OH-selected AGB and post-AGB objects I.Infrared and maser properties

Using 766 compact objects from a survey of the galactic Plane in the 1612-MHz OH line, new light is cast on the infrared properties of evolved stars on the TP-AGB and beyond. The usual mid-infrared selection criteria, based on IRAS colours, largely fail to distinguish early post-AGB stages. A two-colour diagram from narrower-band MSX flux densities, with bimodal distributions, provides a better tool to do the latter. Four mutually consistent selection criteria for OH-masing red PPNe are given, as well as two for early post-AGB masers and one for all post--AGB masers, including the earliest ones. All these criteria miss a group of blue, high-outflow post-AGB sources with 60-mum excess; these will be discussed in detail in Paper II. The majority of post-AGB sources show regular double-peaked spectra in the OH 1612-MHz line, with fairly low outflow velocities, although the fractions of single peaks and irregular spectra may vary with age and mass. The OH flux density shows a fairly regular relation with the stellar flux and the envelope optical depth, with the maser efficiency increasing with IRAS colour R21. The OH flux density is linearly correlated with the 60-mum flux density.Comment: 16 pages, LaTex, 22 figures, AJ (accepted

Spectral Characteristics of Greenland Lichens

Spectral reflectance measurements conducted during two field campaigns in west Greenland, and in the laboratory using samples collected during those campaigns, are discussed to evaluate the spectral signature of lichens. Given the diversity in lichen species, colors, and appearance — ranging from crust-like (crustose) to almost like mini shrubs (fructicose) — it is not surprising that no single signature was found. Some of the brighter fructicose lichens have reflectance characteristics very similar to those of green vegetation, with a pronounced rise in reflectivity around 750 nm. However, the most abundant lichen species covering rocks in the ice-marginal zone of west Greenland are dark grey to black crustose and foliose ephilithic (rock-growing) lichens and the shape of the reflectance spectrum for these lichens is generally very different from that of other surface types and landcovers, with near-zero reflectance at visible wavelengths, and a maximum around 1 600 nm. This characteristic allows rock-covered lichen to be identified on multispectral satellite imagery.L’évaluation de la signature spectrale des lichens est effectuée à partir de mesures prises en laboratoire et sur le terrain, au Groenland occidental. Aucune signature spécifique ne peut être identifiée, en raison de la diversité des espèces quant à leur couleur et à leur port, allant de la croûte à l’arbuste nain. Les lichens arbustifs les plus brillants montrent une signature spectrale semblable à celle des plantes vertes, avec un pic très prononcé autour de 750 nm. Toutefois, les lichens les plus abondants sur les roches à proximité des glaces sont gris foncés à noirs et du type crustacé ou foliacé; leur spectre de réflectance montre une allure très différente de celle des autres types de surface et de couverture, et se rapproche de zéro dans le spectre visible avec un pic autour de 1 600 nm. Cette caractéristique permet l’identification des roches recouvertes de lichens par l’imagerie satellitaire multispectrale

Patterns of calculated basal drag on ice streams B and C, Antarctica

This is the published version.Patterns of strain rate and slope on the ice streams are unusual. They cannot be accounted for in the usual way as due to standing waves in ice flow over a basal obstruction to flow (such as a sticky spot) . The features are studied using the force-budget technique. The conventional flow law is used, together with measurements of surface strain rate and shape of the glacier, to compute basal drag. The results for Ice Stream C are as expected, in that the drag varies from site to site but is directed inland, restraining the flow. The calculated drag at the base of Ice Stream B, on the other hand, is in places such that it acts to propel the glacier forward. This result is untenable. Either the conventional flow law is not applicable to Ice Stream B or there are large spatial variations in ice stiffness, perhaps associated with foliation, or both

Force budget: I. Theory and numerical methods

This is the published version, also available here: http://dx.doi.org/10.3189/002214389793701581.A practical method is developed for calculating stresses and velocities at depth using field measurements of the geometry and surface velocity of glaciers. To do this, it is convenient to partition full stresses into lithostatic and resistive components. The horizontal gradient in vertically integrated lithostatic stress is the driving stress and it describes the horizontal action of gravity. The horizontal resistive stress gradients describe the reactions. Resistive stresses are simply related to deviatoric stresses and hence to strain-rates through a constitutive relation. A numerical scheme can be used to calculate stresses and velocities from surface velocities and slope, and from ice thickness. There is no mathematical requirement that the variations in these quantities be small

