Diffuse Thermal X-Ray Emission in the Core of the Young Massive Cluster Westerlund 1

We present an analysis of the diffuse hard X-ray emission in the core of the young massive Galactic cluster Westerlund 1 based on a 48 ks XMM-Newton observation. Chandra results for the diffuse X-ray emission have indicated a soft thermal component together with a hard component that could be either thermal or non-thermal. We seek to resolve this ambiguity regarding the hard component exploiting the higher sensitivity of XMM-Newton to diffuse emission. Our new X-ray spectra from the central (2' radius) diffuse emission are found to exhibit He-like Fe 6.7 keV line emission, demonstrating that the hard emission in the cluster core is predominantly thermal in origin. Potential sources of this hard component are reviewed, namely an unresolved Pre-Main Sequence population, a thermalized cluster wind and Supernova Remnants interacting with stellar winds. We find that the thermalized cluster wind likely contributes the majority of the hard emission with some contribution from the Pre-Main Sequence population. It is unlikely that Supernova Remnants are contributing significantly to the Wd1 diffuse emission at the current epoch

Quantum Monte Carlo simulation of thin magnetic films

The stochastic series expansion quantum Monte Carlo method is used to study thin ferromagnetic films, described by a Heisenberg model including local anisotropies. The magnetization curve is calculated, and the results compared to Schwinger boson and many-body Green's function calculations. A transverse field is introduced in order to study the reorientation effect, in which the magnetization changes from out-of-plane to in-plane. Since the approximate theoretical approaches above differ significantly from each other, and the Monte Carlo method is free of systematic errors, the calculation provides an unbiased check of the approximate treatments. By studying quantum spin models with local anisotropies, varying spin size, and a transverse field, we also demonstrate the general applicability of the recent cluster-loop formulation of the stochastic series expansion quantum Monte Carlo method.Comment: 9 pages, 12 figure

Field-induced magnetic reorientation and effective anisotropy of a ferromagnetic monolayer within spin wave theory

The reorientation of the magnetization of a ferromagnetic monolayer is calculated with the help of many-body Green's function theory. This allows, in contrast to other spin wave theories, a satisfactory calculation of magnetic properties over the entire temperature range of interest since interactions between spin waves are taken into account. A Heisenberg Hamiltonian plus a second-order uniaxial single-ion anisotropy and an external magnetic field is treated by the Tyablikov (Random Phase Approximation: RPA) decoupling of the exchange interaction term and the Anderson-Callen decoupling of the anisotropy term. The orientation of the magnetization is determined by the spin components $\langle S^\alpha\rangle$ ($\alpha=x,y,z$), which are calculated with the help of the spectral theorem. The knowledge of the orientation angle $\Theta_0$ allows a non-perturbative determination of the temperature dependence of the effective second-order anisotropy coefficient. Results for the Green's function theory are compared with those obtained with mean-field theory (MFT). We find significant differences between these approaches

Structure and dynamics of cationic van der Waals clusters. I. Binding and structure of protonated argon clusters

The geometric structure and properties of medium sized protonated ArnH clusters n 2 35 are investigated with a DIM model with ab initio input data generated by means of multi reference CI computations. For the electronic ground state, the global minimum as well as secondary minima are found and analyzed. Low lying excited states are also calculated. They are all globally repulsiv

Cationic van der Waals complexes Theoretical study of Ar2H structure and stability

The electronic and geometric structure, stability and molecular properties of the cationic van der Waals complex Ar 2H in its ground electronic state are studied by means of two ab initio quantum chemical approaches conventional configuration interaction multi reference and coupled cluster methods and a diatomics in molecules model with ab initio input data. To ensure consistency between the two approaches, one and the same one electron atomic basis set aug cc pVTZ by Dunning is employed in both. The topography of the ground state potential energy surface is examined with respect to the nature of the binding and the stability of structures corresponding to stationary points. In accordance with most earlier theoretical work, there are two local minima at linear arrangements a strongly bound centro symmetric moiety, Ar H Ar , and a weakly bound van der Waals complex, Ar...ArH . These are separated by a low barrier. Only the centro symmetric molecule is significantly stable De 0.68 eV against fragmentation into Ar ArH and should have structural and dynamical relevance. A fairly simple diatomics in molecules model taking into account only the few lowest electronic fragment states yields a qualitatively correct description of the ground state but shows quantitative deviations from the more accurate configuration interaction data in detail. Nevertheless, it should provide a good starting point for the treatment of larger complexes Ar nH with n gt;