Shocked H2 and Fe+ Dynamics in the Orion Bullets

Observations of H2 velocity profiles in the two most clearly defined Orion bullets are extremely difficult to reconcile with existing steady-state shock models. We have observed [FeII] 1.644um velocity profiles of selected bullets and H2 1-0 S(1) 2.122um velocity profiles for a series of positions along and across the corresponding bow-shaped shock fronts driven into the surrounding molecular cloud. Integrated [FeII] velocity profiles of the brightest bullets are consistent with theoretical bow shock predictions. However, observations of broad, singly-peaked H2 1-0 S(1) profiles at a range of positions within the most clearly resolved bullet wakes are not consistent with molecular shock modelling. A uniform, collisionally broadened background component which pervades the region in both tracers is inconsistent with fluorescence due to the ionizing radiation of the Trapezium stars alone.Comment: 20 pages including 18 figures, published in MNRA

Evaluation of image tubes in astronomy

It is to be hoped that the methods described in the first part of this thesis will be extensively applied to imaging devices and to photographic emulsions, since this is the only way in which an unambiguous figure for the performance of the detector may be derived. Lack of attention to such concepts has produced many pitfalls, notably the enormous (and imaginary) gains reported earlier for various types of image tube, which, when they were not realised in practice, produced some considerable despondency.The work of Shaw(60) on aerial films is an out- standing example of thorough application of Information Theory to the photographic process. His results are formally the same as those of Chapter I of this thesis.The results derived enable a prediction of signal-to-noise-ratio (accuracy) and information gain for the image tube under any conditions of exposure. The results also point to the necessity of knowing, for instance, the IMF of the spectrograph used with the detector, in order to fully exploit the storage capacity of the detector.Part II of this work has described the manner in which the Spectracon, as a particular example of an astronomical image tube, may be brought into routine use as part of the observational astronomer's instrument capability.Much is still to be achieved in this field. In particular, the emulsions are in no way comparable in standard (understandably, they are still a laboratory - rather than commercial - commodity) with the best astronomical emulsions. Similarly, such is the experimental complexity in building a tube, that to produce a flawless cathode and window ( and both are focal surfaces) is asking a great deal. But it is not asking for the impossible, and it is foreseeable that in time such difficulties will be eliminated.There remains the problem of the small area of the cathode. In effect, while the information storage capacity of the image tube is considerably greater than that of astronomical emulsions, its limited area rather nullifies this advantage in that, with suitable coding, all the information in the small cathode area could be spread onto one large astronomical plate. However, to do this the exposing intensity must be reduced, and this, because of the nonlinearity of the photographic process, reduces the efficiency.Thus the crucial advantage in using an image tube must be in exploiting its higher detecting efficiency. If programs consist in looking at small regions of spectrum (or of sky), then the advantages of the image tube are obvious. Some of this work has already been reported in a paper read to the 3rd Symposium on Photoelectronic Imaging Devices

Constraints in Non-Boolean Contexts

In high-level constraint modelling languages, constraints can occur in non-Boolean contexts: implicitly, in the form of partial functions, or more explicitly, in the form of constraints on local variables in non-Boolean expressions. Specifications using these facilities are often more succinct. However, these specifications are typically executed on solvers that only support questions of the form of existentially quantified conjunctions of constraints. We show how we can translate expressions with constraints appearing in non-Boolean contexts into conjunctions of ordinary constraints. The translation is clearly structured into constrained type elimination, local variable lifting and partial function elimination. We explain our approach in the context of the modelling language Zinc. An implementation of it is an integral part of our Zinc compiler

On the photodissociation of H2 by the first stars

The first star formation in the universe is expected to take place within small protogalaxies, in which the gas is cooled by molecular hydrogen. However, if massive stars form within these protogalaxies, they may suppress further star formation by photodissociating the H2. We examine the importance of this effect by estimating the timescale on which significant H2 is destroyed. We show that photodissociation is significant in the least massive protogalaxies, but becomes less so as the protogalactic mass increases. We also examine the effects of photodissociation on dense clumps of gas within the protogalaxy. We find that while collapse will be inhibited in low density clumps, denser ones may survive to form stars.Comment: 13 pages, 10 figures. Minor revisions to match version accepted by MNRA

Synthesizing Field and Experimental Observations to Investigate the Behavior of Pyroclastic Density Currents

One of the major hazards associated with volcanic eruptions are pyroclastic density currents (PDCs), which are fast-moving volcanic avalanches consisting of ash, boulders, and gas. Because of their unpredictability, studying PDCs in real time is dangerous and difficult. Therefore, we investigate the deposits produced by PDCs and use granular flow experiments to simulate PDCs in the laboratory. The experimental results allow us to understand sediment transport and erosional processes at small scales, and then we can extrapolate those results to natural PDCs. By better understanding what controls PDC behavior, we hope to ultimately improve risk assessment for these dangerous flows