africa

We present a new upper mantle seismic model for southern Africa based on the fitting of a large (3622 waveforms) multi-mode surface wave data set with propagation paths significantly shorter (≤ 6000 km) than those in globally-derived surface wave models. The seismic lithosphere beneath the cratonic region of southern Africa in this model is about 175 ± 25 km thick, consistent with other recent surface wave models, but significantly thinner than indicated by teleseismic body-wave tomography. We determine the in situ geotherm from kimberlite nodules from beneath the same region and find the thermal lithosphere model that best fits the nodule data has a mechanical boundary layer thickness of 186 km and a thermal lithosphere thickness of 204 km, in very good agreement with the seismic measurement. The shear wave velocity determined from analyzes of the kimberlite nodule compositions agree with the seismic shear wave velocity to a depth of not, vert, similar150 km. However, the shear wave velocity decrease at the base of the lid seen in the seismic model does not correspond to a change in mineralogy. Recent experimental studies of the shear wave velocity in olivine as a function of temperature and period of oscillation demonstrate that this wave speed decrease can result from grain boundary relaxation at high temperatures at the period of seismic waves. This decrease in velocity occurs where the mantle temperature is close to the melting temperature (within not, vert, similar100 °C)

An efficient method for determining the chemical evolution of gravitationally collapsing prestellar cores

We develop analytic approximations to the density evolution of prestellar cores, based on the results of hydrodynamical simulations. We use these approximations as input for a time-dependent gas-grain chemical code to investigate the effects of differing modes of collapse on the molecular abundances in the core. We confirm that our method can provide reasonable agreement with an exact numerical solution of both the hydrodynamics and chemistry while being significantly less computationally expensive, allowing a large grid of models varying multiple input parameters to be run. We present results using this method to illustrate how the chemistry is affected not only by the collapse model adopted, but also by the large number of unknown physical and chemical parameters. Models which are initially gravitationally unstable predict similar abundances despite differing densities and collapse timescales, while ambipolar diffusion produces more extended inner depleted regions which are not seen in observations of prestellar cores. Molecular observations are capable of discriminating between modes of collapse despite the unknown values of various input parameters. We also investigate the evolution of the ambipolar diffusion timescale for a range of collapse modes, metallicities and cosmic ray ionization rates, finding that it remains comparable to or larger than the collapse timescale during the initial stages for all models we consider, but becomes smaller at later evolutionary stages. This confirms that ambipolar diffusion is an important process for diffuse gas, but becomes less significant as cores collapse to higher densities.Comment: Accepted by AJ. Author accepted manuscript. Accepted 29/05/2018, deposited 05/06/201