6,478 research outputs found
On the Structure of the Orion A Cloud and the Formation of the Orion Nebula Cluster
We suggest that the Orion A cloud is gravitationally collapsing on large
scales, and is producing the Orion Nebula Cluster due to the focusing effects
of gravity acting within a finite cloud geometry. In support of this
suggestion, we show how an elliptical rotating sheet of gas with a modest
density gradient along the major axis can collapse to produce a structure
qualitatively resembling Orion A, with a fan-shaped structure at one end,
ridges or filaments along the fan, and a narrow curved filament at the other
end reminiscent of the famous integral-shaped filament. The model produces a
local concentration of mass within the narrow filament which in principle could
form a dense cluster of stars like that of the Orion Nebula. We suggest that
global gravitational contraction might be a more common feature of molecular
clouds than previously recognized, and that the formation of star clusters is a
dynamic process resulting from the focusing effects of gravity acting upon the
geometry of finite clouds.Comment: 23 pages, 6 figures, to appear in the Astrophysical Journa
Probing the structure of Nucleons in Electromagbetic Interactions
Open problems in the study of the nucleon structure using electromagnetic
probes are discussed. The focus is on experimental aspects in the regime of
strong interaction QCD. Significant progress in our understanding of the
nucleon structure in this domain of QCD may be expected in the first decade of
the next millenium. This is due to major experimental and theoretical efforts
currently underway in this field.Comment: 9 pages, 6 figures, plenary talk at PANIC9
Recent Results from Jefferson Lab
Recent results on studies of the structure of nucleons and nuclei in the
regime of strong interaction QCD are discussed. Use of high current polarized
electron beams, polarized targets, and recoil polarimeters, in conjunction with
modern spectrometers and detector instrumentation allow much more detailed
studies of nucleon and nuclear structure than has been possible in the past.
The CEBAF accelerator at Jefferson Lab was build to study the internal
structure of hadrons in a regime where confinement is important and strong
interaction QCD is the relevant theory. I discuss how the first experiments
already make significant contributions towards an improved understanding of
hadronic structure.Comment: Lecture presented at the International School of Nuclear Physics,
Erice, Sicily, Italy, September 17 - 25, 199
The Formation of the Milky Way in the Cosmological Context
The formation of the Milky Way is discussed within the context of the cold
dark matter scenario. Several problems arise which can be solved if the Galaxy
experienced an early phase of gas heating and decoupling from the dark matter
substructure. This model combines the Eggen, Lynden-Bell and Sandage picture of
a monolithic protogalactic collapses with the Searle and Zinn picture of an
early merging phase of substructures into one consistent scenario of Galactic
formation.Comment: 5 pages, conference proceeding. to appear in "Cosmic Evolution", eds.
M. Lemoine and E. Vangioni-Fla
Thermal Instability and the Formation of Clumpy Gas Clouds
The radiative cooling of optically thin gaseous regions and the formation of
a two-phase medium and of cold gas clouds with a clumpy substructure is
investigated. In optically thin clouds, the growth rate of small isobaric
density perturbations is independent of their length scale. However, the growth
of a perturbation is limited by its transition from isobaric to isochoric
cooling. The temperature at which this transition occurs decreases with the
length scale of the perturbation. Consequently small scale perturbations have
the potential to reach higher amplitudes than large scale perturbations. When
the amplitude becomes nonlinear, advection overtakes the pressure gradient in
promoting the compression resulting in an accelerated growth of the
disturbance. The critical temperature for transition depends on the initial
amplitude. The fluctuations which can first reach nonlinearity before their
isobaric to isochoric transition will determine the characteristic size and
mass of the cold dense clumps which would emerge from the cooling of an
initially nearly homogeneous region of gas. Thermal conduction is in general
very efficient in erasing isobaric, small-scale fluctuations, suppressing a
cooling instability. A weak, tangled magnetic field can however reduce the
conductive heat flux enough for low-amplitude fluctuations to grow isobarically
and become non-linear if their length scales are of order 0.01 pc. Finally, we
demonstrate how a 2-phase medium, with cold clumps being pressure confined in a
diffuse hot residual background component, would be sustained if there is
adequate heating to compensate the energy loss.Comment: 26 pages, Latex, 10 postscript figures, ApJ, in pres
The geometry and origin of ultra-diffuse ghost galaxies
The geometry and intrinsic ellipticity distribution of ultra diffuse galaxies
(UDGs) is determined from the line-of-sight distribution of axial ratios q of a
large sample of UDGs, detected by Koda et al. (2015) in the Coma cluster. With
high significance the data rules out an oblate, disk-like geometry,
characterised by major axi a=b>c. The data is however in good agreement with
prolate shapes, corresponding to a=b<c. This indicates that UDGs are not
thickened, rotating, axisymmetric disks, puffed up by violent processes.
Instead they are anisotropic elongated cigar- or bar-like structures, similar
to the prolate dwarf spheroidal galaxy population of the Local Group. The
intrinsic distribution of axial ratios of the Coma UDGs is flat in the range of
0.4 <= a/c <= 0.9 which will provide important constraints for theoretical
models of their origin. Formation scenarios that could explain the extended
prolate nature of UDGs are discussed.Comment: 13 pages, 4 figures, ApJ, in pres
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