3D Metrology for Process Control and Structure Integrity for Evaluating MEMS Reliability for Space Applications
MEMS fabrication processes are as diverse as their applications.
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Engineering topics
Koester, D.
Publications and source records attributed to Koester, D..
The pulsation index, effective temperature, and thickness of the hydrogen layer in the pulsating DA white dwarf G117-B15A
We have measured the amplitude of the 215 s pulsation of the pulsating DA white dwarf, or ZZ Ceti star, G117-B15A in six passbands with effective wavelengths from 1570 to 6730 A. We find that the index of the pulsation is l = 1 with a high degree of confidence, the first unambiguous determination of l for a pulsation of a ZZ Ceti star. We also find that log g and T(sub eff) are tightly correlated for model atmospheres that fit the data, such that at log g = 7.5 the temperature is 11,750 K and at log g = 8.0 the temperature is 12,375 K. Adopting log g = 7.97 +/- 0.06 from published observations of the optical spectrum of G117-B15A, the correlation yields T(sub eff) = 12,375 +/- 125 K. This temperature is free of flux calibration errors and should be substantially more reliable than temperatures derived for IUE spectra. Since G117-B15A is thought to lie close to the blue edge of the ZZ Ceti instability strip, this low temperature also implies a low temperature for the blue edge. Using pulsation models calculated by Fontaine et al. (1992) and Bradley (1994), we find that the mass of the hydrogen layer in G117-B15A lies between 1.0 x 10(exp -6) solar mass (for k = 1) and 8 x 10(exp -5) solar mass (for k = 2). This range of masses is (barely) consistent with the masses predicted by recent models for the ejection of planetary nebulae, (8-13) x 10(exp -5) solar mass. The mass is too large to be consistent with models invoking thin hydrogen layers to explain the spectral evolution of white dwarfs.
Robinson, E. L.↗
Atmospheric parameters of field subdwarf B stars
High signal-to-noise ratio optical spectrophotometry of a sample of field subluminous B stars drawn largely from the Palomar Green ultraviolet-excess survey is analyzed with a new grid of model atmospheres and synthetic spectra. The stellar effective temperatures, surface gravities, and photospheric helium abundances are determined simultaneously from a detailed analysis of hydrogen and helium absorption line profiles. The derived temperatures and gravities place the subluminous B stars in the theoroetical Hertzsprung-Russell (H-R) diagram along and bounded below by theoretical sequences of the zero-age extended horizontal branch, lending strong support to the hypothesis that these stars are composed of helium-burning cores of approximately 0.5 solar mass overlain by very thin layers of hydrogen (approximately less than 0.02 solar mass). Various scenarios for their past evolutionary history are examined in the context of their probable future evolution into white dwarfs of lower than average mass.
Saffer, Rex A.↗
ROSAT EUV and soft X-ray studies of atmospheric composition and structure in G191-B2B
Previous studies of the hot DA white dwarf GI91-B2B have been unable to determine whether the observed soft X-ray and EUV opacity arises from a stratified hydrogen and helium atmosphere or from the presence of trace metals in the photosphere. New EUV and soft X-ray photometry of this star, made with the ROSAT observatory, when analyzed in conjunction with the earlier data, shows that the stratified models cannot account for the observed fluxes. Consequently, we conclude that trace metals must be a substantial source of opacity in the photosphere of G191-B2B.
Barstow, M. A.↗
Theoretical profiles of Lyman-alpha satellites and application to synthetic spectra of DA white dwarfs
We present new theoretical calculations for the red wing of the Lyman-alpha profile. Close collisions with neutral and ionized hydrogen lead to the formation of the pseudomolecules H-H and H-H(+) with the appearance of satellite features near 1600 and 1400 A. The calculations include multiperturber effects, which are responsible for the formation of H3 and H3 with features near 1950 and 2600 A. The theoretical absorption profiles are included in stellar atmosphere codes and used to predict synthetic spectra for DA white dwarfs of intermediate temperatures (20,000 to 8000 K). These new calculations offer a unique opportunity to determine accurate effective temperatures and surface gravities for the variable ZZ Ceti stars.
