OH-selected AGB and post-AGB stellar objects II.Blue versus red evolution off the AGB

Using objects found in a systematic survey of the galactic Plane in the 1612-MHz OH line, we discuss in detail two ``sequences'' of post-AGB evolution, a red and a blue. We argue that the red and the blue groups separate by initial mass at 4Msun, based on evolutionary-sequence turn-off colours, spectral energy distributions, outflow velocities and scaleheight. The higher-mass (blue) objects may have earlier AGB termination. The lower-mass (red) objects undergo very sudden reddening for IRAS colour R21\sim1.2; these sources must all undergo a very similar process at AGB termination. The transition colour corresponds to average initial masses of 1.7Msun. A combined IRAS-MSX colour proves a very sensitive tool to distinguish lower-mass, early post-AGB objects from sources still on the AGB and also to distinguish more evolved post-AGB objects from star-forming regions. The high-mass blue objects are the likely precursors of bipolar planetary nebulae, whereas the low-mass red objects will evolve into elliptical planetary nebulae.Comment: 12 pages, LaTex, 7 figures (1 colour), AJ (accepted

Microcomputer-controlled polarographic instrumentation and its use in the determination of stability constants of crown ether complexes

A computer-controlled polarographic system is described, based on a commercially available polarograph interfaced to a microcomputer. Experiments are controlled and monitored entirely from software, including automatic evaluation of the Tast polarograms and addition of solutions to the polarographic cell from a motor burette. The program was written in FORTH, a computer language especially apt for laboratory automation. The system is used in the determination of stability constants of crown ether complexes

Polarographic determination of malononitrile

A procedure is proposed for the determination of malononitrile by differential pulse polarography in methanolic 0.1M tetraethylammonium iodide/0.001M tetramethylammonium hydroxide as the supporting electrolyte. In this medium malononitrile is chemically converted into an electroactive species. With close control of timing of the steps in the procedure the error of the method is ±1.5% in the concentration range 0.0001–0.001M. Acrylonitrile, benzonitrile and succinic acid dinitrile do not interfere

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

Blocking a wave: Frequency band gaps in ice shelves with periodic crevasses

We assess how the propagation of high-frequency elastic-flexural waves through an ice shelf is modified by the presence of spatially periodic crevasses. Analysis of the normal modes supported by the ice shelf with and without crevasses reveals that a periodic crevasse distribution qualitatively changes the mechanical response. The normal modes of an ice shelf free of crevasses are evenly distributed as a function of frequency. In contrast, the normal modes of a crevasse-ridden ice shelf are distributed unevenly. There are "band gaps", frequency ranges over which no eigenmodes exist. A model ice shelf that is 50 km in lateral extent and 300 m thick with crevasses spaced 500 m apart has a band gap from 0.2 to 0.38 Hz. This is a frequency range relevant for ocean wave/ice-shelf interactions. When the outermost edge of the crevassed ice shelf is oscillated at a frequency within the band gap, the ice shelf responds very differently from a crevasse-free ice shelf. The flexural motion of the crevassed ice shelf is confined to a small region near the outermost edge of the ice shelf and effectively "blocked" from reaching the interior.Comment: 6 pages, 4 figures, accepted to Annals of Glaciolog

Cost-efficient staffing under annualized hours

We study how flexibility in workforce capacity can be used to efficiently match capacity and demand. Flexibility in workforce capacity is introduced by the annualized hours regime. Annualized hours allow organizations to measure working time per year, instead of per month or per week. An additional source of flexibility is hiring employees with different contract types, like full-time, part-time, and min-max, and by hiring subcontractors. We propose a mathematical programming formulation that incorporates annualized hours and shows to be very flexible with regard to modeling various contract types. The objective of our model is to minimize salary cost, thereby covering workforce demand, and using annualized hours. Our model is able to address various business questions regarding tactical workforce planning problems, e.g., with regard to annualized hours, subcontracting, and vacation planning. In a case study for a Dutch hospital two of these business questions are addressed, and we demonstrate that applying annualized hours potentially saves up to 5.2% in personnel wages annually