Simulation of stellar instabilities with vastly different timescales using domain decomposition

Strange mode instabilities in the envelopes of massive stars lead to shock waves, which can oscillate on a much shorter timescale than that associated with the primary instability. The phenomenon is studied by direct numerical simulation using a, with respect to time, implicit Lagrangian scheme, which allows for the variation by several orders of magnitude of the dependent variables. The timestep for the simulation of the system is reduced appreciably by the shock oscillations and prevents its long term study. A procedure based on domain decomposition is proposed to surmount the difficulty of vastly different timescales in various regions of the stellar envelope and thus to enable the desired long term simulations. Criteria for domain decomposition are derived and the proper treatment of the resulting inner boundaries is discussed. Tests of the approach are presented and its viability is demonstrated by application to a model for the star P Cygni. In this investigation primarily the feasibility of domain decomposition for the problem considered is studied. We intend to use the results as the basis of an extension to two dimensional simulations.Comment: 15 pages, 10 figures, published in MNRA

Spin-polarized electronic structure of the core-shell ZnO/ZnO:Mn nanowires probed by x-ray absorption and emission spectroscopy

The combination of x-ray spectroscopy methods complemented with theoretical analysis unravels the coexistence of paramagnetic and antiferromagnetic phases in the Zn_0.9Mn_0.1O shell deposited onto array of wurtzite ZnO nanowires. The shell is crystalline with orientation toward the ZnO growth axis, as demonstrated by X-ray linear dichroism. EXAFS analysis confirmed that more than 90% of Mn atoms substituted Zn in the shell while fraction of secondary phases was below 10%. The value of manganese spin magnetic moment was estimated from the Mn K{\beta} X-ray emission spectroscopy to be 4.3{\mu}B which is close to the theoretical value for substitutional Mn_Zn. However the analysis of L_2,3 x-ray magnetic circular dichroism data showed paramagnetic behaviour with saturated spin magnetic moment value of 1.95{\mu}B as determined directly from the spin sum rule. After quantitative analysis employing atomic multiplet simulations such difference was explained by a coexistence of paramagnetic phase and local antiferromagnetic coupling of Mn magnetic moments. Finally, spin-polarized electron density of states was probed by the spin-resolved Mn K-edge XANES spectroscopy and consequently analyzed by band structure calculations.Comment: Supplementary information available at http://www.rsc.org/suppdata/ja/c3/c3ja50153a/c3ja50153a.pdf J. Anal. At. Spectrom., 201

Galactic Twins of the Ring Nebula Around SN1987A and a Possible LBV-like Phase for Sk-69 202

Some core-collapse supernovae show clear signs of interaction with dense circumstellar material that often appears to be non-spherical. Circumstellar nebulae around supernova progenitors provide clues to the origin of that asymmetry in immediate pre-supernova evolution. Here I discuss outstanding questions about the formation of the ring nebula around SN1987A and some implications of similar ring nebulae around Galactic B supergiants. Several clues hint that SN1987A's nebula may have been ejected in an LBV-like event, rather than through interacting winds in a transition from a red supergiant to a blue supergiant.Comment: 2 pages, to appear in procedings of "Massive stars: fundamental parameters and circumstellar interactions", conference in honor of Virpi Niemela's 70th birthda

New Constraints on the Origin of the Short-Term Cyclical Variability of the Wolf-Rayet Star WR 46

The Wolf-Rayet star WR 46 is known to exhibit a very complex variability pattern on relatively short time scales of a few hours. Periodic but intermittent radial velocity shifts of optical lines as well as multiple photometric periods have been found in the past. Non-radial pulsations, rapid rotational modulation or the presence of a putative low-mass companion have been proposed to explain the short-term behaviour. In an effort to unveil its true nature, we observed WR 46 with FUSE (Far Ultraviolet Spectroscopic Explorer) over several short-term variability cycles. We found significant variations on a time scale of ~8 hours in the far-ultraviolet (FUV) continuum, in the blue edge of the absorption trough of the OVI {\lambda}{\lambda}1032, 1038 doublet P Cygni profile and in the SVI {\lambda}{\lambda}933, 944 P Cygni absorption profile. We complemented these observations with X-ray and UV light-curves and an X-ray spectrum from archival XMM-Newton (X-ray Multi-Mirror Mission - Newton Space Telescope) data. The X-ray and UV light-curves show variations on a time scale similar to the variability found in the FUV. We discuss our results in the context of the different scenarios suggested to explain the short-term variability of this object and reiterate that non-radial pulsations is the most likely to occur.Comment: 36 pages, 11 figures. Accepted for publication in Ap

Formation of the physical vapor deposited CdS Cu In,Ga Se2 interface in highly efficient thin film solar cells

