2,023 research outputs found
Experimental rate coefficients for collisional excitation of lithium-like ions
Collisional excitation rates for lithium-like ions derived from diagnosed plasma produced in theta pinch device and line intensities emitted by these ion
Simplified models of electromagnetic and gravitational radiation damping
In previous work the authors analysed the global properties of an approximate
model of radiation damping for charged particles. This work is put into context
and related to the original motivation of understanding approximations used in
the study of gravitational radiation damping. It is examined to what extent the
results obtained previously depend on the particular model chosen. Comparisons
are made with other models for gravitational and electromagnetic fields. The
relation of the kinetic model for which theorems were proved to certain
many-particle models with radiation damping is exhibited
High-Resolution Imaging of Molecular Gas and Dust in the Antennae (NGC 4038/39): Super Giant Molecular Complexes
We present new aperture synthesis CO maps of the Antennae (NGC 4038/39)
obtained with the Caltech Millimeter Array. These sensitive images show
molecular emission associated with the two nuclei and a partial ring of star
formation to the west of NGC 4038, as well as revealing the large extent of the
extra-nuclear region of star formation (the ``overlap region''), which
dominates the CO emission from this system. The largest molecular complexes
have masses of 3-6x10^8 M_sun, typically an order of magnitude larger than the
largest structures seen to date in more quiescent galaxy disks. The extremely
red luminous star clusters identified previously with HST are well-correlated
with the CO emission, which supports the conclusion that they are highly
embedded young objects rather than old globular clusters. There is an excellent
correlation between the CO emission and the 15 micron emission seen with ISO,
particularly for the brightest regions. The most massive complexes in the
overlap region have similar [NeIII]/[NeII] ratios, which implies that all these
regions are forming many massive stars. However, only the brightest
mid-infrared peak shows strong, rising continuum emission longward of 10
microns, indicative of very small dust grains heated to high temperatures by
their proximity to nearby luminous stars. Since these grains are expected to be
removed rapidly from the immediate environment of the massive stars, it is
possible that this region contains very young (< 1 Myr) sites of star
formation. Alternatively, fresh dust grains could be driven into the sphere of
influence of the massive stars, perhaps by the bulk motions of two giant
molecular complexes. The kinematics and morphology of the CO emission in this
region provide some support for this second scenario.Comment: Accepted for publication in The Astrophysical Journal, 13 pages, 5
figures, higher quality color images available at
http://www.astro.cornell.edu/staff/vassilis/papers/ngc4038_co.ps.g
What Powers Ultra-luminous IRAS Galaxies?
We present an ISO SWS and ISOPHOT-S, mid-infrared spectroscopic survey of 15
ultra-luminous IRAS galaxies. We combine the survey results with a detailed
case study, based on near-IR and mm imaging spectroscopy, of one of the sample
galaxies (UGC 5101). We compare the near- and mid-IR characteristics of these
ultra-luminous galaxies to ISO and literature data of thirty starburst and
active galactic nuclei (AGN), template galaxies. We find that
1) 70-80% of the ultra-luminous IRAS galaxies in our sample are predominantly
powered by recently formed massive stars. 20-30% are powered by a central AGN.
These conclusions are based on a new infrared 'diagnostic diagram' involving
the ratio of high to low excitation mid-IR emission lines on the one hand, and
on the strength of the 7.7um PAH feature on the other hand.
2) at least half of the sources probably have simultaneously an active
nucleus and starburst activity in a 1-2 kpc diameter circum-nuclear disk/ring.
3) the mid-infrared emitting regions are highly obscured. After correction
for these extinctions, we estimate that the star forming regions in ULIRGs have
ages between 10^7 and 10^8 years, similar to but somewhat larger than those
found in lower luminosity starburst galaxies.
4) in the sample we have studied there is no obvious trend for the AGN
component to dominate in the most compact, and thus most advanced mergers.
Instead, at any given time during the merger evolution, the time dependent
compression of the circum-nuclear interstellar gas, the accretion rate onto the
central black hole and the associated radiation efficiency may determine
whether star formation or AGN activity dominates the luminosity of the system.Comment: 63 pages postscript (ex. MS Word), 11 postscript and 2 gif figures,
submitted to ApJ. See also
http://www.mpe-garching.mpg.de/ISO/preprint/MPE-IR-97003.htm
The Geoff Egan Memorial Lecture 2011. Artefacts, art and artifice: reconsidering iconographic sources for archaeological objects in early modern Europe
A first systematic analysis of historic domestic material culture depicted in contemporaneous Western painting and prints, c.1400-1800. Drawing on an extensive data set, the paper proposes to methodologies and hermeneutics for historical analysis and archaeological correspondence
Bose-Einstein condensates in standing waves: The cubic nonlinear Schroedinger equation with a periodic potential
We present a new family of stationary solutions to the cubic nonlinear
Schroedinger equation with a Jacobian elliptic function potential. In the limit
of a sinusoidal potential our solutions model a dilute gas Bose-Einstein
condensate trapped in a standing light wave. Provided the ratio of the height
of the variations of the condensate to its DC offset is small enough, both
trivial phase and nontrivial phase solutions are shown to be stable. Numerical
simulations suggest such stationary states are experimentally observable.Comment: 4 pages, 4 figure
Formation of a stable deacagonal quasicrystalline Al-Pd-Mn surface layer
We report the in situ formation of an ordered equilibrium decagonal Al-Pd-Mn
quasicrystal overlayer on the 5-fold symmetric surface of an icosahedral
Al-Pd-Mn monograin. The decagonal structure of the epilayer is evidenced by
x-ray photoelectron diffraction, low-energy electron diffraction and electron
backscatter diffraction. This overlayer is also characterized by a reduced
density of states near the Fermi edge as expected for quasicrystals. This is
the first time that a millimeter-size surface of the stable decagonal Al-Pd-Mn
is obtained, studied and compared to its icosahedral counterpart.Comment: Submitted to Phys. Ref. Lett. (18 July 2001
Interaction of photons with plasmas and liquid metals: photoabsorption and scattering
Formulas to describe the photoabsorption and the photon scattering by a
plasma or a liquid metal are derived in a unified manner with each other. It is
shown how the nuclear motion, the free-electron motion and the core-electron
behaviour in each ion in the system determine the structure of photoabsorption
and scattering in an electron-ion mixture. The absorption cross section in the
dipole approximation consists of three terms which represent the absorption
caused by the nuclear motion, the absorption owing to the free-electron motion
producing optical conductivity or inverse Bremsstrahlung, and the absorption
ascribed to the core-electron behaviour in each ion with the Doppler
correction. Also, the photon scattering formula provides an analysis method for
experiments observing the ion-ion dynamical structure factor (DSF), the
electron-electron DSF giving plasma oscillations, and the core-electron DSF
yielding the X-ray Raman (Compton) scattering with a clear definition of the
background scattering for each experiment, in a unified manner. A formula for
anomalous X-ray scattering is also derived for a liquid metal. At the same
time, Thomson scattering in plasma physics is discussed from this general point
of view.Comment: LaTeX file: 18 pages without figur
- …