Predicting X-ray emission from wind-blown bubbles - Limitations of fits to ROSAT spectra

Wind-blown bubbles, from those around massive O and Wolf-Rayet stars, to superbubbles around OB associations and galactic winds in starburst galaxies, have a dominant role in determining the structure of the Interstellar Medium. X-ray observations of these bubbles are particularly important as most of their volume is taken up with hot gas, 1E5 < T (K) < 1E8. However, it is difficult to compare X-ray observations, usually analysed in terms of single or two temperature spectral model fits, with theoretical models, as real bubbles do not have such simple temperature distributions. In this introduction to a series of papers detailing the observable X-ray properties of wind-blown bubbles, we describe our method with which we aim to solve this problem, analysing a simulation of a wind-blown bubble around a massive star. We model a wind of constant mass and energy injection rate, blowing into a uniform ISM, from which we calculate X-ray spectra as would be seen by the ROSAT PSPC. We compare the properties of the bubble as would be inferred from the ROSAT data with the true properties of the bubble in the simulation. We find standard spectral models yield inferred properties that deviate significantly from the true properties, even though the spectral fits are statistically acceptable, and give no indication that they do not represent to true spectral distribution. Our results suggest that in any case where the true source spectrum does not come from a simple single or two temperature distribution the "observed" X-ray properties cannot naively be used to infer the true properties.Comment: 14 pages, LaTeX with 13 eps figures, condensed abstract. MNRAS in pres

The Impact of Welfare Reform on Leaver Characteristics, Employment and Recidivism: An Analysis of Maryland and Missouri

State and federal reforms of the 1990s transformed the U.S. cash assistance program for single parents and their children. Despite an extensive literature examining these changes and their impacts, there have been few studies that consider the effects of these reforms from the perspective of the recent period. The analysis here focuses on the characteristics and employment of welfare recipients in Maryland and Missouri, 1991-2004. We find that there has been only modest change in the observable characteristics of those entering, remaining on or leaving welfare, but the importance of employment has grown for each of these groups. We also examine the dynamics of employment and welfare recidivism, comparing cohorts of leavers prior to and after welfare reform. We find that after welfare reform leavers are much more likely to be working. Although in Maryland those working have earnings that are somewhat below employed leavers prior to reform, in Missouri earnings for employed leavers are unchanged. In both states, the types of jobs leavers hold have not changed substantially, and leavers are less likely to return to welfare following reform.welfare reform

Eroding ribbon thermocouples: impulse response and transient heat flux analysis

We have investigated a particular type of fast-response surface thermocouple to determine if it is appropriate to use a one dimensional transient heat conduction model to derive the transient surface heat flux from the measurements of surface temperature. With these sensors, low thermal inertia thermocouple junctions are formed near the surface by abrasive wear. Using laser excitation, we obtained the impulse response of these commercially available devices. The response of particular sensors can vary if new junctions are created by abrasive wear. Furthermore, the response of these sensors was found to deviate substantially from the one dimensional model and varied from sensor to sensor. The impulse response was simulated with greater fidelity using a two dimensional finite element model, but three dimensional effects also appear to be significant. The impact of these variations on the derived heat flux is assessed for the case of measurements in an internal combustion engine. When the measured impulse response is used to derive the surface heat flux, the apparent reversal of heat flux during the expansion stroke does not occur

Galaxies in Clusters: the Observational Characteristics of Bow-Shocks, Wakes and Tails

The dynamical signatures of the interaction between galaxies in clusters and the intracluster medium (ICM) can potentially yield significant information about the structure and dynamical history of clusters. To develop our understanding of this phenomenon we present results from numerical modelling of the galaxy/ICM interaction, as the galaxy moves through the cluster. The simulations have been performed for a broad range, of ICM temperatures (kT = 1,4 and 8 keV), representative of poor clusters or groups through to rich clusters. There are several dynamical features that can be identified in these simulations; for supersonic galaxy motion, a leading bow-shock is present, and also a weak gravitationally focussed wake or tail behind the galaxy (analogous to Bondi-Hoyle accretion). For galaxies with higher mass-replenishment rates and a denser interstellar medium (ISM), the dominant feature is a dense ram-pressure stripped tail. In line with other simulations, we find that the ICM/galaxy ISM interaction can result in complex time- dependent dynamics, with ram-pressure stripping occurring in an episodic manner. In order to facilitate this comparison between the observational consequences of dynamical studies and X-ray observations we have calculated synthetic X-ray flux and hardness maps from these simulations. These calculations predict that the ram-pressure stripped tail will usually be the most visible feature, though in nearby galaxies the bow-shock preceding the galaxy should also be apparent in deeper X-ray observations. We briefly discuss these results and compare with X-ray observations of galaxies where there is evidence of such interactions.Comment: 14 pages, 8 diagrams, MNRAS (in press

The Energetics and Mass-loss of Mrk33

We present ROSAT HRI X-ray data and optical imaging of the important dwarf starburst Markarian 33. We find an extended, complex, shell-like morphology in the X-ray emission, with an extent of 2.3 x 1.9kpc, coincident with the bright star-forming regions at the centre of the galaxy. The physical extent of this X-ray emission from Mrk 33 is very similar to the observed Halpha emission, and suggests that the bulk of the X-ray emission is coming from an expanding superbubble. We estimate the age and mass of Mrk 33's starburst to be 5.8 Myr and 6.9 x 10^{6} Msolar respectively with the energy injection rate in the central regions of the galaxy being 10^{41} erg/s, while the associated mass-loss rate from the star-forming regions is estimated to be 0.2 Msolar/yr. We suggest that the X-ray emission is predominantly powered by starburst type activity and argue that a blowout in the form of a galactic wind is the most likely fate for Mrk 33 resulting in the loss of most of the galaxy's metal-enriched material and a small fraction (<1 per cent) of the ISM.Comment: 13 pages, 6 figures, accepted for publication in MNRA

