Design of electromagnetic bearing for vibration control of flexible transmission shaft

Recently magnetic bearings were proposed by several researchers and shown to be viable on a variety of rotor assemblies. The design and construction of such a bearing, which employs features hitherto not used by other workers is examined. These include an original approach to the design of the electromagnets and their amplifiers, and to software in a digital control system, to condition the control signals so as to make the magnets appear to be linear and uncoupled. The resulting system is used to control a rotor-bearing assembly, whose speed range covers two flexural-critical speeds

Superconducting rebalance acceleration and rate sensor

The goal of this program is the development of a high precision multisensor based on a high T(sub c) superconducting proof mass. The design of a prototype is currently underway. Key technical issues appear resolvable. High temperature superconductors have complicated, hysteretic flux dynamics but the forces on them can be linearly controlled for small displacements. Current data suggests that the forces on the superconductors decay over a short time frame and then stabilize, though very long term data is not available. The hysteretic force characteristics are substantial for large scale excursions, but do not appear to be an issue for the very small displacements required in this device. Sufficient forces can be exerted for non-contact suspension of a centimeter sized proof mass in a vacuum sealed nitrogen jacket cryostat. High frequency capacitive sensing using stripline technology will yield adequate position resolution for 0.1 micro-g measurements at 100 Hz. Overall, a reasonable cost, but very high accuracy, system is feasible with this technology

Electrostatically suspended and sensed micro-mechanical rate gyroscope

The goal of this work is development of fully electrostatically suspended and rebalancing angular rate sensing micro-gyroscope fabricated according to standard VLSI techniques. Fabrication of test structures is proceeding. Off chip electronics for the electrostatic sensing and driving circuits has been tested. The prototype device will be assembled in a hybrid construction including the FET input stages of the sensors

Do the Broad Emission Line Clouds See the Same Continuum that We See?

Recent observations of quasars, Mrk 335 and the HST quasar composite spectrum, have indicated that many of them have remarkably soft ionizing continua (fnu ~ nu^-2.0, 13.6 eV -- 100 eV). We point out that the number of E > 54.4 eV photons is insufficient to create the observed strengths of the He II emission lines. While the numbers of photons which energize C IV 1549 and O VI 1034 are sufficient, even the most efficiently emitting clouds for these two lines must each cover at least 20% -- 40% of the source. If the typical quasar ionizing continuum is indeed this soft, then we must conclude that the broad emission line clouds must see a very different (harder) continuum than we see. The other viable possibility is that the UV -- EUV SED is double-peaked, with the second peaking near 54 eV, its Wien tail the observed soft X-ray excess.Comment: 11 pages AAS-LATeX aaspp4.sty format, including 1 figure; accepted for publication in Ap

Supernova remnant S147 and its associated neutron star(s)

The supernova remnant S147 harbors the pulsar PSR J0538+2817 whose characteristic age is more than an order of magnitude greater than the kinematic age of the system (inferred from the angular offset of the pulsar from the geometric center of the supernova remnant and the pulsar proper motion). To reconcile this discrepancy we propose that PSR J0538+2817 could be the stellar remnant of the first supernova explosion in a massive binary system and therefore could be as old as its characteristic age. Our proposal implies that S147 is the diffuse remnant of the second supernova explosion (that disrupted the binary system) and that a much younger second neutron star (not necessarily manifesting itself as a radio pulsar) should be associated with S147. We use the existing observational data on the system to suggest that the progenitor of the supernova that formed S147 was a Wolf-Rayet star (so that the supernova explosion occurred within a wind bubble surrounded by a massive shell) and to constrain the parameters of the binary system. We also restrict the magnitude and direction of the kick velocity received by the young neutron star at birth and find that the kick vector should not strongly deviate from the orbital plane of the binary system.Comment: 9 pages, 5 figures, revised version accepted for publication in A&

The Chemical Composition and Gas-to-Dust Mass Ratio of Nearby Interstellar Matter

