156 research outputs found
Planetary population synthesis
In stellar astrophysics, the technique of population synthesis has been
successfully used for several decades. For planets, it is in contrast still a
young method which only became important in recent years because of the rapid
increase of the number of known extrasolar planets, and the associated growth
of statistical observational constraints. With planetary population synthesis,
the theory of planet formation and evolution can be put to the test against
these constraints. In this review of planetary population synthesis, we first
briefly list key observational constraints. Then, the work flow in the method
and its two main components are presented, namely global end-to-end models that
predict planetary system properties directly from protoplanetary disk
properties and probability distributions for these initial conditions. An
overview of various population synthesis models in the literature is given. The
sub-models for the physical processes considered in global models are
described: the evolution of the protoplanetary disk, the planets' accretion of
solids and gas, orbital migration, and N-body interactions among concurrently
growing protoplanets. Next, typical population synthesis results are
illustrated in the form of new syntheses obtained with the latest generation of
the Bern model. Planetary formation tracks, the distribution of planets in the
mass-distance and radius-distance plane, the planetary mass function, and the
distributions of planetary radii, semimajor axes, and luminosities are shown,
linked to underlying physical processes, and compared with their observational
counterparts. We finish by highlighting the most important predictions made by
population synthesis models and discuss the lessons learned from these
predictions - both those later observationally confirmed and those rejected.Comment: 47 pages, 12 figures. Invited review accepted for publication in the
'Handbook of Exoplanets', planet formation section, section editor: Ralph
Pudritz, Springer reference works, Juan Antonio Belmonte and Hans Deeg, Ed
Brane gravity, higher derivative terms and non-locality
In brane world scenarios with a bulk scalar field between two branes it is
known that 4-dimensional Einstein gravity is restored at low energies on either
brane. By using a gauge-invariant gravitational and scalar perturbation
formalism we extend the theory of weak gravity in the brane world scenarios to
higher energies, or shorter distances. We argue that weak gravity on either
brane is indistinguishable from 4-dimensional higher derivative gravity,
provided that the inter-brane distance (radion) is stabilized, that the
background bulk scalar field is changing near the branes and that the
background bulk geometry near the branes is warped. This argument holds for a
general conformal transformation to a frame in which matter on the branes is
minimally coupled to the metric. In particular, Newton's constant and the
coefficients of curvature-squared terms in the 4-dimensional effective action
are determined up to an ambiguity of adding a Gauss-Bonnet topological term. In
other words, we provide the brane-world realization of the so called
-model without utilizing a quantum theory. We discuss the appearance of
composite spin-2 and spin-0 fields in addition to the graviton on the brane and
point out a possibility that the spin-0 field may play the role of an effective
inflaton to drive brane-world inflation. Finally, we conjecture that the
sequence of higher derivative terms is an infinite series and, thus, indicates
non-locality in the brane world scenarios.Comment: Latex, 18 pages; a comment on the spurious tensor mode was added;
recovery condition of higher derivative gravity clarifie
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Refining the accuracy of validated target identification through coding variant fine-mapping in type 2 diabetes.
We aggregated coding variant data for 81,412 type 2 diabetes cases and 370,832 controls of diverse ancestry, identifying 40 coding variant association signals (P < 2.2 × 10-7); of these, 16 map outside known risk-associated loci. We make two important observations. First, only five of these signals are driven by low-frequency variants: even for these, effect sizes are modest (odds ratio ≤1.29). Second, when we used large-scale genome-wide association data to fine-map the associated variants in their regional context, accounting for the global enrichment of complex trait associations in coding sequence, compelling evidence for coding variant causality was obtained for only 16 signals. At 13 others, the associated coding variants clearly represent 'false leads' with potential to generate erroneous mechanistic inference. Coding variant associations offer a direct route to biological insight for complex diseases and identification of validated therapeutic targets; however, appropriate mechanistic inference requires careful specification of their causal contribution to disease predisposition
Control of star formation by supersonic turbulence
Understanding the formation of stars in galaxies is central to much of modern
astrophysics. For several decades it has been thought that stellar birth is
primarily controlled by the interplay between gravity and magnetostatic
support, modulated by ambipolar diffusion. Recently, however, both
observational and numerical work has begun to suggest that support by
supersonic turbulence rather than magnetic fields controls star formation. In
this review we outline a new theory of star formation relying on the control by
turbulence. We demonstrate that although supersonic turbulence can provide
global support, it nevertheless produces density enhancements that allow local
collapse. Inefficient, isolated star formation is a hallmark of turbulent
support, while efficient, clustered star formation occurs in its absence. The
consequences of this theory are then explored for both local star formation and
galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28
figures, in pres
Stationary Black Holes: Uniqueness and Beyond
The spectrum of known black-hole solutions to the stationary Einstein
equations has been steadily increasing, sometimes in unexpected ways. In
particular, it has turned out that not all black-hole-equilibrium
configurations are characterized by their mass, angular momentum and global
charges. Moreover, the high degree of symmetry displayed by vacuum and
electro-vacuum black-hole spacetimes ceases to exist in self-gravitating
non-linear field theories. This text aims to review some developments in the
subject and to discuss them in light of the uniqueness theorem for the
Einstein-Maxwell system.Comment: Major update of the original version by Markus Heusler from 1998.
Piotr T. Chru\'sciel and Jo\~ao Lopes Costa succeeded to this review's
authorship. Significantly restructured and updated all sections; changes are
too numerous to be usefully described here. The number of references
increased from 186 to 32
Hundreds of variants clustered in genomic loci and biological pathways affect human height
Most common human traits and diseases have a polygenic pattern of inheritance: DNA sequence variants at many genetic loci influence the phenotype. Genome-wide association (GWA) studies have identified more than 600 variants associated with human traits, but these typically explain small fractions of phenotypic variation, raising questions about the use of further studies. Here, using 183,727 individuals, we show that hundreds of genetic variants, in at least 180 loci, influence adult height, a highly heritable and classic polygenic trait. The large number of loci reveals patterns with important implications for genetic studies of common human diseases and traits. First, the 180 loci are not random, but instead are enriched for genes that are connected in biological pathways (P = 0.016) and that underlie skeletal growth defects (P < 0.001). Second, the likely causal gene is often located near the most strongly associated variant: in 13 of 21 loci containing a known skeletal growth gene, that gene was closest to the associated variant. Third, at least 19 loci have multiple independently associated variants, suggesting that allelic heterogeneity is a frequent feature of polygenic traits, that comprehensive explorations of already-discovered loci should discover additional variants and that an appreciable fraction of associated loci may have been identified. Fourth, associated variants are enriched for likely functional effects on genes, being over-represented among variants that alter amino-acid structure of proteins and expression levels of nearby genes. Our data explain approximately 10% of the phenotypic variation in height, and we estimate that unidentified common variants of similar effect sizes would increase this figure to approximately 16% of phenotypic variation (approximately 20% of heritable variation). Although additional approaches are needed to dissect the genetic architecture of polygenic human traits fully, our findings indicate that GWA studies can identify large numbers of loci that implicate biologically relevant genes and pathways.
Discovery and Fine-Mapping of Glycaemic and Obesity-Related Trait Loci Using High-Density Imputation
Reference panels from the 1000 Genomes (1000G) Project Consortium provide near complete coverage of common and low-frequency genetic variation with minor allele frequency ≥0.5% across European ancestry populations. Within the European Network for Genetic and Genomic Epidemiology (ENGAGE) Consortium, we have undertaken the fi
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