Stochastic and deterministic models for age-structured populations with genetically variable traits

Understanding how stochastic and non-linear deterministic processes interact is a major challenge in population dynamics theory. After a short review, we introduce a stochastic individual-centered particle model to describe the evolution in continuous time of a population with (continuous) age and trait structures. The individuals reproduce asexually, age, interact and die. The 'trait' is an individual heritable property (d-dimensional vector) that may influence birth and death rates and interactions between individuals, and vary by mutation. In a large population limit, the random process converges to the solution of a Gurtin-McCamy type PDE. We show that the random model has a long time behavior that differs from its deterministic limit. However, the results on the limiting PDE and large deviation techniques \textit{\`a la} Freidlin-Wentzell provide estimates of the extinction time and a better understanding of the long time behavior of the stochastic process. This has applications to the theory of adaptive dynamics used in evolutionary biology. We present simulations for two biological problems involving life-history trait evolution when body size is plastic and individual growth is taken into account.Comment: This work is a proceeding of the CANUM 2008 conferenc

Assessing the risks of changing ongoing management of endangered species

Recovery programmes for endangered species can become increasingly demanding over time, but managers may be reluctant to change ongoing actions that are believed to be assisting recovery. We used a quantitative risk assessment to choose support strategies for a reintroduced population of Mauritius olive white‐eyes Zosterops chloronothos. Facing increasing costs, managers considered changing the ongoing supplementary feeding strategy, but at the same time worried this could jeopardize the observed positive population trend. We used a feeding experiment to compare the current feeding regime and a cheaper alternative (a simple sugar/water mix). Results suggested the cheaper alternative would only marginally reduce population vital rates. We assessed the influence of these results and the associated uncertainty on population recovery and management costs using two decision‐analytic criteria, incremental cost‐effectiveness ratio and stochastic dominance. The new feeding regime was expected to be, on average, more cost‐effective than the status quo. Moreover, even negative outcomes would only likely mean a slower growing population, not a declining one, whereas not changing feeding regime actually entailed greater risk. Because shifting from the current regime to a cheaper sugar/water mixture was both a risk‐averse and a cost‐effective choice, we decided to implement this change. Four years after the experiment, the population continues to grow and costs have been contained, matching predictions almost exactly. In this case, the field experiment provided useful empirical information about prospective actions; the risk analysis then helped us understand the real implications of changing the feeding regime. We encourage managers of recovery plans facing a similar situation to explicitly recognize trade‐offs and risk aversion, and address them by combining targeted research and formal decision analysis

Two-dimensional AMR simulations of colliding flows

Colliding flows are a commonly used scenario for the formation of molecular clouds in numerical simulations. Due to the thermal instability of the warm neutral medium, turbulence is produced by cooling. We carry out a two-dimensional numerical study of such colliding flows in order to test whether statistical properties inferred from adaptive mesh refinement (AMR) simulations are robust with respect to the applied refinement criteria. We compare probability density functions of various quantities as well as the clump statistics and fractal dimension of the density fields in AMR simulations to a static-grid simulation. The static grid with 2048^2 cells matches the resolution of the most refined subgrids in the AMR simulations. The density statistics is reproduced fairly well by AMR. Refinement criteria based on the cooling time or the turbulence intensity appear to be superior to the standard technique of refinement by overdensity. Nevertheless, substantial differences in the flow structure become apparent. In general, it is difficult to separate numerical effects from genuine physical processes in AMR simulations.Comment: 6 pages, 6 figures, submitted to A&

Multiwavelength Campaign on Mrk 509 X. Lower limit on the distance of the absorber from HST COS and STIS spectroscopy

Active Galactic Nuclei often show evidence of photoionized outflows. A major uncertainty in models for these outflows is the distance ($R$) to the gas from the central black hole. In this paper we use the HST/COS data from a massive multi-wavelength monitoring campaign on the bright Seyfert I galaxy Mrk 509, in combination with archival HST/STIS data, to constrain the location of the various kinematic components of the outflow. We compare the expected response of the photoionized gas to changes in ionizing flux with the changes measured in the data using the following steps: 1) We compare the column densities of each kinematic component measured in the 2001 STIS data with those measured in the 2009 COS data; 2) We use time-dependent photionization calculations with a set of simulated lightcurves to put statistical upper limits on the hydrogen number density that are consistent with the observed small changes in the ionic column densities; 3) From the upper limit on the number density, we calculate a lower limit on the distance to the absorber from the central source via the prior determination of the ionization parameter. Our method offers two improvements on traditional timescale analysis. First, we account for the physical behavior of AGN lightcurves. Second, our analysis accounts for the quality of measurement in cases where no changes are observed in the absorption troughs. The very small variations in trough ionic column densities (mostly consistent with no change) between the 2001 and 2009 epochs allow us to put statistical lower limits on the distance between 100--200 pc for all the major UV absorption components at a confidence level of 99%. These results are mainly consistent with the independent distance estimates derived for the warm absorbers from the simultaneous X-ray spectra.Comment: Accepted to A&A (06 APR 2012