Recent results on nearby interstellar gas and interstellar byproducts within the solar system are used to select among the equilibrium radiative transfer models of the nearest interstellar material of Slavin and Frisch (2002). Assuming O/H~400 ppm, Models 2 and 8 are found to yield good fits to available data on local interstellar material, and pickup ions and anomalous cosmic rays inside of the heliosphere, with the exception of Ne. For these models, the density of interstellar gas at the entry point to the heliosphere is n(HI)~0.20 /cc and n(e-)~0.1 /cc. These models suggest the chemical composition of the nearby ISM is ~60--70% subsolar if S is undepleted (where HI and HII must be included for abundance calculations). Gas-to-dust mass ratios of 178-183 for solar abundances, or 611-657 for 70% solar abundances are found. The percentage of the dust mass that is carried by iron is directly correlated with the gas-to-dust mass ratio implying an Fe-rich grain core remains after grain destruction.Comment: 36 pages, 6 figures, accepted by Ap

Time Dependence of the Ultraviolet Radiation Field in the Local Interstellar Medium

Far Ultraviolet radiation has been recognized as the main source of heating of the neutral interstellar gas, and, as a consequence, it determines whether the thermal balance of the neutral gas results in cold ($T\sim 50 - 100 K$) clouds (CNM), warm ($T \sim 10^4 K$) clouds (WNM), or a combination of the two. High FUV fields convert the neutral gas to WNM, while low fields result in CNM. The sources of FUV radiation are the short-lived massive stars that generally originate in associations that form in Giant Molecular Clouds present in the galactic disk. Using McKee & Williams' (1997) distribution of birthrates for OB associations in the Galaxy, we determine the expected behavior of the time-dependent FUV field for random positions in the local ISM. The FUV field is calculated in two bands (912-1100 \AA and 912-2070 \AA) and at the wavelength 1400 \AA. Our median value for the [912-2070 \AA] band is $G_0=1.6$ times Habing's (1968) value for the radiation field at the solar circle in this band, and quite close to Draine's (1976) value, $G_0=1.7$. Due to attenuation by dust, only associations within about 500 pc contribute significantly to the energy density at a given point. The FUV field is asymmetric at a given point, and the asymmetry grows for higher fields. The FUV field fluctuates with a variety of amplitudes, the larger ones being less frequent. The mean field is about twice the median field because of these fluctuations, or spikes, in the radiation field. These spikes, which last $\sim 30$ Myr, are caused by the infrequent birth of nearby associations. We also model shorter duration spikes caused by runaway OB stars. The presence of a fluctuating heating rate created by the fluctuating FUV field converts CNM to WNM and vice versa.Comment: 43 pages, 23 figures, submitted to Ap

Evidence for a High Carbon Abundance in the Local Interstellar Cloud

The nature of the Local Interstellar Cloud (LIC) is highly constrained by the combination of in situ heliospheric and line-of-sight data towards nearby stars. We present a new interpretation of the LIC components of the absorption line data towards epsilon CMa, based on recent atomic data that include new rates for the Mg+ to Mg0 dielectronic recombination rate, and using in situ measurements of the temperature and density of neutral helium inside of the heliosphere. With these data we are able to place interesting limits on the gas phase abundance of carbon in the LIC. If the C/S abundance ratio is solar, ~20, then no simultaneous solution exists for the N(Mg I), N(Mg II), N(C II) and N(C II*) data. The combined column density and in situ data favor an abundance ratio A(C)/A(S) = 47 +22 -26. We find that the most probable gas phase C abundance is in the range 400 to 800 ppm with a lower limit of ~330. We speculate that such a supersolar abundance could have come to be present in the LIC via destruction of decoupled dust grains. Similar enhanced C/H ratios are seen in very low column density material, N(H) < 10^19 cm^-2, towards several nearby stars.Comment: 11 pages, 1 figure, Accepted for publication in ApJ Letter