9,874 research outputs found
Red Runaways II: Low mass Hills stars in SDSS Stripe 82
Stars ejected from the Galactic centre can be used to place important
constraints on the Milky Way potential. Since existing hypervelocity stars are
too distant to accurately determine orbits, we have conducted a search for
nearby candidates using full three-dimensional velocities. Since the efficacy
of such studies are often hampered by deficiencies in proper motion catalogs,
we have chosen to utilize the reliable, high-precision SDSS Stripe 82 proper
motion catalog. Although we do not find any candidates which have velocities in
excess of the escape speed, we identify 226 stars on orbits that are consistent
with Galactic centre ejection. This number is significantly larger than what we
would expect for halo stars on radial orbits and cannot be explained by disk or
bulge contamination. If we restrict ourselves to metal-rich stars, we find 29
candidates with [Fe/H] > -0.8 dex and 10 with [Fe/H] > -0.6 dex. Their
metallicities are more consistent with what we expect for bulge ejecta, and so
we believe these candidates are especially deserving of further study. We have
supplemented this sample using our own radial velocities, developing an
algorithm to use proper motions for optimizing candidate selection. This
technique provides considerable improvement on the blind spectroscopic sample
of SDSS, being able to identify candidates with an efficiency around 20 times
better than a blind search.Comment: 13 pages, accepted for publication in Ap
Nearby Low-Mass Hypervelocity Stars
Hypervelocity stars are those that have speeds exceeding the escape speed and
are hence unbound from the Milky Way. We investigate a sample of low-mass
hypervelocity candidates obtained using data from the high-precision SDSS
Stripe 82 catalogue, which we have combined with spectroscopy from the 200-inch
Hale Telescope at Palomar Observatory. We find four good candidates, but
without metallicities it is difficult to pin-down their distances and therefore
total velocities. Our best candidate has a significant likelihood that it is
escaping the Milky Way for a wide-range of metallicities.Comment: 5 pages; Contribution to proceedings for "The Milky Way Unravelled by
Gaia" conference, Barcelona, Dec 201
Linear laser diode arrays for improvement in optical disk recording for space stations
The design and fabrication of individually addressable laser diode arrays for high performance magneto-optic recording systems are presented. Ten diode arrays with 30 mW cW light output, linear light vs. current characteristics and single longitudinal mode spectrum were fabricated using channel substrate planar (CSP) structures. Preliminary results on the inverse CSP structure, whose fabrication is less critically dependent on device parameters than the CSP, are also presented. The impact of systems parameters and requirements, in particular, the effect of feedback on laser design is assessed, and techniques to reduce feedback or minimize its effect on systems performance, including mode-stabilized structures, are evaluated
High-speed civil transport flight- and propulsion-control technological issues
Technology advances required in the flight and propulsion control system disciplines to develop a high speed civil transport (HSCT) are identified. The mission and requirements of the transport and major flight and propulsion control technology issues are discussed. Each issue is ranked and, for each issue, a plan for technology readiness is given. Certain features are unique and dominate control system design. These features include the high temperature environment, large flexible aircraft, control-configured empennage, minimizing control margins, and high availability and excellent maintainability. The failure to resolve most high-priority issues can prevent the transport from achieving its goals. The flow-time for hardware may require stimulus, since market forces may be insufficient to ensure timely production. Flight and propulsion control technology will contribute to takeoff gross weight reduction. Similar technology advances are necessary also to ensure flight safety for the transport. The certification basis of the HSCT must be negotiated between airplane manufacturers and government regulators. Efficient, quality design of the transport will require an integrated set of design tools that support the entire engineering design team
Should we attempt global (inlet engine airframe) control design?
The feasibility of multivariable design of the entire airplane control system is briefly addressed. An intermediate step in that direction is to design a control for an inlet engine augmentor system by using multivariable techniques. The supersonic cruise large scale inlet research program is described which will provide an opportunity to develop, integrate, and wind tunnel test a control for a mixed compression inlet and variable cycle engine. The integrated propulsion airframe control program is also discussed which will introduce the problem of implementing MVC within a distributed processing avionics architecture, requiring real time decomposition of the global design into independent modules in response to hardware communication failures
Improving the Reliability and Modal Stability of High Power 870 nm AlGaAs CSP Laser Diodes for Applications to Free Space Communication Systems
The operating characteristics (power-current, beam divergence, etc.) and reliability assessment of high-power CSP lasers is discussed. The emission wavelength of these lasers was optimized at 860 to 880 nm. The operational characteristics of a new laser, the inverse channel substrate planar (ICSP) laser, grown by metalorganic chemical vapor deposition (MOCVD), is discussed and the reliability assessment of this laser is reported. The highlights of this study include a reduction in the threshold current value for the laser to 15 mA and a degradation rate of less than 2 kW/hr for the lasers operating at 60 mW of peak output power
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