10,921 research outputs found
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
Diversity among Bi-ethnic students and differences in educational outcomes and social functioning
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
- …