Determination of a flow center on an ice cap

This is the published version.A method for identifying the center of ice flow is developed and applied using results from surveys of a strain grid near the summit of Dunde Ice Cap (central China). Strain rates are used to compute stresses. These are used with a consideration of the balance of forces to compute basal friction. The flow center at the bed occurs where this friction changes sign. For Dunde Ice Cap, the basal flow center nearly underlies the summit

The role of lateral drag in the dynamics of Ice Stream B, Antarctica

The partitioning of resistive force between the bed and sides of Ice Stream B, Antarctica , is obtained for three large areas that have bee n measured using repeat aerial photogrammetry. Problems associated with data errors and local variations in ice strength and velocity are reduced by considering the a really ave raged budget of forces for each photo block. Results indicate that the bed under Ice Stream B must be very weak and unable to provide much res instance. Mechanical l control on this ice stream emanates almost entirely from the lateral margins

Flow laws for glacier ice: comparison of numerical predictions and field measurements

This is the published version, also available here: http://dx.doi.org/10.3189/002214390793701372.Ice flow along the 20 km long strain network up-stream of the Dye 3 bore hole in Greenland is studied in detail. By solving the force—balance equations and using selected flow laws, stresses and strain-rates are calculated throughout the section of the ice sheet. The validity of the results is evaluated by comparison with the velocity profile derived from bore-hole-tilting measurements, and with observed surface strain-rates. A number of constitutive relations are tried and most predict a velocity profile at the bore-hole site that is in good agreement with that observed, if appropriate enhancement factors are used. However, there are major discrepancies between modeled and measured surface strain-rates. Use of Nye's generalization of Glen's flow law, or an anisotropic constitutive relation, requires unrealistically large along-flow variations in the enhancement factor. Inclusion of normal stress effects can lead to much better agreement, but it is possible that other processes, such as dynamic recrystallization or primary creep, should be included in the constitutive relation of polar ice

Force Budget: II. Application to Two-Dimensional Flow Along Byrd Station Strain Network, Antarctica

This is the published version, also available here: http://dx.doi.org/10.3189/002214389793701455.Resistive stresses and velocities at depth are calculated along the Byrd Station Strain Network, Antarctica, using field data. There are found to be large longitudinal variations in basal drag and this result is little affected by errors in the input data or by uncertainties in the constitutive relation for ice. Basal drag varies by a factor of about 2 along the strain network, and is usually equal to the driving stress to within 10–20%. Sites of high drag are not always correlated with basal topographic highs, indicating that some process such as basal water drainage is involved in controlling the friction at the bed. Basal sliding velocities are very sensitive to errors in measured surface velocities and the rate factor in Glen's flow law. As a result, calculated sliding velocities are much less reliable than deep stresses, and need to be interpreted with caution

New and improved determinations of velocity of ice streams B and C, West Antarctica

This is the publisher's version, copyright by the International Glaciological Society.Measurements of velocity have been made on and next to Ice Streams B and C, West Antarctica. The results are more precise than previous work and constitute a 93% increase in the number of values. These velocities are used to describe the confluence of flow into the ice streams and the development of fast icestream flow. The onset of fast-streaming flow occurs in many separate tributaries that coalesce down-glacier into the major ice streams. For those inter-stream ridges that have been studied, the flow is consistent with steady state. Along Ice Stream B, gradients in longitudinal stress offer little resistance to the ice flow. The transition from basal-drag control to ice-shelf flow is achieved through reduced drag at the glacier base and increased resistance associated with lateral drag. Velocities in the trunk of Ice Stream C are nearly zero but those at the up-glacial head are similar to those at the head of Ice Stream B

Reply to Lliboutry's letter 'Why calculated basal drags of ice streams can be fallacious'

This is the publisher's version, copyright by the International Glaciological Society.No abstract is available for this item