How planetary growth outperforms migration

Planetary migration is a major challenge for planet formation theories. The speed of Type I migration is proportional to the mass of a protoplanet, while the final decade of growth of a pebble-accreting planetary core takes place at a rate that scales with the mass to the two-thirds power. This results in planetary growth tracks (i.e., the evolution of a protoplanet's mass versus its distance from the star) that become increasingly horizontal (migration-dominated) with rising mass of the protoplanet. It has been shown recently that the migration torque on a protoplanet is reduced proportional to the relative height of the gas gap carved by the growing planet. Here we show from 1-D simulations of planet-disc interaction that the mass at which a planet carves a 50% gap is approximately 2.3 times the pebble isolation mass. Our measurements of the pebble isolation mass from 1-D simulations match published 3-D results relatively well, except at very low viscosities where the 3-D pebble isolation mass is significantly higher, possibly due to gap edge instabilities not captured in 1-D. The pebble isolation mass demarks the transition from pebble accretion to gas accretion. Gas accretion to form gas-giant planets therefore takes place over a few astronomical units of migration after reaching first the pebble isolation mass and, shortly after, the 50% gap mass. Our results demonstrate how planetary growth can outperform migration, both during core accretion and during gas accretion, even when the Stokes number of the pebbles is small, St~0.01, and the pebble-to-gas flux ratio in the protoplanetary disc is in the nominal range of 0.01-0.02. We find that planetary growth is very rapid in the first million years of the protoplanetary disc and that the probability for forming gas-giant planets increases with the initial size of the protoplanetary disc and with decreasing turbulent diffusion.Comment: Accepted for publication in Astronomy & Astrophysic

The growth of planets by pebble accretion in evolving protoplanetary discs

The formation of planets depends on the underlying protoplanetary disc structure, which influences both the accretion and migration rates of embedded planets. The disc itself evolves on time-scales of several Myr during which both temperature and density profiles change as matter accretes onto the central star. Here we use a detailed model of an evolving disc to determine the growth of planets by pebble accretion and their migration through the disc. Cores that reach their pebble isolation mass accrete gas to finally form giant planets with extensive gas envelopes, while planets that do not reach pebble isolation mass are stranded as ice giants and ice planets containing only minor amounts of gas in their envelopes. Unlike earlier population synthesis models, our model works without any artificial reductions in migration speed and for protoplanetary discs with gas and dust column densities similar to those inferred from observations. We find that in our nominal disc model the emergence of planetary embryos preferably occurs after approximately 2 Myr in order to not exclusively form gas giants, but also ice giants and smaller planets. The high pebble accretion rates ensure that critical core masses for gas accretion can be reached at all orbital distances. Gas giant planets nevertheless experience significant reduction in semi-major axes by migration. Considering instead planetesimal accretion for planetary growth, we show that formation time-scales are too long to compete with the migration time-scales and the dissipation time of the protoplanetary disc. Altogether, we find that pebble accretion overcomes many of the challenges in the formation of ice and gas giants in evolving protoplanetary discs.Comment: Accepted by A&A, now with language editin

Internal migration and rural service provision in northern Ghana:

This paper uses a two-stage conditional maximum likelihood procedure and new data from Ghana to identify the determinants of rural-urban migration at the individual, household and community levels, with a particular focus on rural services. The econometric evidence supports the theoretical expectation that human-capital and network variables as well as assets are important determinants of migration. Taking the possible endogeneity of rural services into account, the evidence suggests that rural service improvements aimed at reducing economic isolation can enhance labor mobility and free up on-farm labor for migration by lowering transaction costs.Rural-urban migration, rural services, Development strategies,

Spotlight on islands.On the origin and diversification of an ancient lineage of the Italian wall lizard Podarcis siculus in the western Pontine Islands

Groups of proximate continental islands may conceal more tangled phylogeographic patterns than oceanic archipelagos as a consequence of repeated sea level changes, which allow populations to experience gene flow during periods of low sea level stands and isolation by vicariant mechanisms during periods of high sea level stands. Here, we describe for the first time an ancient and diverging lineage of the Italian wall lizard Podarcis siculus from the western Pontine Islands. We used nuclear and mitochondrial DNA sequences of 156 individuals with the aim of unraveling their phylogenetic position, while microsatellite loci were used to test several a priori insular biogeographic models of migration with empirical data. Our results suggest that the western Pontine populations colonized the islands early during their Pliocene volcanic formation, while populations from the eastern Pontine Islands seem to have been introduced recently. The inter-island genetic makeup indicates an important role of historical migration, probably due to glacial land bridges connecting islands followed by a recent vicariant mechanism of isolation. Moreover, the most supported migration model predicted higher gene flow among islands which are geographically arranged in parallel. Considering the threatened status of small insular endemic populations, we suggest this new evolutionarily independent unit be given priority in conservation efforts

Genetic and phenotypic divergence in an island bird: isolation by distance, by colonization or by adaptation?

Discerning the relative roles of adaptive and nonadaptive processes in generating differences among populations and species, as well as how these processes interact, is a fundamental aim in biology. Both genetic and phenotypic divergence across populations can be the product of limited dispersal and gradual genetic drift across populations (isolation by distance), of colonization history and founder effects (isolation by colonization) or of adaptation to different environments preventing migration between populations (isolation by adaptation). Here, we attempt to differentiate between these processes using island populations of Berthelot's pipit (Anthus berthelotii), a passerine bird endemic to three Atlantic archipelagos. Using microsatellite markers and approximate Bayesian computation, we reveal that the northward colonization of this species ca. 8500years ago resulted in genetic bottlenecks in the colonized archipelagos. We then show that high levels of genetic structure exist across archipelagos and that these are consistent with a pattern of isolation by colonization, but not with isolation by distance or adaptation. Finally, we show that substantial morphological divergence also exists and that this is strongly concordant with patterns of genetic structure and bottleneck history, but not with environmental differences or geographic distance. Overall, our data suggest that founder effects are responsible for both genetic and phenotypic changes across archipelagos. Our findings provide a rare example of how founder effects can persist over evolutionary timescales and suggest that they may play an important role in the early stages of speciation

The reproduction trajectories of institutions in relation to social isolation of individual population groups in regions of Russia

This article deals with spatial socio-demographic isolation as a factor of the existing settlement system in Russian regions. Theoretically, the phenomena of solitude and social isolation have their own demographic, socio-economic, and psychological origin. Conceptually, they are reflected in theories of nucleation of society’s family structure, families losing its core functions, and the deprivation of personality (friends and family) in the context of the second demographic transition. In fact, the trending reluctance to having children, increases in cohabitation and divorces, men’s premature deaths resulting in widowhood are supplemented and accompanied by institutional factors that enhance a singular lifestyle, including various forms of personal isolation from social environment in post-industrial society. We examine the increasingly higher number of private households with single individuals ranging from young to old, the incidence of widowhood institution (effects of death rate risks on marriage and family), effects of labor, academic, survival, and consumer migration (both reversible and irreversible) on replacement levels as the major reasons of socio-demographic isolation. Psychological, demographic isolation of any given local communities favors the feel of solitude as the controller of communication and interaction intensity between individuals. In socio-economic terms, isolation of local communities is related to the factor of families’ living away from communication centers and real markets, to new Russian and global logistics, deprivation of traditional sources of income of households as specific forms of survival on the vast living space of Russian society.The article has been prepared with the support of the Russian Foundation for Basic Research Grant 13-06-00008a “Creation and Enhancement of Quality of Life as a Priority for Socio-Economic Development of Russian Regions.