10,698 research outputs found
Development of low-frequency kernel-function aerodynamics for comparison with time-dependent finite-difference methods
Finite difference methods for unsteady transonic flow frequency use simplified equations in which certain of the time dependent terms are omitted from the governing equations. Kernel functions are derived for two dimensional subsonic flow, and provide accurate solutions of the linearized potential equation with the same time dependent terms omitted. These solutions make possible a direct evaluation of the finite difference codes for the linear problem. Calculations with two of these low frequency kernel functions verify the accuracy of the LTRAN2 and HYTRAN2 finite difference codes. Comparisons of the low frequency kernel function results with the Possio kernel function solution of the complete linear equations indicate the adequacy of the HYTRAN approximation for frequencies in the range of interest for flutter calculations
The Search for Celestial Positronium via the Recombination Spectrum
Positronium is the short-lived atom consisting of a bound electron-positron
pair. In the triplet state, when the spins of both particles are parallel,
radiative recombination lines will be emitted prior to annihilation. The
existence of celestial positronium is revealed through gamma-ray observations
of its annihilation products. These observations however have intrinsically low
angular resolution. In this paper we examine the prospects for detecting the
positronium recombination spectrum. Such observations have the potential to
reveal discrete sources of positrons for the first time and will allow the
acuity of optical telescopes and instrumentation to be applied to observations
of high energy phenomena.
We review the theory of the positronium recombination spectrum and provide
formulae to calculate expected line strengths from the positrons production
rate and for different conditions in the interstellar medium. We estimate the
positronium emission line strengths for several classes of Galactic and
extragalactic sources. These are compared to current observational limits and
to current and future sensitivities of optical and infrared instrumentation. We
find that observations of the Ps-alpha line should soon be possible due to
recent advances in near-infrared spectroscopy.Comment: Accepted for publication in Ap
MMTF: The Maryland-Magellan Tunable Filter
This paper describes the Maryland-Magellan Tunable Filter (MMTF) on the
Magellan-Baade 6.5-meter telescope. MMTF is based on a 150-mm clear aperture
Fabry-Perot (FP) etalon that operates in low orders and provides transmission
bandpass and central wavelength adjustable from ~5 to ~15 A and from ~5000 to
over ~9200 A, respectively. It is installed in the Inamori Magellan Areal
Camera and Spectrograph (IMACS) and delivers an image quality of ~0.5" over a
field of view of 27' in diameter (monochromatic over ~10'). This versatile and
easy-to-operate instrument has been used over the past three years for a wide
variety of projects. This paper first reviews the basic principles of FP
tunable filters, then provides a detailed description of the hardware and
software associated with MMTF and the techniques developed to observe with this
instrument and reduce the data. The main lessons learned in the course of the
commissioning and implementation of MMTF are highlighted next, before
concluding with a brief outlook on the future of MMTF and of similar facilities
which are soon coming on line.Comment: 38 pages, 12 figures, 3 tables, now accepted for publication to the
Astronomical Journa
GALAH Survey: Chemically Tagging the Thick Disk
The GALAH survey targets one million stars in the southern hemisphere down to
a limiting magnitude of V = 14 at the Anglo- Australian Telescope. The project
aims to measure up to 30 elemental abundances and radial velocities (~1 km/s
accuracy) for each star at a resolution of R = 28000. These elements fall into
8 independent groups (e.g. alpha, Fe peak, r-process). For all stars, Gaia will
provide distances to 1% and transverse velocities to 1 km/s or better, giving
us a 14D set of parameters for each star, i.e. 6D phase space and 8D abundance
space. There are many scientic applications but here we focus on the prospect
of chemically tagging the thick disk and making a direct measurement of how
stellar migration evolves with cosmic time.Comment: Barcelona conference (Dec 1-5, 2014): The Milky Way Unravelled by
Gaia, eds. Soubiran, Figueras, Walton; 8 page
Galactic Winds
Galactic winds are the primary mechanism by which energy and metals are
recycled in galaxies and are deposited into the intergalactic medium. New
observations are revealing the ubiquity of this process, particularly at high
redshift. We describe the physics behind these winds, discuss the observational
evidence for them in nearby star-forming and active galaxies and in the
high-redshift universe, and consider the implications of energetic winds for
the formation and evolution of galaxies and the intergalactic medium. To
inspire future research, we conclude with a set of observational and
theoretical challenges.Comment: Paper to be published in 2005 Annual Review of Astronomy &
Astrophysics; revision based on comments from readers and production editor.
