836 research outputs found
PopCORN: Hunting down the differences between binary population synthesis codes
Binary population synthesis (BPS) modelling is a very effective tool to study
the evolution and properties of close binary systems. The uncertainty in the
parameters of the model and their effect on a population can be tested in a
statistical way, which then leads to a deeper understanding of the underlying
physical processes involved. To understand the predictive power of BPS codes,
we study the similarities and differences in the predicted populations of four
different BPS codes for low- and intermediate-mass binaries. We investigate
whether the differences are caused by different assumptions made in the BPS
codes or by numerical effects. To simplify the complex problem of comparing BPS
codes, we equalise the inherent assumptions as much as possible. We find that
the simulated populations are similar between the codes. Regarding the
population of binaries with one WD, there is very good agreement between the
physical characteristics, the evolutionary channels that lead to the birth of
these systems, and their birthrates. Regarding the double WD population, there
is a good agreement on which evolutionary channels exist to create double WDs
and a rough agreement on the characteristics of the double WD population.
Regarding which progenitor systems lead to a single and double WD system and
which systems do not, the four codes agree well. Most importantly, we find that
for these two populations, the differences in the predictions from the four
codes are not due to numerical differences, but because of different inherent
assumptions. We identify critical assumptions for BPS studies that need to be
studied in more detail.Comment: 13 pages, +21 pages appendix, 35 figures, accepted for publishing in
A&A, Minor change to match published version, most important the added link
to the website http://www.astro.ru.nl/~silviato/popcorn for more detailed
figures and informatio
If larvae were smart: a simple model for optimal settlement behavior of competent larvae
Much research has been done on larval settlement cues. Rather than having simple fixed responses to constant environmental stimuli, it seems likely that settlement decisions made by individual larvae should vary depending on the individual and the conditions under which it encounters that cue. Here, we present a simple stochastic dynamic programming model that explores the conditions under which larvae may maximize their lifetime fitness by accepting lower quality habitat rather than continuing to search for superior habitat. Our model predicts that there is a relatively narrow range of parameter values over which larval selectivity among habitat types changes dramatically from 1 (larvae accept only optimal substrata) to 0 (indiscriminant settlement). This narrow range coincides with our best estimate of parameter values gleaned from empirical studies, and the model output matches data for the polychaete worm Hydroides dianthus remarkably well. The relative availability of habitats and the total time available to search for high quality habitat (i.e. the ability to delay metamorphosis) had the greatest effects on larval selectivity. In contrast, intuitive factors, including larval energetics and mortality, showed little effect on larval habitat preference, but could still alter the proportion of larvae settling in different habitats by reducing search time. Our model predicts that a given larva may behave differently depending on where it falls in the optimality decision matrix at the instant in which it locates substrata. This model provides a conceptual framework in which to conduct future studies involving variability in settlement decisions among individual larvae, and in which to consider the selective forces driving the evolution of specific larval settlement cues. Our results suggest that a combination of the maximum search period and the relative frequency and quality of optimal habitat likely exert the greatest influence on the evolution of larval selectivity in the field
Gigahertz repetition rate thermionic electron gun concept
We present a novel concept for the generation of gigahertz repetition rate
high brightness electron bunches. A custom design 100 kV thermionic gun
provides a continuous electron beam, with the current determined by the
filament size and temperature. A 1 GHz rectangular RF cavity deflects the beam
across a knife-edge, creating a pulsed beam. Adding a higher harmonic mode to
this cavity results in a flattened magnetic field profile which increases the
duty cycle to 30%. Finally, a compression cavity induces a negative
longitudinal velocity-time chirp in a bunch, initiating ballistic compression.
