42,722 research outputs found
Reverse engineering a spectrum: using fluorescent spectra of molecular hydrogen to recreate the missing Lyman-α line of pre-main sequence stars
The hydrogen Lyman-α (Lyα) line, a major source of ionization of metals in the circumstellar disks of pre-main sequence (PMS) stars, is usually not observed due to absorption by interstellar and circumstellar hydrogen. We have developed a technique to reconstruct the intrinsic Lyα line using the observed emission in the H2 B-X lines that are fluoresced by Lyα. We describe this technique and the subsequent analysis of the ultraviolet (UV) spectra of the TW Hya, RU Lupi and other PMS stars. We find that the reconstructed Lyα lines are indeed far brighter than any other feature in the UV spectra of these stars and therefore play an important role in the ionization and heating of the outer layers of circumstellar disks
B-field Determination from Magnetoacoustic Oscillations in kHz QPO Neutron Star Binaries: Theory and Observations
We present a method for determining the B-field around neutron stars based on
observed kHz and viscous QPO frequencies used in combination with the best-fit
optical depth and temperature of a Comptonization model. In the framework of
the transition layer QPO model, we analyze magnetoacoustic wave formation in
the layer between a neutron star surface and the inner edge of a Keplerian
disk. We derive formulas for the magnetoacoustic wave frequencies for different
regimes of radial transition layer oscillations. We demonstrate that our model
can use the QPO as a new kind of probe to determine the magnetic field
strengths for 4U 1728-42, GX 340+0, and Sco X-1 in the zone where the QPOs
occur. Observations indicate that the dependence of the viscous frequency on
the Keplerian frequency is closely related to the inferred dependence of the
magnetoacoustic wave frequency on the Keplerian frequency for a dipole magnetic
field. The magnetoacoustic wave dependence is based on a single parameter, the
magnetic moment of the star as estimated from the field strength in the
transition layer. The best-fit magnetic moment parameter is about (0.5-1)x
10^{25} G cm^3 for all studied sources. From observational data, the magnetic
fields within distances less 20 km from neutron star for all three sources are
strongly constrained to be dipole fields with the strengths 10^{7-8} G on the
neutron star surface.Comment: 10 pages, 1 figure, accepted for the Astrophysical Journal Letter
Nondestructive tests of regenerative chambers
The capabilities and limitations of nondestructive evaluation methods were studied to detect and locate bond deficiencies in regeneratively cooled thrust chambers for rocket engines. Flat test panels and a cylinder were produced to simulate regeneratively cooled thrust chamber walls. Planned defects with various bond integrities were produced in the panels to evaluate the sensitivity, accuracy, and limitations of nondestructive methods to define and locate bond anomalies. Holography, acoustic emission, and ultrasonic scan were found to yield sufficient data to discern bond quality when used in combination and in selected sequences. Bonding techniques included electroforming and brazing. Materials of construction included electroformed nickel bonded to Nickel 200 and OFHC copper, electroformed copper bonded to OFHC copper, and 300 series stainless steel brazed to OFHC copper. Variations in outer wall strength, wall thickness, and defect size were evaluated for nondestructive test response
Nondestructive tests of regenerative chambers, appendix
For abstract, see N75-12058
THE USE OF AN IONIZATION GAGE AS A QUANTITATIVE ANALYZER FOR BIGASEOUS MIXTURES
Helium & air - ionization gauge for determining percentage composition of gaseous mixture
Gas analyzer for bi-gaseous mixtures Patent
Gas analyzer for bi-gaseous mixtures suitable for use in test facilitie
Photographic investigation of propellant stream behavior in a firing rocket engine, volume I Technical summary report, 1 Aug. 1964 - 28 Feb. 1966
Photographic study to obtain injection and combustion patterns of F-1 rocket engine injector element
SPH Simulations of Direct Impact Accretion in the Ultracompact AM CVn Binaries
The ultracompact binary systems V407 Vul (RX J1914.4+2456) and HM Cnc (RX
J0806.3+1527) - a two-member subclass of the AM CVn stars - continue to pique
interest because they defy unambiguous classification. Three proposed models
remain viable at this time, but none of the three is significantly more
compelling than the remaining two, and all three can satisfy the observational
constraints if parameters in the models are tuned. One of the three proposed
models is the direct impact model of Marsh & Steeghs (2002), in which the
accretion stream impacts the surface of a rapidly-rotating primary white dwarf
directly but at a near-glancing angle. One requirement of this model is that
the accretion stream have a high enough density to advect its specific kinetic
energy below the photosphere for progressively more-thermalized emission
downstream, a constraint that requires an accretion spot size of roughly
1.2x10^5 km^2 or smaller. Having at hand a smoothed particle hydrodynamics code
optimized for cataclysmic variable accretion disk simulations, it was
relatively straightforward for us to adapt it to calculate the footprint of the
accretion stream at the nominal radius of the primary white dwarf, and thus to
test this constraint of the direct impact model. We find that the mass flux at
the impact spot can be approximated by a bivariate Gaussian with standard
deviation \sigma_{\phi} = 164 km in the orbital plane and \sigma_{\theta} = 23
km in the perpendicular direction. The area of the the 2\sigma ellipse into
which 86% of the mass flux occurs is roughly 47,400 km^2, or roughly half the
size estimated by Marsh & Steeghs (2002). We discuss the necessary parameters
of a simple model of the luminosity distribution in the post-impact emission
region.Comment: 24 pages, 5 figures, Accepted for publication in Ap
On-board timeline validation and repair : a feasibility study
We report on the progress and outcome of a recent ESAfunded project (MMOPS) designed to explore the feasibility of on-board reasoning about payload timelines. The project sought to examine the role of on-board timeline reasoning and the operational context into which it would fit. We framed a specification for an on-board service that fits with existing practices and represents a plausible advance within sensible constraints on the progress of operations planning. We have implemented a prototype to demonstrate the feasibility of such a system and have used it to show how science gathering operations might be improved by its deployment
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