High-Resolution X-ray Spectroscopy of the Interstellar Medium

The interstellar medium (ISM) has a multiphase structure characterized by gas, dust and molecules. The gas can be found in different charge states: neutral, low-ionized (warm) and high-ionized (hot). It is possible to probe the multiphase ISM through the observation of its absorption lines and edges in the X-ray spectra of background sources. We present a high-quality RGS spectrum of the low-mass X-ray binary GS 1826-238 with an unprecedent detailed treatment of the absorption features due to the dust and both the neutral and ionized gas of the ISM. We constrain the column density ratios within the different phases of the ISM and measure the abundances of elements such as O, Ne, Fe and Mg. We found significant deviations from the proto-Solar abundances: oxygen is over-abundant by a factor 1.23 +/- 0.05, neon 1.75 +/- 0.11, iron 1.37 +/- 0.17 and magnesium 2.45 +/- 0.35. The abundances are consistent with the measured metallicity gradient in our Galaxy: the ISM appears to be metal-rich in the inner regions. The spectrum also shows the presence of warm/hot ionized gas. The gas column has a total ionization degree less than 10%. We also show that dust plays an important role as expected from the position of GS 1826-238: most iron appears to be bound in dust grains, while 10-40% of oxygen consists of a mixture of dust and molecules

Supernova Remnants as Clues to Their Progenitors

Supernovae shape the interstellar medium, chemically enrich their host galaxies, and generate powerful interstellar shocks that drive future generations of star formation. The shock produced by a supernova event acts as a type of time machine, probing the mass loss history of the progenitor system back to ages of $\sim$ 10 000 years before the explosion, whereas supernova remnants probe a much earlier stage of stellar evolution, interacting with material expelled during the progenitor's much earlier evolution. In this chapter we will review how observations of supernova remnants allow us to infer fundamental properties of the progenitor system. We will provide detailed examples of how bulk characteristics of a remnant, such as its chemical composition and dynamics, allow us to infer properties of the progenitor evolution. In the latter half of this chapter, we will show how this exercise may be extended from individual objects to SNR as classes of objects, and how there are clear bifurcations in the dynamics and spectral characteristics of core collapse and thermonuclear supernova remnants. We will finish the chapter by touching on recent advances in the modeling of massive stars, and the implications for observable properties of supernovae and their remnants.Comment: A chapter in "Handbook of Supernovae" edited by Athem W. Alsabti and Paul Murdin (18 pages, 6 figures

Disordered Environments in Spatial Games

The Prisoner's dilemma is the main game theoretical framework in which the onset and maintainance of cooperation in biological populations is studied. In the spatial version of the model, we study the robustness of cooperation in heterogeneous ecosystems in spatial evolutionary games by considering site diluted lattices. The main result is that due to disorder, the fraction of cooperators in the population is enhanced. Moreover, the system presents a dynamical transition at $\rho^*$, separating a region with spatial chaos from one with localized, stable groups of cooperators.Comment: 6 pages, 5 figure

Gamma-Rays and the Far-Infrared-Radio Continuum Correlation Reveal a Powerful Galactic Centre Wind

We consider the thermal and non-thermal emission from the inner 200 pc of the Galaxy. The radiation from this almost star-burst-like region is ultimately driven dominantly by on-going massive star formation. We show that this region's radio continuum (RC) emission is in relative deficit with respect to the expectation afforded by the Far- infrared-Radio Continuum Correlation (FRC). Likewise we show that the region's gamma-ray emission falls short of that expected given its star formation and resultant supernova rates. These facts are compellingly explained by positing that a powerful (400-1200 km/s) wind is launched from the region. This wind probably plays a number of important roles including advecting positrons into the Galactic bulge thus explaining the observed ~kpc extension of the 511 keV positron annihilation signal around the GC. We also show that the large-scale GC magnetic field falls in the range ~100-300 microG and that - in the time they remain in the region - GC cosmic rays do not penetrate into the region's densest molecular material.Comment: Version accepted for publication in MNRAS Letters. Discussion extended and references adde

On the Origin of Cosmic Magnetic Fields

We review the literature concerning how the cosmic magnetic fields pervading nearly all galaxies actually got started. some observational evidence involves the chemical abundance of the light elements Be and B, while another one is based on strong magnetic fields seen in high red shift galaxies. Seed fields, whose strength is of order 10^{-20} gauss, easily sprung up in the era preceding galaxy formation. Several mechanisms are proposed to amplify these seed fields to microgauss strengths. The standard mechanism is the Alpha-Omega dynamo theory. It has a major difficulty that makes unlikely to provide the sole origin. The difficulty is rooted in the fact that the total flux is constant. This implies that flux must be removed from the galactic discs. This requires that the field and flux be separated, for otherwise interstellar mass must be removed from the deep galactic gravitational and then their strength increased by the alpha omega theory.Comment: 90 pages and 6 figures; accepted for publication in Reports of Progress in Physics as an invited revie

The Interstellar Environment of our Galaxy

We review the current knowledge and understanding of the interstellar medium of our galaxy. We first present each of the three basic constituents - ordinary matter, cosmic rays, and magnetic fields - of the interstellar medium, laying emphasis on their physical and chemical properties inferred from a broad range of observations. We then position the different interstellar constituents, both with respect to each other and with respect to stars, within the general galactic ecosystem.Comment: 39 pages, 12 figures (including 3 figures in 2 parts