Adding a higher harmonic mode to this cavity increases the linearity of this
chirp and thus decreases the final bunch length. Charged particle simulations
show that with a 0.15 mm radius LaB6 filament held at 1760 K, this method can
create 279 fs, 3.0 pC electron bunches with a radial rms core emittance of
0.089 mm mrad at a repetition rate of 1 GHz.Comment: 12 pages, 12 figure
Progenitors of Supernovae Type Ia
Despite the significance of Type Ia supernovae (SNeIa) in many fields in
astrophysics, SNeIa lack a theoretical explanation. The standard scenarios
involve thermonuclear explosions of carbon/oxygen white dwarfs approaching the
Chandrasekhar mass; either by accretion from a companion or by a merger of two
white dwarfs. We investigate the contribution from both channels to the SNIa
rate with the binary population synthesis (BPS) code SeBa in order to constrain
binary processes such as the mass retention efficiency of WD accretion and
common envelope evolution. We determine the theoretical rates and delay time
distribution of SNIa progenitors and in particular study how assumptions affect
the predicted rates.Comment: 6 pages, 6 figures, appeared in proceedings for "The 18th European
White Dwarf Workshop
Prospects for detection of detached double white dwarf binaries with Gaia, LSST and LISA
Double white dwarf (DWD) binaries are expected to be very common in the Milky
Way, but their intrinsic faintness challenges the detection of these systems.
Currently, only a few tens of detached DWDs are know. Such systems offer the
best chance of extracting the physical properties that would allow us to
address a wealth of outstanding questions ranging from the nature of white
dwarfs, over stellar and binary evolution to mapping the Galaxy. In this paper
we explore the prospects for detections of ultra-compact (with binary
separations of a few solar radii or less) detached DWDs in: 1) optical
radiation with Gaia and the LSST and 2) gravitational wave radiation with LISA.
We show that Gaia, LSST and LISA have the potential to detect respectively
around a few hundreds, a thousand, and 25 thousand DWD systems. Moreover, Gaia
and LSST data will extend by respectively a factor of two and seven the
guaranteed sample of binaries detected in electromagnetic and gravitational
wave radiation, opening the era of multi-messenger astronomy for these sources.Comment: submitted to MNRA
Rare coral under the genomic microscope: timing and relationships among Hawaiian Montipora
Background
Evolutionary patterns of scleractinian (stony) corals are difficult to infer given the existence of few diagnostic characters and pervasive phenotypic plasticity. A previous study of Hawaiian Montipora (Scleractinia: Acroporidae) based on five partial mitochondrial and two nuclear genes revealed the existence of a species complex, grouping one of the rarest known species (M. dilatata, which is listed as Endangered by the International Union for Conservation of Nature - IUCN) with widespread corals of very different colony growth forms (M. flabellata and M. cf. turgescens). These previous results could result from a lack of resolution due to a limited number of markers, compositional heterogeneity or reflect biological processes such as incomplete lineage sorting (ILS) or introgression.
Results
All 13 mitochondrial protein-coding genes from 55 scleractinians (14 lineages from this study) were used to evaluate if a recent origin of the M. dilatata species complex or rate heterogeneity could be compromising phylogenetic inference. Rate heterogeneity detected in the mitochondrial data set seems to have no significant impacts on the phylogenies but clearly affects age estimates. Dating analyses show different estimations for the speciation of M. dilatata species complex depending on whether taking compositional heterogeneity into account (0.8 [0.05–2.6] Myr) or assuming rate homogeneity (0.4 [0.14–0.75] Myr). Genomic data also provided evidence of introgression among all analysed samples of the complex. RADseq data indicated that M. capitata colour morphs may have a genetic basis.
Conclusions
Despite the volume of data (over 60,000 SNPs), phylogenetic relationships within the M. dilatata species complex remain unresolved most likely due to a recent origin and ongoing introgression. Species delimitation with genomic data is not concordant with the current taxonomy, which does not reflect the true diversity of this group. Nominal species within the complex are either undergoing a speciation process or represent ecomorphs exhibiting phenotypic polymorphisms.info:eu-repo/semantics/publishedVersio
Shape-shifting corals: Molecular markers show morphology is evolutionarily plastic in Porites
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