123224123
Distorted wave theory of the 50 MeV (p, 2p) reaction
Abstract An attempt is made to answer the question whether distorting potentials that give really good fits to elastic scattering cross sections and total reaction cross sections for experiments in the relevant two-body subsystems can correctly give positions of maxima and minima in the (p, 2p) angular correlation. The 50 MeV energy range is chosen because the angular correlation is very sensitive to the distortion. It is found that the positions are correctly reproduced by good elastic scattering parameters for the reaction 89 Y(p, 2p) 88 Sr, Q = 7.44 MeV, at 46 MeV. The shortcoming of the distorted wave theory at this energy, as at higher energies, is that the calculated ratio of the cross section at small angles to that at large angles is too large. Apart from this serious defect, the shape of the angular correlation for 12 C(p, 2p) 11 B, Q = 16 MeV, at 50 MeV is also encouraging. Previous studies suggest that the defect can be corrected if a better description of the p-p t -matrix is used in the distorted wave theory.
119482740
Observing electron motion in solids
We describe an experimental technique that allows for the direct determination of the motion (i.e., the momentum) of electrons in atoms, molecules, and solids. This ({ital e},2{ital e}) technique centers on a fast electron`s ejecting a second electron from a target. Precise spectroscopy of both electrons` final energies and momenta provides very significant information on the second electron`s initial state. Decades of past ({ital e},2{ital e}) studies on single-molecule states have now progressed into studies of condensed matter. The interpretation of these experiments is mediated in this colloquium by analysis of intermediate models in the form of chainlike molecules.
121500029
Time-Dependent Scattering Experiment
Theory is developed for a time-dependenit scatternng experiment that consists in following the time development of a photon wave packet scattered resonantly from the metastable state of an appropriate nucleus. (C.E.S.)
120471006
Analytic distorted waves for intermediate-energy alpha particles
119722071
Off-shell effects in the (p, 2p) reaction
Abstract It is shown that, within the limitations of the distorted-wave off-shell impulse approximation, future accurate (p, 2p) experiments near 150 MeV incident energy can be helpful in distinguishing between some of six phenomenological nuclear potentials that are examined. On-shell differences between the Reid soft-core and five different one-boson-exchange potentials are amplified in the (p, 2p) context in such a way that an accurately measurable feature, the peak height ratio for 1p knockout, is sensitive to them.
18498536
A high-resolution electron momentum spectroscopy and density functional theory study into the complete valence electronic structure of allene
A study of the electronic structure of the complete valence shell of allene (1,2-propadiene) is reported. New high-resolution binding-energy spectra were measured in the energy regime 6–34.5 eV over a range of different target electron momenta, so that momentum distributions (MDs) could be determined for each molecular orbital. These data supersede the low-resolution work of Braidwood et al. [J Phys B 27 (1994) 2075–2087], and also clarify some ambiguities with the earlier results. Theoretical MDs were calculated using a plane wave impulse approximation (PWIA) model for the reaction mechanism and density functional theory (DFT) for the wave function. Three basis sets, at the local spin density (LSD) approximation level and, additionally, incorporating nonlocal corrections such as the generalized gradient approximation (GGA), were studied. A critical comparison between the experimental and theoretical MDs was made, and it allowed us to determine the “optimum” wave function for allene from the basis sets we considered. This wave function is then used to derive allene's chemically interesting molecular properties. A summary of some of these results and a comparison of them with those of other workers is also presented with the level of agreement typically being good. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1321–1333, 2001
14941452
The influence of baryons on the mass distribution of dark matter halos
Using a set of high-resolution N-body/SPH cosmological simulations with identical initial conditions but run with different numerical setups, we investigate the influence of baryonic matter on the mass distribution of dark halos when radiative cooling is not included. We compare the concentration parameters of about 400 massive halos with virial mass from 1013 to 7.1 × 1014 h-1 M☉. We find that the concentration parameters for the total mass and dark matter distributions in nonradiative simulations are on average larger by ~3% and 10% than those in a pure dark matter simulation. Our results indicate that the total mass density profile is little affected by a hot gas component in the simulations. After carefully excluding the effects of resolutions and spurious two-body heating between dark matter (DM) and gas particles, we conclude that the increase of the DM concentration parameters is due to interactions between baryons and DM. We demonstrate this with the aid of idealized simulations of two-body mergers. The results of individual halos simulated with different mass resolutions show that in the gas profiles of densities, temperature and entropy are subjects of mass resolution of SPH particles. In particular, we find that in the inner parts of halos, as the SPH resolution increases the gas density becomes higher but both the entropy and temperature decrease.
4526351
On the Relationship between Cooling Flows and Bubbles
A common feature of the X-ray bubbles observed in Chandra images of some "cooling flow" clusters is that they appear to be surrounded by bright, cool shells. Temperature maps of a few nearby luminous clusters reveal that the shells consist of the coolest gas in the clusters—much cooler than the surrounding medium. Using simple models, we study the effects of this cool emission on the inferred cooling flow properties of clusters. We find that the introduction of bubbles into model clusters that do not have cooling flows results in temperature and surface brightness profiles that resemble those seen in nearby "cooling flow" clusters. They also approximately reproduce the recent XMM-Newton and Chandra observations of a high minimum temperature of ~1-3 keV. Hence, bubbles, if present, must be taken into account when inferring the physical properties of the intracluster medium. In the case of some clusters, bubbles may account entirely for these observed features, calling into question their designation as clusters with cooling flows. However, since not all nearby "cooling flow" clusters show bubble-like features, we suggest that there may be a diverse range of physical phenomena that give rise to the same observed features.