Allard, N. F.↗
The CI pressure shift and gravitational redshift of the cool DBQA5 white dwarf LDS678A
A high resolution ultraviolet spectrum of the helium rich degenerate LDS 678A, obtained with the International Ultraviolet Explorer (IUE) satellite is presented. LDS 678A is the coolest metallic line generate (DQ or DZ) yet observed with the IUE scale. These observations provide a detailed line profile of the strong C I 2479 absorption line with equivalent width (W sub 2479 = 2.35 plus or minus 0.06 angstroms) from which theoretical line profile fits yield a C abundance (log C/He = 6.4). The presence of carbon in a helium rich atmosphere lends credence to the notion that LDS 678A is a transitional case between the DB white dwarfs with nearly pure helium atmospheres and the helium rich DQ white dwarfs which exhibit carbon bands. Corrected for an inferred pressure shift for the C I line, a gravitational redshift is deduced from which a most probable mass of 0.55 solar mass is derived.
Sion, E. M.↗
Model atmosphere analysis of the DZ white dwarf K789 - 37
This paper presents new observations and an analysis using model atmosphere techniques of the recently discovered DZA white dwarf K789 - 37. The effective temperature derived is 9500 + or - 500 K. Abundances are determined for the elements hydrogen, magnesium, calcium, and iron, as well as upper limits for carbon and silicon. The abundances are very low compared to solar values and within the range typical for this spectral class. The implications of the results for the currently accepted explanation of trace elements in cool white dwarfs in terms of the diffusion/accretion scenario are discussed. It is concluded that the accreted matter cannot have had solar composition.
Koester, D.↗
Can the Exosat observations of DA white dwarfs be explained by layered atmospheric structures?
It is demonstrated that the Exosat broadband filter observations of DA white dwarfs, which have recently been interpreted with homogeneously mixed H/He atmospheres (Jordan et al, 1987; Paerels and Heise, 1989), can be explained with stratified theoretical models. In these models a very thin H layer is floating on top of the underlying He. The transition zone is determined by diffusive equilibrium. The resulting total hydrogen-layer masses are in the range (3-500) x 10 to the -16th solar mass. Several arguments are discussed that could lead to a decision about the correct interpretation, but a final answer is not possible.
Koester, D.↗
Chemical stratification in white dwarf atmospheres and envelopes
Theoretical arguments supporting a new mechanism maintaining a homogeneously mixed composition in white dwarf atmospheres with traces of helium are presented. Diffusion time scales, meridional circulation, mass loss, accretion of interstellar matter, convection, and radiative levitation are discussed. Theoretically, layered envelopes, with hydrogen on top of helium and an abundance profile in the transition layer determined by diffusion equilibrium, are expected. In cases with observed helium and hydrogen in the atmosphere this means that the total hydrogen mass must be very small. The empirical evidence for such atmospheres are assessed, using a new grid of model atmospheres with stratified element abundances and applying it to typical mixed abundance cases at the hot end of the white dwarf temperature sequence.
Koester, D.↗
Temperatures and surface gravities of DB white dwarfs
Multichannel observations or reobservations of all DB white dwarfs accessible from Palomar Observatory, reduced and calibrated with the AB79 program of Oke and Gunn (1983), have been compared by least squares fitting procedures with energy distributions calculated from new model atmospheres with He:H = 100,000 and reduced metal content He:C = 1,000,000. The surface gravities and temperatures derived reveal for the first time a definitive cooling sequence for the DB stars, with a narrow mass distribution comparable to that of the DA white dwarfs. The mass range, 0.55 + or - 0.10 solar mass, almost coincides with the range derived for DA, DC, and C2 stars and therefore suggests that the progenitors for all these stars are the same. Spectroscopic distance determinations and comparison with cooling ages demonstrate that DB stars account for about one-tenth of the total white dwarf birthrate.
Oke, J. B.↗