Figure 1 was replaced to show the proper density scale. A PDF file combining
both text and figures is available at
http://www.astro.umd.edu/~veilleux/pubs/araa.pd
Calculation of unsteady aerodynamics for four AGARD standard aeroelastic configurations
Calculated unsteady aerodynamic characteristics for four Advisory Group for Aeronautical Research Development (AGARD) standard aeroelastic two-dimensional airfoils and for one of the AGARD three-dimensional wings are reported. Calculations were made using the finite-difference codes XTRAN2L (two-dimensional flow) and XTRAN3S (three-dimensional flow) which solve the transonic small disturbance potential equations. Results are given for the 36 AGARD cases for the NACA 64A006, NACA 64A010, and NLR 7301 airfoils with experimental comparisons for most of these cases. Additionally, six of the MBB-A3 airfoil cases are included. Finally, results are given for three of the cases for the rectangular wing
Airfoil shape and thickness effects on transonic airloads and flutter
A transient pulse technique is used to obtain harmonic forces from a time-marching solution of the complete unsteady transonic small perturbation potential equation. The unsteady pressures and forces acting on a model of the NACA 64A010 conventional airfoil and the MBB A-3 supercritical airfoil over a range of Mach numbers are examined in detail. Flutter calculations at constant angle of attack show a similar flutter behavior for both airfoils, except for a boundary shift in Mach number associated with corresponding Mach number shift in the unsteady aerodynamic forces. Differences in the static aeroelastic twist behavior for the two airfoils are significant
The Magellanic Stream: break up and accretion onto the hot Galactic corona
The Magellanic HI Stream (~2x10^9 Msun [d/55 kpc]^2) encircling the Galaxy at
a distance 'd' is arguably the most important tracer of what happens to gas
accreting onto a disk galaxy. Recent observations reveal that the Stream's mass
is in fact dominated (3:1) by its ionised component. Here we revisit the origin
of the mysterious H-alpha recombination emission observed along much of its
length that is overly bright (~150-200 milli-Rayleigh) for the known Galactic
ultraviolet background (~20-40 mR / [d/55 kpc]^2). In an earlier model, we
proposed that a slow shock cascade was operating along the Stream due to its
interaction with the extended Galactic hot corona. We find that, for a smooth
coronal density profile, this model can explain the bright H-alpha emission if
the coronal density satisfies 2 < (n / 10^{-4} cm^{-3}) < 4 at d = 55 kpc. But
in view of updated parameters for the Galactic halo and mounting evidence that
most of the Stream must lie far beyond the Magellanic Clouds (d>55 kpc), we
revisit the shock cascade model in detail. At lower densities, the HI gas is
broken down by the shock cascade but mostly mixes with the hot corona without
significant recombination. At higher densities, the hot coronal mass (including
the other baryonic components) exceeds the baryon budget of the Galaxy. If the
H-alpha emission arises from the shock cascade, the upper limit on the smooth
coronal density constrains the Stream's mean distance to < 75 kpc. If, as some
models indicate, the Stream is even further out, either the shock cascade is
operating in a regime where the corona is substantially mass-loaded with recent
gas debris, or an entirely different ionization mechanism is responsible.Comment: Significant expansion of the parameter space explored in response to
referee's comments. ApJ accepte
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