Missing Iron Problem and Type Ia Supernova Enrichment of Hot Gas in Galactic Spheroids

Type Ia supernovae (Ia SNe) provide a rich source of iron for hot gas in galactic stellar spheroids. However, the expected super-solar iron abundance of the hot gas is not observed. Instead, X-ray observations often show decreasing iron abundance toward galactic central regions, where the Ia SN enrichment is expected to be the highest. We examine the cause of this missing iron problem by studying the enrichment process and its effect on X-ray abundance measurements of the hot gas. The evolution of Ia SN iron ejecta is simulated in the context of galaxy-wide hot gas outflows, in both supersonic and subsonic cases, as may be expected for hot gas in galactic bulges or elliptical galaxies of intermediate masses. SN reverse-shock heated iron ejecta is typically found to have a very high temperature and low density, hence producing little X-ray emission. Such hot ejecta, driven by its large buoyancy, can quickly reach a substantially higher outward velocity than the ambient medium, which is dominated by mass loss from evolved stars. The ejecta is gradually and dynamically mixed with the medium at large galactic radii. The ejecta is also slowly diluted and cooled by {\sl insitu} mass injection from evolved stars. These processes together naturally result in the observed positive gradient in the average radial iron abundance distribution of the hot gas, even if mass-weighted. This trend is in addition to the X-ray measurement bias that tends to underestimate the iron abundance for the hot gas with a temperature distribution.Comment: 18 pages, 9 figures, MNRAS in pres

X-ray emission from dense plasma in CTTSs: Hydrodynamic modeling of the accretion shock

High spectral resolution X-ray observations of CTTSs demonstrate the presence of plasma at T~2-3X10^6 K and n_e~10^11-10^13 cm^-3, unobserved in non-accreting stars. Stationary models suggest that this emission is due to shock-heated accreting material, but they do not allow to analyze the stability of such material and its position in the stellar atmosphere. We investigate the dynamics and the stability of shock-heated accreting material in CTTSs and the role of the stellar chromosphere in determining the position and the thickness of the shocked region. We perform 1-D HD simulations of the impact of the accretion flow onto chromosphere of a CTTS, including the effects of gravity, radiative losses from optically thin plasma, thermal conduction and a well tested detailed model of the stellar chromosphere. Here we present the results of a simulation based on the parameters of the CTTS MP Mus. We find that the accretion shock generates an hot slab of material above the chromosphere with a maximum thickness of 1.8X10^9 cm, density n_e~10^11-10^2 cm^-3, temperature T~3X10^6 K and uniform pressure equal to the ram pressure of the accretion flow (~450 dyn cm^-2). The base of the shocked region penetrates the chromosphere and stays where the ram pressure is equal to the thermal pressure. The system evolves with quasi-periodic instabilities of the material in the slab leading to cyclic disappearance and re-formation of the slab. For an accretion rate of ~10^-10 M_sun yr^-1, the shocked region emits a time-averaged X-ray luminosity L_X~7X10^29 erg s^-1, which is comparable to the X-ray luminosity observed in CTTSs of the same mass. Furthermore, the X-ray spectrum synthesized from the simulation matches in detail all the main features of the O VIII and O VII lines of the star MP Mus.Comment: Accepted for publication as a Letter in Astronomy & Astrophysic

Mixing between high velocity clouds and the Galactic halo

In the Galactic halo, metal-bearing Galactic halo material mixes into high velocity clouds (HVCs) as they hydrodynamically interact. This interaction begins long before the clouds completely dissipate and long before they slow to the velocity of the Galactic material. In order to make quantitative estimates of the mixing efficiency and resulting metal enrichment of HVCs, we made detailed two- and three-dimensional simulations of cloud-interstellar medium interactions. Our simulations track the hydrodynamics and time-dependent ionization levels. They assume that the cloud originally has a warm temperature and extremely low metallicity while the surrounding medium has a high temperature, low density, and substantial metallicity, but our simulations can be generalized to other choices of initial metallicities. In our simulations, mixing between cloud and halo gas noticeably raises the metallicity of the high velocity material. We present plots of the mixing efficiency and metal enrichment as a function of time.open0

On the observability of T Tauri accretion shocks in the X-ray band

Context. High resolution X-ray observations of classical T Tauri stars (CTTSs) show a soft X-ray excess due to high density plasma (n_e=10^11-10^13 cm^-3). This emission has been attributed to shock-heated accreting material impacting onto the stellar surface. Aims. We investigate the observability of the shock-heated accreting material in the X-ray band as a function of the accretion stream properties (velocity, density, and metal abundance) in the case of plasma-beta<<1 in the post-shock zone. Methods. We use a 1-D hydrodynamic model describing the impact of an accretion stream onto the chromosphere, including the effects of radiative cooling, gravity and thermal conduction. We explore the space of relevant parameters and synthesize from the model results the X-ray emission in the [0.5-8.0] keV band and in the resonance lines of O VII (21.60 Ang) and Ne IX (13.45 Ang), taking into account the absorption from the chromosphere. Results. The accretion stream properties influence the temperature and the stand-off height of the shocked slab and its sinking in the chromosphere, determining the observability of the shocked plasma. Our model predicts that X-ray observations preferentially detect emission from low density and high velocity shocked accretion streams due to the large absorption of dense post-shock plasma. In all the cases examined, the post-shock zone exhibits quasi-periodic oscillations due to thermal instabilities, but in the case of inhomogeneous streams and beta<<1, the shock oscillations are hardly detectable. Conclusions. We suggest that, if accretion streams are inhomogeneous, the selection effect introduced by the absorption on observable plasma components may explain the discrepancy between the accretion rate measured by optical and X-ray data as well as the different densities measured using different He-like triplets in the X-ray band.Comment: 12 pages, 7 figures. Accepted for publication on A&

