189 research outputs found
van der Waals coefficients for positronium-atom interactions
The van der Waals coefficients for positronium interactions with a number of rare gases (He, Ne, Ar, Kr, and Xe) and alkali-metal atoms (Li, Na, K, and Rb) are estimated using a variety of ab initio and semiempirical methods. Dispersion coefficients are also presented for atomic hydrogen and a number of rare-gas and alkali-metal atoms for validation purposes
New constraints on the evolution of the stellar-to-dark matter connection : a combined analysis of galaxy-galaxy lensing, clustering, and stellar mass functions from z = 0.2 to z = 1
Using data from the COSMOS survey, we perform the first joint analysis of galaxy-galaxy weak lensing, galaxy spatial clustering, and galaxy number densities. Carefully accounting for sample variance and for scatter between stellar and halo mass, we model all three observables simultaneously using a novel and self-consistent theoretical framework. Our results provide strong constraints on the shape and redshift evolution of the stellar-to-halo mass relation (SHMR) from z = 0.2 to z = 1. At low stellar mass, we find that halo mass scales as Mh vpropM 0.46 * and that this scaling does not evolve significantly with redshift from z = 0.2 to z = 1. The slope of the SHMR rises sharply at M * > 5 × 1010 M ☉ and as a consequence, the stellar mass of a central galaxy becomes a poor tracer of its parent halo mass. We show that the dark-to-stellar ratio, Mh /M *, varies from low to high masses, reaching a minimum of Mh /M * ~ 27 at M * = 4.5 × 1010 M ☉ and Mh = 1.2 × 1012 M ☉. This minimum is important for models of galaxy formation because it marks the mass at which the accumulated stellar growth of the central galaxy has been the most efficient. We describe the SHMR at this minimum in terms of the "pivot stellar mass," M piv *, the "pivot halo mass," M piv h , and the "pivot ratio," (Mh /M *)piv. Thanks to a homogeneous analysis of a single data set spanning a large redshift range, we report the first detection of mass downsizing trends for both M piv h and M piv *. The pivot stellar mass decreases from M piv * = 5.75 ± 0.13 × 1010 M ☉ at z = 0.88 to M piv * = 3.55 ± 0.17 × 1010 M ☉ at z = 0.37. Intriguingly, however, the corresponding evolution of M piv h leaves the pivot ratio constant with redshift at (Mh /M *)piv ~ 27. We use simple arguments to show how this result raises the possibility that star formation quenching may ultimately depend on Mh /M * and not simply on Mh , as is commonly assumed. We show that simple models with such a dependence naturally lead to downsizing in the sites of star formation. Finally, we discuss the implications of our results in the context of popular quenching models, including disk instabilities and active galactic nucleus feedback
Fertility, Living Arrangements, Care and Mobility
There are four main interconnecting themes around which the contributions in this book are based. This introductory chapter aims to establish the broad context for the chapters that follow by discussing each of the themes. It does so by setting these themes within the overarching demographic challenge of the twenty-first century – demographic ageing. Each chapter is introduced in the context of the specific theme to which it primarily relates and there is a summary of the data sets used by the contributors to illustrate the wide range of cross-sectional and longitudinal data analysed
Origin of the Local Bubble
We present a new unbiased search for OB associations in the Solar neighbourhood
which have hosted the progenitor stars of the core collapse supernovae responsible for the
Local Bubble in the interstellar gas. For this purpose we have analyzed a volume complete
set (with a diameter of 400 pc) of B stars drawn from the Hipparcos catalogue and the Arivel
data base, from which candidate members were selected by a kinematical criterion. After
careful dereddening the star colours we have constructed a colour-magnitude diagram and
confirmed that the Upper Scorpius, Upper Centaurus Lupus, and Lower Centaurus Crux
subgroups of the Sco OB2 association are the youngest nearby OB associations. We dated
their ages with theoretical isochrones in the range of 20–30 Myr, in agreement with previous
work. We have traced backwards in time the paths of the stars and found that they entered
the volume of the present bubble at 10 to 15 Myr ago. We argue that the Local Bubble began
to form then and estimate that 14 to 20 supernovae have exploded since. The implied energy
input into the ambient medium can be shown to be sufficient to excavate a bubble of the
presently observed size
Computing pseudotriangulations via branched coverings
We describe an efficient algorithm to compute a pseudotriangulation of a
finite planar family of pairwise disjoint convex bodies presented by its
chirotope. The design of the algorithm relies on a deepening of the theory of
visibility complexes and on the extension of that theory to the setting of
branched coverings. The problem of computing a pseudotriangulation that
contains a given set of bitangent line segments is also examined.Comment: 66 pages, 39 figure
What Physical Processes Drive the Interstellar Medium in the Local Bubble?
Recent 3D high-resolution simulations of the interstellar medium in a star form-
ing galaxy like the Milky Way show that supernova explosions are the main driver of the
structure and evolution of the gas. Its physical state is largely controlled by turbulence due
to the high Reynolds numbers of the average flows. For a constant supernova rate a dynam-
ical equilibrium is established within 200 Myr of simulation as a consequence of the setup
of a galactic fountain. The resulting interstellar medium reveals a typical density/pressure
pattern, i.e. distribution of so-called gas phases, on scales of 500–700 pc, with interstellar
bubbles being a common phenomenon just like the Local Bubble and the Loop I superbub-
ble, which are assumed to be interacting. However, modeling the Local Bubble is special,
because it is driven by a moving group, passing through its volume, as it is inferred from
the analysis of Hipparcos data. A detailed analysis reveals that between 14 and 19 super-
novae have exploded during the last 15 Myr. The age of the Local Bubble is derived from
comparison with HI and UV absorption line data to be 14.5±0.7
Myr. We further predict the
0.4merging of the two bubbles in about 3 Myr from now, when the interaction shell starts to
fragment. The Local Cloud and its companion HI clouds are the consequence of a dynamical
instability in the interaction shell between the Local and the Loop I bubble
A high precision n-p scattering measurement at 14.9 MeV
The n-p scattering angular distribution was measured with 14.9 MeV incident neutrons using the traditional time-of-flight technique with neutron-gamma discrimination. The scattering angle varied from 20o to 65o (laboratory system) in 5o incremental steps. The efficiency of the neutron detectors was measured in the energy range 2–9 MeV relative to the 252Cf-standard, and was calculated using Monte Carlo methods in the 2–14 MeV energy range. Two methods of analysis were applied for experimental and simulated data: a traditional approach with a fixed threshold, and a dynamic threshold approach. The present data agree with the ENDF/B-VII evaluation for the shape of n-p angular distribution within about 1.5%
Astronomical Distance Determination in the Space Age: Secondary Distance Indicators
The formal division of the distance indicators into primary and secondary leads to difficulties in description of methods which can actually be used in two ways: with, and without the support of the other methods for scaling. Thus instead of concentrating on the scaling requirement we concentrate on all methods of distance determination to extragalactic sources which are designated, at least formally, to use for individual sources. Among those, the Supernovae Ia is clearly the leader due to its enormous success in determination of the expansion rate of the Universe. However, new methods are rapidly developing, and there is also a progress in more traditional methods. We give a general overview of the methods but we mostly concentrate on the most recent developments in each field, and future expectations. © 2018, The Author(s)
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