University of St Andrews

About: University of St Andrews is a education organization based out in St Andrews, Fife, United Kingdom. It is known for research contribution in the topics: Population & Laser. The organization has 16260 authors who have published 43364 publications receiving 1636072 citations. The organization is also known as: St Andrews University & University of St. Andrews.

Ionizing feedback from massive stars in massive clusters – II. Disruption of bound clusters by photoionization

Abstract: We present a smoothed particle hydrodynamics parameter study of the dynamical effect of photoionization from O-type stars on star-forming clouds of a range of masses and sizes during the time window before supernovae explode. Our model clouds all have the same degree of turbulent support initially, the ratio of turbulent kinetic energy to gravitational potential energy being set to Ekin/|Epot|= 0.7. We allow the clouds to form stars and study the dynamical effects of the ionizing radiation from the massive stars or clusters born within them. We find that dense filamentary structures and accretion flows limit the quantities of gas that can be ionized, particularly in the higher density clusters. More importantly, the higher escape velocities in our more massive (106 M⊙) clouds prevent the H ii regions from sweeping up and expelling significant quantities of gas, so that the most massive clouds are largely dynamically unaffected by ionizing feedback. However, feedback has a profound effect on the lower density 104 and 105 M⊙ clouds in our study, creating vast evacuated bubbles and expelling tens of per cent of the neutral gas in the 3-Myr time-scale before the first supernovae are expected to detonate, resulting in clouds highly porous to both photons and supernova ejecta.

Perspective:is human cultural evolution darwinian? evidence reviewed from the perspective of the origin of species

Abstract: The claim that human culture evolves through the differential adoption of cultural variants, in a manner analogous to the evolution of biological species, has been greeted with much resistance and confusion. Here we demonstrate that as compelling a case can now be made that cultural evolution has key Darwinian properties, as Darwin himself presented for biological evolution in The Origin of Species. Culture is shown to exhibit variation, competition, inheritance, and the accumulation of successive cultural modifications over time. Adaptation, convergence, and the loss or change of function can also be identified in culture. Just as Darwin knew nothing of genes or particulate inheritance, a case for Darwinian cultural evolution can be made irrespective of whether unitary cultural replicators exist or whether cultural transmission mechanisms are well understood.

How Cooper pairs vanish approaching the Mott insulator in Bi 2 Sr 2 CaCu 2 O 8+δ

Abstract: The antiferromagnetic ground state of copper oxide Mott insulators is achieved by localizing an electron at each copper atom in real space (r-space). Removing a small fraction of these electrons (hole doping) transforms this system into a superconducting fluid of delocalized Cooper pairs in momentum space (k-space). During this transformation, two distinctive classes of electronic excitations appear. At high energies, the mysterious 'pseudogap' excitations are found, whereas, at lower energies, Bogoliubov quasi-particles-the excitations resulting from the breaking of Cooper pairs-should exist. To explore this transformation, and to identify the two excitation types, we have imaged the electronic structure of Bi(2)Sr(2)CaCu(2)O(8+delta) in r-space and k-space simultaneously. We find that although the low-energy excitations are indeed Bogoliubov quasi-particles, they occupy only a restricted region of k-space that shrinks rapidly with diminishing hole density. Concomitantly, spectral weight is transferred to higher energy r-space states that lack the characteristics of excitations from delocalized Cooper pairs. Instead, these states break translational and rotational symmetries locally at the atomic scale in an energy-independent way. We demonstrate that these unusual r-space excitations are, in fact, the pseudogap states. Thus, as the Mott insulating state is approached by decreasing the hole density, the delocalized Cooper pairs vanish from k-space, to be replaced by locally translational- and rotational-symmetry-breaking pseudogap states in r-space.

Improving intergroup relations through direct, extended and other forms of indirect contact:

Abstract: The benefits of direct, personal contact with members of another group are well established empirically. This Special Issue complements that body of work by demonstrating the effects of various for...

Optical Continuum and Emission-Line Variability of Seyfert 1 Galaxies

Abstract: We present the light curves obtained during an 8 yr program of optical spectroscopic monitoring of nine Seyfert 1 galaxies: 3C 120, Akn 120, Mrk 79, Mrk 110, Mrk 335, Mrk 509, Mrk 590, Mrk 704, and Mrk 817. All objects show significant variability in both the continuum and emission-line fluxes. We use cross-correlation analysis to derive the sizes of the broad Hβ-emitting regions based on emission-line time delays, or lags. We successfully measure time delays for eight of the nine sources and find values ranging from about 2 weeks to a little over 2 months. Combining the measured lags and widths of the variable parts of the emission lines allows us to make virial mass estimates for the active nucleus in each galaxy. The virial masses are in the range 107-108 M☉.