118385462
The physics driving the cosmic star formation history
We investigate the physics driving the cosmic star formation (SF) history using the more than 50 large, cosmological, hydrodynamical simulations that together comprise the OverWhelmingly Large Simulations project. We systematically vary the parameters of the model to determine which physical processes are dominant and which aspects of the model are robust. Generically, we find that SF is limited by the build-up of dark matter haloes at high redshift, reaches a broad maximum at intermediate redshift and then decreases as it is quenched by lower cooling rates in hotter and lower density gas, gas exhaustion and self-regulated feedback from stars and black holes. The higher redshift SF is therefore mostly determined by the cosmological parameters and to a lesser extent by photoheating from reionization. The location and height of the peak in the SF history, and the steepness of the decline towards the present, depend on the physics and implementation of stellar and black hole feedback. Mass loss from intermediate-mass stars and metal-line cooling both boost the SF rate at late times. Galaxies form stars in a self-regulated fashion at a rate controlled by the balance between, on the one hand, feedback from massive stars and black holes and, on the other hand, gas cooling and accretion. Paradoxically, the SF rate is highly insensitive to the assumed SF law. This can be understood in terms of self-regulation: if the SF efficiency is changed, then galaxies adjust their gas fractions so as to achieve the same rate of production of massive stars. Self-regulated feedback from accreting black holes is required to match the steep decline in the observed SF rate below redshift 2, although more extreme feedback from SF, for example in the form of a top-heavy initial stellar mass function at high gas pressures, can help.
14677179
On the origin of cores in simulated galaxy clusters
The diffuse plasma that fills galaxy groups and clusters (the intracluster medium) is a by-product of galaxy formation. The present thermal state of this gas results from a competition between gas cooling and heating. The heating comes from two distinct sources: gravitational heating associated with the collapse of the dark matter halo and additional thermal input from the formation of galaxies and their black holes. A long-term goal of this research is to decode the observed temperature, density and entropy profiles of clusters and to understand the relative roles of these processes. However, a long-standing problem has been that cosmological simulations based on smoothed particle hydrodynamics (SPH) and Eulerian mesh-based codes predict different results even when cooling and galaxy/black hole heating are switched off. Clusters formed in SPH simulations show near power-law entropy profiles, while those formed in Eulerian simulations develop a core and do not allow gas to reach such low entropies. Since the cooling rate is closely connected to the minimum entropy of the gas distribution, the differences are of potentially key importance. 
 
In this paper, we investigate the origin of this discrepancy. By comparing simulations run using the GADGET-2 SPH code and the FLASH adaptive Eulerian mesh code, we show that the discrepancy arises during the idealized merger of two clusters and that the differences are not the result of the lower effective resolution of Eulerian cosmological simulations. The difference is not sensitive to the minimum mesh size (in Eulerian codes) or the number of particles used (in SPH codes). We investigate whether the difference is the result of the different gravity solvers, the Galilean non-invariance of the mesh code or an effect of unsuitable artificial viscosity in the SPH code. Instead, we find that the difference is inherent to the treatment of vortices in the two codes. Particles in the SPH simulations retain a close connection to their initial entropy, while this connection is much weaker in the mesh simulations. The origin of this difference lies in the treatment of eddies and fluid instabilities. These are suppressed in the SPH simulations, while the cluster mergers generate strong vortices in the Eulerian simulations that very efficiently mix the fluid and erase the low-entropy gas. We discuss the potentially profound implications of these results.
118956459
The similarity of observed X-ray coronae associated with L* disc and elliptical galaxies
The existence of hot, X-ray luminous gaseous coronae surrounding present day L* galaxies is a generic prediction of galaxy formation theory in the cold dark matter cosmogony. While extended X-ray emission has been known to exist around elliptical galaxies for a long time, diffuse extra-planar emission has only recently been detected around disc galaxies. We compile samples of elliptical and disc galaxies that have Chandra and XMM-Newton measurements, and compare the scaling of the coronal X-ray luminosity (L_X) with both the K-band luminosity (L_K) and the coronal X-ray temperature (T_X). The X-ray flux measurements are corrected for non-thermal point source contamination by spatial excision and spectral subtraction for resolved and unresolved sources respectively. We find that the properties of the extended X-ray emission from galaxies of different morphological types are similar: for both elliptical and disc galaxies, the L_X - L_K and L_X - T_X relations have similar slope, normalisation and scatter. The observed universality of coronal X-ray properties suggests that the bulk of this emission originates from gas that has been accreted, shock-heated and compressed during the assembly of the galaxy and that outflows triggered by stellar processes make only a minor contribution to the X-ray emission. This reservoir of cooling gas is a potential source of morphological transformation; it provides a fresh supply of material for discs to grow around galaxies of all morphological types.