Atmospheric dispersion and the implications for phase calibration

The success of any ALMA phase-calibration strategy, which incorporates phase transfer, depends on a good understanding of how the atmospheric path delay changes with frequency (e.g. Holdaway & Pardo 2001). We explore how the wet dispersive path delay varies for realistic atmospheric conditions at the ALMA site using the ATM transmission code. We find the wet dispersive path delay becomes a significant fraction (>5 per cent) of the non-dispersive delay for the high-frequency ALMA bands (>160 GHz, Bands 5 to 10). Additionally, the variation in dispersive path delay across ALMA's 4-GHz contiguous bandwidth is not significant except in Bands 9 and 10. The ratio of dispersive path delay to total column of water vapour does not vary significantly for typical amounts of water vapour, water vapour scale heights and ground pressures above Chajnantor. However, the temperature profile and particularly the ground-level temperature are more important. Given the likely constraints from ALMA's ancillary calibration devices, the uncertainty on the dispersive-path scaling will be around 2 per cent in the worst case and should contribute about 1 per cent overall to the wet path fluctuations at the highest frequencies.Comment: 13 pages, 10 figures, ALMA Memo 59

On the Radial Distribution of White Dwarfs in the Globular Cluster NGC 6397

We have examined the radial distribution of white dwarfs over a single HST/ACS field in the nearby globular cluster NGC 6397. In relaxed populations, such as in a globular cluster, stellar velocity dispersion, and hence radial distribution, is directly dependent on stellar masses. The progenitors of very young cluster white dwarfs had a mass of ~0.8 solar masses, while the white dwarfs themselves have a mass of ~0.5 solar masses. We thus expect young white dwarfs to have a concentrated radial distribution (like that of their progenitors) that becomes more extended over several relaxation times to mimic that of ~0.5 solar mass main-sequence stars. However, we observe young white dwarfs to have a significantly extended radial distribution compared to both the most massive main sequence stars in the cluster and also to old white dwarfs.Comment: 13 pages including 1 table and 3 figures. Accepted for publication in the MNRAS Letter

Power-law nature of field-effect transistor experimental characteristics

In making experimental measurements of field-effect transistor static drain characteristics in the pinch-off region, determination of the effective pinch-off voltage is not possible by direct measurement because of the presence of spurious drain current at and beyond pinch-off. Further, indirect measurement is hampered by the absence of a theoretical straight-line function from which the pinch-off voltage may be obtained as an intercept or a slope. In this communication a power-law relation for the transfer characteristic is assumed, from which values of both the pinch-off voltage and the exponent may be obtained directly from a straight-line plot of experimental quantities

[Authors' Comment on] Power-Law Nature of Field-Effect Transistor Characteristics

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Comment on "FET input capacitance"

In a recent correspondence, a formula for the input capacitance of a field-effect transistor (FET) was derived. The formula is incorrect because of an error in theory, the identical error having been committed by several other authors. The source of the error has previously been noted, and this correspondence repeats some of the pertinent arguments of Richer

Ethyl 6-Bromo-3indolcarboxylate and 3-Hydroxyacetal-6-bromoindole, novel bromoindoles from the sponge Pleroma menoui of the Coral Sea

L'éponge #Plemora menoui (Démosponge, #O. Lithistida, ss. #O. Trienosina, #Desmophorina, Fam. #Pleromidae$) récoltée à une profondeur de 500 m dans la Mer de Corail, au sud-est de Nouméa contient deux nouveaux alcaloïdes : ethyl 6-bromo-3-indolcarboxylate and 3-hydroxyacétal-6-bromoindole. (Résumé d'auteur