Simulations of the Effects of Stripping and Accretion on Galaxy Haloes in Clusters

We present results from a series of hydrodynamic simulations investigating ram pressure stripping of galactic haloes as the host galaxy falls radially into a cluster. We perform a parameter study comprising of variations in initial gas content, gas injection rate (via stellar mass loss processes), galaxy mass and amplitude of infall. From the simulation results we track variations in both physical quantities (e.g. gas mass) and directly observable quantities (e.g. X-ray luminosities). The luminosity of the galaxy's X-ray halo is found to compare favourably with the observationally determined correlation with optical blue band luminosity (L_X:L_B) relation. Factors affecting the X-ray luminosity are explored and it is found that the gas injection rate is a dominant factor in determining the integrated luminosity. Observational properties of the material stripped from the galaxy, which forms an X-ray wake, are investigated and it is found that wakes are most visible around galaxies with a substantial initial gas content, during their first passage though the cluster. We define a statistical skewness measure which may be used to determine the direction of motion of a galaxy using X-ray observations. Structures formed in these simulations are similar to the cold fronts seen in observation of cluster mergers where a sharp increase in surface brightness is accompanied by a transition to a cooler region.Comment: Accepted for publication in MNRAS. 19 pages, 21 figure

Short-term climate response to a freshwater pulse in the Southern Ocean

The short-term response of the climate system to a freshwater anomaly in the Southern Ocean is investigated using a coupled global climate model. As a result of the anomaly, ventilation of deep waters around Antarctica is inhibited, causing a warming of the deep ocean, and a cooling of the surface. The surface cooling causes Antarctic sea-ice to thicken and increase in extent, and this leads to a cooling of Southern Hemisphere surface air temperature. The surface cooling increases over the first 5 years, then remains constant over the next 5 years. There is a more rapid response in the Pacific Ocean, which transmits a signal to the Northern Hemisphere, ultimately causing a shift to the negative phase of the North Atlantic Oscillation in years 5–10

The Adaptive Nature of Impulsivity

An old joke circulates among animal behavior instructors. One can, the joke goes, divide the topics of animal behavior into four Fs: fighting, fleeing, feeding, and reproduction. This somewhat tired joke carries considerable truth. Animals behaving in nature surely must make decisions about conflicts, predator avoidance, feeding, and mating. Male crickets, for example, are notoriously combative. Studies have shown, however, that they escalate fights in some situations and retreat to fight another day in others (Beaugrand, 1997; Parker, 1974; Parker & Rubenstein, 1981). Squirrels, like many small animals, respond to the presence or absence of protective cover; for example, they will carry large food items into the safety of the bushes to consume them but eat small items immediately (Lima, Valone, & Caraco, 1985). Female widow birds prefer males with long tails, and evolutionary theorists have argued that tail length correlates with male quality (Andersson, 1982, 1994). So a female confronted with a short-tailed male faces a dilemma: mate now or keep looking. Notice that in all of these choice situations, time complicates the animal’s problem: Risk injury by fighting now or retreat to fight later; stay exposed to possible predation or invest time in moving to a safer place; settle for the short-tailed male or keep looking. Each of these situations, and indeed virtually any naturally occurring choice situation one can imagine, is an intertemporal choice problem. We define these as choice situations in which an animal’s alternatives vary in the time at which the animal realizes consequences and in the quality of those consequences once the animal secures them. Although intertemporal choice applies to many domains (and all four Fs), we need to focus on a specific situation to make scientific headway, and for virtually all behavioral ecologists interested in intertemporal choice that focal situation is foraging. We can observe animal foraging choices easily (e.g., animals eat more often than they reproduce), and we can manipulate the time and magnitude of foraging options much more easily than we can manipulate mate quality or predation risk. Moreover, we have a large base of theoretical and empirical results that help us frame the intertemporal choice problem in the context of animal foraging behavior. Foraging is not only a convenient topic but also a fundamentally important one; actively seeking food is a basic part of animal existence that deserves our attention. In the first part of this chapter, we focus on adaptive aspects of intertemporal choice in animal foraging behavior, and especially on the problem of impulsivity, which we see as a central problem in intertemporal choice. In the second part of this chapter, we take a broader perspective, including domains other than food and extending beyond impulsivity to a more encompassing view of intertemporal choice. Within this general view, we explore the adaptive nature of impulsivity

DOSE-RESPONSE MODELING WITH MARGINAL INFORMATION ON A MISSING CATEGORICAL COVARIATE

When the relationship between a dosage-type variable and a binary outcome depends on a categorical variable, a common analysis would employ a dose-response model with the categorical variable as a covariate. When the level of the categorical variable is not known for all subjects, however, the standard dose-response model alone cannot provide useful inference. We present an EM-based approach to account for the missing covariate in a dose-response model setting when additional knowledge about the marginal distribution of the covariate is available. This approach is motivated by a study of the beetle Rhyzopertha dominica, a pest of stored grain in Australia. Certain genotypes of this beetle have developed inheritable resistance to a widely-used insecticidal fumigant. In this study, the effects of various dosage levels of the fumigant were considered, and it was feasible to genotype only the surviving beetles