26,422 research outputs found
Observational Tests Of Intermediate Mass Star Yields Using Planetary Nebulae
This paper summarizes a project designed to study abundances in a sample of
planetary nebulae representing a broad range in progenitor mass and
metallicity. We collect abundances of C, N, and O determined for the entire
sample and compare them with theoretical predictions of planetary nebula
abundances from a grid of intermediate-mass star models. We find very good
agreement between observations and theory, lending strong support to our
current understanding of nucleosynthesis in stars with progenitor masses below
8 solar masses. This agreement between observation and theory also supports the
validity of published stellar yields of C and N in the study of the abundance
evolution of these two elements.Comment: 2 pages, to appear in the proceedings of ``Cosmic Evolution'', held
at Institut d'Astrophysique de Paris, November 13-17, 200
Aerodynamics, Stability and Control of the 1903 Wright Flyer
The Los Angeles Chapter of the American Institute of Aero and
Astronautics is building two replicas of the 1903 Wright Flyer airplane;
one to wind-tunnel test and display, and a modified one to fly. As part
of this project the aerodynamic characteristics of the Flyer are being
analyzed by modern wind-tunnel and analytical techniques. Tnis paper
describes the Wright Flyer Project, and compares key results from
small-scale wind-tunnel tests and from vortex-lattice computations for
this multi-biplane canard configuration. Analyses of the stability and
control properties are summarized and their implications for closed-loop
control by a pilot are derived using quasilinear pilot-vehicle analysis
and illustrated by simulation time histories.
It is concluded that, although the Wrights were very knowledgeable
and ingenious with respect to aircraft controls and their interactions
(e.g., the good effects of their wing-warp-to-rudder linkage are
validated), they were largely ignorant of dynamic stability
considerations. The paper shows that the 1903 Flyer was readily
controllable about all axes but was intrinsically unstable in pitch and
roll, and it could barely be stabilized by a skilled pilot
The Local Interstellar Ultraviolet Radiation Field
I have used the Hipparcos Input Catalog, together with Kurucz model stellar
atmospheres, and information on the strength of the interstellar extinction, to
create a model of the expected intensity and spectral distribution of the local
interstellar ultraviolet radiation field, under various assumptions concerning
the albedo a of the interstellar grains. (This ultraviolet radiation field is
of particular interest because of the fact that ultraviolet radiation is
capable of profoundly affecting the chemistry of the interstellar medium.) By
comparing my models with the observations, I am able to conclude that the
albedo a of the interstellar grains in the far ultraviolet is very low, perhaps
a = 0.1. I also advance arguments that my present determination of this albedo
is much more reliable than any of the many previous (and conflicting)
ultraviolet interstellar grain albedo determinations. Beyond this, I show that
the ultraviolet background radiation that is observed at high galactic
latitudes must be extragalactic in origin, as it cannot be backscatter of the
interstellar radiation field.Comment: Astrophysical Journal, in press; 9 figures + 16 text page
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