3,506 research outputs found
Space-Based Thermal Infrared Studies of Asteroids
Large-area surveys operating at mid-infrared wavelengths have proven to be a
valuable means of discovering and characterizing minor planets. Through the use
of radiometric models, it is possible to derive physical properties such as
diameters, albedos, and thermal inertia for large numbers of objects. Modern
detector array technology has resulted in a significant improvement in spatial
resolution and sensitivity compared with previous generations of space-based
infrared telescopes, giving rise to a commensurate increase in the number of
objects that have been observed at these wavelengths. Space-based infrared
surveys of asteroids therefore offer an effective means of rapidly gathering
information about small body populations' orbital and physical properties. The
AKARI, WISE/NEOWISE, Spitzer, and Herschel missions have significantly
increased the number of minor planets with well-determined diameters and
albedos.Comment: Chapter for Asteroids IV book (accepted for publication
Exciton-exciton scattering: Composite boson versus elementary boson
This paper introduces a new quantum object, the ``coboson'', for composite
particles, like the excitons, which are made of two fermions. Although commonly
dealed with as elementary bosons, these composite bosons -- ``cobosons'' in
short -- differ from them due to their composite nature which makes the
handling of their many-body effects quite different from the existing
treatments valid for elementary bosons. Due to this composite nature, it is not
possible to correctly describe the interaction between cobosons as a potential
. Consequently, the standard Fermi golden rule, written in terms of ,
cannot be used to obtain the transition rates between exciton states. Through
an unconventional expression for this Fermi golden rule, which is here given in
terms of the Hamiltonian only, we here give a detailed calculation of the time
evolution of two excitons. We compare the results of this exact approach with
the ones obtained by using an effective bosonic exciton Hamiltonian. We show
that the relation between the inverse lifetime and the sum of transition rates
for elementary bosons differs from the one of composite bosons by a factor of
1/2, whatever the mapping from composite bosons to elementary bosons is. The
present paper thus constitutes a strong mathematical proof that, in spite of a
widely spread belief, we cannot forget the composite nature of these cobosons,
even in the extremely low density limit of just two excitons. This paper also
shows the (unexpected) cancellation, in the Born approximation, of the
two-exciton transition rate for a finite value of the momentum transfer
Aerodynamic investigation of an air-cooled axial-flow turbine. Part 2: Rotor blade tip-clearance effects on overall turbine performance and internal gas flow conditions: Experimental results and prediction methods
Total turbine blade performance was investigated while changing the blade tip clearance in three ways. The internal flow at the moving blade outlet point was measured. Experimental results were compared with various theoretical methods. Increased blade clearance leads to decreased turbine efficiency
Photometric Observations of Star Formation Activity in Early Type Spirals
We observationally study the current star formation activities of early type
spiral galaxies. We construct a complete sample of 15 early type spirals having
far-infrared (FIR) to optical B band luminosity ratios, L(FIR)/L(B), larger
than the average of the type, and make their CCD imaging of the R and H-alpha
bands. The equivalent widths of H-alpha emission increase with increasing
L(FIR)/L(B), indicating that L(FIR)/L(B) can be an indicator of star formation
for such early type spirals with star formation activities higher than the
average. For all of the observed early type spirals, the extended HII regions
exist at the central regions with some asymmetric features. H-alpha emission is
more concentrated to the galactic center than the R band light, and the degree
of the concentration increases with the star formation activity. We also
analyze the relation between the star formation activities and the existence of
companion galaxies in the sample galaxies and other bright early type spirals.
No correlation is found and this suggests that the interaction is not
responsible for all of the star formation activities of early type spirals.Comment: LaTex, 23 pages (2 tables included), plus 9 Postscript figures & 1
table. To be published in AJ (November issue
- …