122 research outputs found
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 ()
clouds (CNM), warm () 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
times Habing's (1968) value for the radiation field at the solar circle in this
band, and quite close to Draine's (1976) value, . 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 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
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