33,586 research outputs found

    Differential Emission Measure Determination of Collisionally Ionized Plasma: II. Application to Hot Stars

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    In a previous paper we have described a technique to derive constraints on the differential emission measure (DEM) distribution, a measure of the temperature distribution, of collisionally ionized hot plasmas from their X-ray emission line spectra. We apply this technique to the Chandra/HETG spectra of all of the nine hot stars available to us at the time this project was initiated. We find that DEM distributions of six of the seven O stars in our sample are very similar but that theta Ori has an X-ray spectrum characterized by higher temperatures. The DEM distributions of both of B stars in our sample have lower magnitudes than those of the O stars and one, tau Sco, is characterized by higher temperatures than the other, beta Cru. These results confirm previous work in which high temperatures have been found for theta Ori and tau Sco and taken as evidence for channeling of the wind in magnetic fields, the existence of which are related to the stars' youth. Our results demonstrate the utility of our method for deriving temperature information for large samples of X-ray emission line spectra.Comment: The contents of this paper were formerly part of astro-ph/0403603 which was split into two paper

    Two harmonically coupled Brownian particles in random media

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    We study the behaviour of two Brownian particles coupled by an elastic harmonic force in a quenched disordered medium. We found that to first order in disorder strength, the relative motion weakens (with respect to the reference state of a Brownian particle with the double mass) the effect of the quenched forces on the centre of mass motion of the Brownian particles, so that the motion will become less subdiffusive (superdiffusive) for potential (solenoidal) disorder. The mean-square relative distance between the particles behaves in a different way depending of whether the particles are free to move or one particle is anchored in the space. While the effect of nonpotential disorder consists in increasing the mean-square distance in both cases, the potential disorder decreases the mean-square distance, when the particles are free to move, and increases it when one particle is anchored in the space.Comment: 8 pages, 3 figure

    Collective Diffusion and a Random Energy Landscape

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    Starting from a master equation in a quantum Hamiltonian form and a coupling to a heat bath we derive an evolution equation for a collective hopping process under the influence of a stochastic energy landscape. There results different equations in case of an arbitrary occupation number per lattice site or in a system under exclusion. Based on scaling arguments it will be demonstrated that both systems belong below the critical dimension dcd_c to the same universality class leading to anomalous diffusion in the long time limit. The dynamical exponent zz can be calculated by an ϵ=dcd\epsilon = d_c-d expansion. Above the critical dimension we discuss the differences in the diffusion constant for sufficient high temperatures. For a random potential we find a higher mobility for systems with exclusion.Comment: 15 pages, no figure

    Robust Appointment Scheduling with Heterogeneous Costs

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    Designing simple appointment systems that under uncertainty in service times, try to achieve both high utilization of expensive medical equipment and personnel as well as short waiting time for patients, has long been an interesting and challenging problem in health care. We consider a robust version of the appointment scheduling problem, introduced by Mittal et al. (2014), with the goal of finding simple and easy-to-use algorithms. Previous work focused on the special case where per-unit costs due to under-utilization of equipment/personnel are homogeneous i.e., costs are linear and identical. We consider the heterogeneous case and devise an LP that has a simple closed-form solution. This solution yields the first constant-factor approximation for the problem. We also find special cases beyond homogeneous costs where the LP leads to closed form optimal schedules. Our approach and results extend more generally to convex piece-wise linear costs. For the case where the order of patients is changeable, we focus on linear costs and show that the problem is strongly NP-hard when the under-utilization costs are heterogeneous. For changeable order with homogeneous under-utilization costs, it was previously shown that an EPTAS exists. We instead find an extremely simple, ratio-based ordering that is 1.0604 approximate

    Ion-by-Ion DEM Determination: I. Method

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    We describe a technique to derive constraints on the differential emission measure (DEM) distribution, a measure of the temperature distribution, of collisionally ionized hot plasmas from their X-ray emission line spectra. This technique involves fitting spectra using a number of components, each of which is the entire X-ray line emission spectrum for a single ion. It is applicable to high-resolution X-ray spectra of any collisionally ionized plasma and particularly useful for spectra in which the emission lines are broadened and blended such as those of the winds of hot stars. This method does not require that any explicit assumptions about the form of the DEM distribution be made and is easily automated.Comment: This paper was split in two. This version is part I. Part II may be found at astro-ph/050343