Investigating the X-ray emission from the massive WR+O binary WR 22 using 3D hydrodynamical models

We examine the dependence of the wind-wind collision and subsequent X-ray emission from the massive WR+O star binary WR~22 on the acceleration of the stellar winds, radiative cooling, and orbital motion. Simulations were performed with instantaneously accelerated and radiatively driven stellar winds. Radiative transfer calculations were performed on the simulation output to generate synthetic X-ray data, which are used to conduct a detailed comparison against observations. When instantaneously accelerated stellar winds are adopted in the simulation, a stable wind-wind collision region (WCR) is established at all orbital phases. In contrast, when the stellar winds are radiatively driven, and thus the acceleration regions of the winds are accounted for, the WCR is far more unstable. As the stars approach periastron, the ram pressure of the WR's wind overwhelms the O star's and, following a significant disruption of the shocks by non-linear thin-shell instabilities (NTSIs), the WCR collapses onto the O star. X-ray calculations reveal that when a stable WCR exists the models over-predict the observed X-ray flux by more than two orders of magnitude. The collapse of the WCR onto the O star substantially reduces the discrepancy in the $2-10\;$keV flux to a factor of $\simeq 6$ at $\phi=0.994$. However, the observed spectrum is not well matched by the models. We conclude that the agreement between the models and observations could be improved by increasing the ratio of the mass-loss rates in favour of the WR star to the extent that a normal wind ram pressure balance does not occur at any orbital phase, potentially leading to a sustained collapse of the WCR onto the O star. Radiative braking may then play a significant r\^{o}le for the WCR dynamics and resulting X-ray emission.Comment: 16 pages, 11 figures, 2 tables. Accepted for publication in A&

Fixed and flexible: coexistence of obligate and facultative migratory strategies in a freshwater fish

Migration is an important event in many animal life histories, but the degree to which individual animals participate in seasonal migrations often varies within populations. The powerful ecological and evolutionary consequences of such partial migration are now well documented, but the underlying mechanisms are still heavily debated. One potential mechanism of partial migration is between-individual variation in body condition, where animals in poor condition cannot pay the costs of migration and hence adopt a resident strategy. However, underlying intrinsic traits may overrule such environmental influence, dictating individual consistency in migratory patterns. Unfortunately, field tests of individual consistency compared to the importance of individual condition on migratory propensity are rare. Here we analyse 6 years of field data on roach migration, gathered by tagging almost 3000 individual fish and monitoring their seasonal migrations over extended periods of time. Our aims were to provide a field test of the role of condition in wild fish for migratory decisions, and also to assess individual consistency in migratory tendency. Our analyses reveal that (1) migratory strategy, in terms of migration/residency, is highly consistent within individuals over time and (2) there is a positive relationship between condition and the probability of migration, but only in individuals that adopt a migratory strategy at some point during their lives. However, life-long residents do not differ in condition to migrants, hence body condition is only a good predictor of migratory tendency in fish with migratory phenotypes and not a more general determinant of migratory tendency for the population. As resident individuals can achieve very high body condition and still remain resident, we suggest that our data provides some of the first field evidence to show that both facultative and obligate strategies can co-exist within populations of migratory animals

The role of the artificial conductivity in SPH simulations of galaxy clusters: effects on the ICM properties

We study the thermal structure of the intracluster medium (ICM) in a set of cosmological hydrodynamical cluster simulations performed with a smoothed particle hydrodynamics (SPH) numerical scheme employing an artificial conductivity (AC) term. We explore the effects of this term on the ICM temperature and entropy profiles, thermal distribution, velocity field and expected X-ray emission. We find that in adiabatic runs, the artificial conductivity favours (i) the formation of an entropy core, raising and flattening the central entropy profiles, in better agreement with findings from Eulerian codes; and (ii) a systematic reduction of the cold gas component. In fact, the cluster large-scale structure and dynamical state are preserved across different runs, but the improved gas mixing enabled by the AC term strongly increases the stripping rate of gas from the cold clumps moving through the ICM. This in turn reduces the production of turbulence generated by the instabilities which develop because of the interaction between clumps and ambient ICM. We then find that turbulent motions, enhanced by the time-dependent artificial viscosity scheme we use, are rather damped by the AC term. The ICM synthetic X-ray emission substantially mirrors the changes in its thermodynamical structure, stressing the robustness of the AC impact. All these effects are softened by the introduction of radiative cooling but still present, especially a partial suppression of cold gas. Therefore, not only the physics accounted for, but also the numerical approach itself can have an impact in shaping the ICM thermodynamical structure and ultimately in the use of SPH cluster simulations for cosmological studies. \ua9 2014 The Authors

