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

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 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

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