The 2.35 year itch of Cygnus OB2 #9: III. X-ray and radio emission analysis based on 3D hydrodynamical modelling

Context. The wind-wind collision in a massive star binary system leads to the generation of high temperature shocks that emit at X-ray wavelengths and, if particle acceleration is effective, may exhibit non-thermal radio emission. Cyg OB2 is one of a small number of massive star binary systems in this class. Aims. X-ray and radio data recently acquired as part of a project to study Cyg OB2 are used to constrain physical models of the binary system, providing in-depth knowledge about the wind-wind collision and the thermal, and non-thermal, emission arising from the shocks. Methods. We use a 3D, adaptive mesh refinement simulation (including wind acceleration, radiative cooling, and the orbital motion of the stars) to model the gas dynamics of the wind-wind collision. The simulation output is used as the basis for radiative transfer calculations considering the thermal X-ray emission and the thermal/non-thermal radio emission. Results. The flow dynamics in the simulation show that wind acceleration (between the stars) is inhibited at all orbital phases by the opposing star's radiation field, reducing pre-shock velocities below terminal velocities. To obtain good agreement with the X-ray observations, our initial mass-loss rate estimates require a down-shift by a factor of ∼7.7 to 6.5 × 10-7 M yr-1 and 7.5 × 10-7 M yr-1 for the primary and secondary star, respectively. Furthermore, the low gas densities and high shock velocities in Cyg OB2 are suggestive of unequal electron and ion temperatures, and the X-ray analysis indicates that an immediately post-shock electron-ion temperature ratio of 0.1 is also required. The radio emission is dominated by non-thermal synchrotron emission. A parameter space exploration provides evidence against models assuming equipartition between magnetic and relativistic energy densities. However, fits of comparable quality can be attained with models having stark contrasts in the ratio of magnetic-to-relativistic energy densities. Both X-ray and radio lightcurves are largely insensitive to viewing angle. The variations in X-ray emission with orbital phase can be traced back to an inverse relation with binary separation and pre-shock velocity. The radio emission also scales with pre-shock velocity and binary separation, but to positive powers (i.e. not inversely). The radio models also reveal a subtle effect whereby inverse Compton cooling leads to an increase in emissivity as a result of the synchrotron characteristic frequency being significantly reduced. Finally, using the results of the radio analysis, we estimate the surface magnetic field strengths to be 0.3-52G

Observing simulated galaxy clusters with PHOX: a novel X-ray photon simulator

We present a novel, virtual X-ray observatory designed to obtain synthetic observations from hydro-numerical simulations, named PHOX. In particular, we provide a description of the code constituting the photon simulator and of the new approach implemented. We apply PHOX to simulated galaxy clusters in order to demonstrate its capabilities. In fact, X-ray observations of clusters of galaxies continue to provide us with an increasingly detailed picture of their structure and of the underlying physical phenomena governing the gaseous component, which dominates their baryonic content. Therefore, it is fundamental to find the most direct and faithful way to compare such observational data with hydrodynamical simulations of cluster-like objects, which can currently include various complex physical processes. Here, we present and analyse synthetic Suzaku observations of two cluster-size haloes obtained by processing with PHOX the hydrodynamical simulation of the large-scale, filament-like region in which they reside. Taking advantage of the simulated data, we test the results inferred from the X-ray analysis of the mock observations against the underlying, known solution. Remarkably, we are able to recover the theoretical temperature distribution of the two haloes by means of the multi-temperature fitting of the synthetic spectra. Moreover, the shapes of the reconstructed distributions allow us to trace the different thermal structure that distinguishes the dynamical state of the two haloes.Comment: 13 pages, 4 figures, 2 tables. Accepted for publication in MNRA

A lake as a microcosm: reflections on developments in aquatic ecology

In the present study, we aim at relating Forbes' remarkable paper on "The lake as a microcosm", published 125 years ago, to the present status of knowledge in our own research group. Hence, we relate the observations Forbes made to our own microcosm, Lake Krankesjon in southern Sweden, that has been intensively studied by several research groups for more than three decades. Specifically, we focus on the question: Have we made any significant progress or did Forbes and colleagues blaze the trail through the unknown wilderness and we are mainly paving that intellectual road? We conclude that lakes are more isolated than many other biomes, but have, indeed, many extensions, for example, input from the catchment, fishing and fish migration. We also conclude that irrespective of whether lakes should be viewed as microcosms or not, the paper by Forbes has been exceptionally influential and still is, especially since it touches upon almost all aspects of the lake ecosystem, from individual behaviour to food web interactions and environmental issues. Therefore, there is no doubt that even if 125 years have passed, Forbes' paper still is a source of inspiration and deserves to be read. Hence, although aquatic ecology has made considerable progress over the latest century, Forbes might be viewed as one of the major pioneers and visionary scientists of limnology