We report on the buffer absorber interface formation in highly efficient 14.5 , AM1.5 ZnO CdS Cu In,Ga Se2 solar cells with a physical vapor deposited CdS buffer. For Se decapped Cu In,Ga Se2 CIGSe absorbers we observe sulfur passivation of the CIGSe grain boundaries during CdS growth and at the interface a thermally stimulated formation of a region with a higher band gap than that of the absorber bulk, determining the height of the potential barrier at the CdS CIGSe interface. For air exposed CIGSe samples the grain boundary passivation is impeded by a native oxide adsorbate layer at the CIGSe surface determining the thermal stability of the potential barrier heigh

Local Radiative Hydrodynamic and Magnetohydrodynamic Instabilities in Optically Thick Media

We examine the local conditions for radiative damping and driving of short wavelength, propagating hydrodynamic and magnetohydrodynamic (MHD) waves in static, optically thick, stratified equilibria. We show that so-called strange modes in stellar oscillation theory and magnetic photon bubbles are intimately related and are both fundamentally driven by the background radiation flux acting on compressible waves. We identify the necessary criteria for unstable driving of these waves, and show that this driving can exist in both gas and radiation pressure dominated media, as well as pure Thomson scattering media in the MHD case. The equilibrium flux acting on opacity fluctuations can drive both hydrodynamic acoustic waves and magnetosonic waves unstable. In addition, magnetosonic waves can be driven unstable by a combination of the equilibrium flux acting on density fluctuations and changes in the background radiation pressure along fluid displacements. We briefly describe the conditions under which these instabilities might be manifested in both main sequence stellar envelopes and accretion disks.Comment: 55 pages, revised version accepted for publication by ApJ. New appendix added justifying WKB analysi

The Nature of the Radiative Hydrodynamic Instabilities in Radiatively Supported Thomson Atmospheres

Atmospheres having a significant radiative support are shown to be intrinsically unstable at luminosities above a critical fraction Gamma_crit ~ 0.5-0.85 of the Eddington limit, with the exact value depending on the boundary conditions. Two different types of absolute radiation-hydrodynamic instabilities of acoustic waves are found to take place even in the electron scattering dominated limit. Both instabilities grow over dynamical time scales and both operate on non radial modes. One is stationary and arises only after the effects of the boundary conditions are taken into account, while the second is a propagating wave and is insensitive to the boundary conditions. Although a significant wind can be generated by these instabilities even below the classical Eddington luminosity limit, quasi-stable configurations can exist beyond the Eddington limit due to the generally reduced effective opacity. The study is done using a rigorous numerical linear analysis of a gray plane parallel atmosphere under the Eddington approximation. We also present more simplified analytical explanations.Comment: 18 Pages, 7 figures, uses emulateapj5.sty, accepted to Ap

Mass-luminosity relation and pulsational properties of Wolf-Rayet stars

Evolution of Population I stars with initial masses from 70M_\odot to 130M_\odot is considered under various assumptions on the mass loss rate \dot M. The mass-luminosity relation of W-R stars is shown to be most sensitive to the mass loss rate during the helium burning phase \dot M_{3\alpha}. Together with the mass-luminosity relation obtained for all evolutionary sequences several more exact relations are determined for the constant ratio f_{3\alpha}=\dot M/\dot M_{3\alpha} with 0.5 \le f_{3\alpha} \le 3. Evolutionary models of W-R stars were used as initial conditions in hydrodynamic computations of radial nonlinear stellar oscillations. The oscillation amplitude is larger in W-R stars with smaller initial mass or with lower mass loss rate due to higher surface abundances of carbon and oxygen. In the evolving W-R star the oscillation amplitude decreases with decreasing stellar mass M and for M < 10M_\odot the sufficiently small nonlinear effects allow us to calculate the integral of the mechanical work W done over the pulsation cycle in each mass zone of the hydrodynamical model. The only positive maximum on the radial dependence of W is in the layers with temperature of T\sim 2e5K where oscillations are excited by the iron Z--bump kappa-mechanism. Radial oscillations of W-R stars with mass of M > 10M_\odot are shown to be also excited by the kappa-mechanism but the instability driving zone is at the bottom of the envelope and pulsation motions exist in the form of nonlinear running waves propagating outward from the inner layers of the envelope.Comment: 15 pages, 10 figures, submitted to Astronomy Letter

The Structure of the Homunculus. III. Forming a Disk and Bipolar Lobes in a Rotating Surface Explosion

We present a semi-analytic model for shaping the nebula around eta Carinae that accounts for the simultaneous production of bipolar lobes and an equatorial disk through a rotating surface explosion. Material is launched normal to the surface of an oblate rotating star with an initial kick velocity that scales approximately with the local escape speed. Thereafter, ejecta follow ballistic orbital trajectories, feeling only a central force corresponding to a radiatively reduced gravity. Our model is conceptually similar to the wind-compressed disk model of Bjorkman & Cassinelli, but we modify it to an explosion instead of a steady line-driven wind, we include a rotationally-distorted star, and we treat the dynamics somewhat differently. Continuum-driving avoids the disk inhibition that normally operates in line-driven winds. Our model provides a simple method by which rotating hot stars can simultaneously produce intrinsically bipolar and equatorial mass ejections, without an aspherical environment or magnetic fields. Although motivated by eta Carinae, the model may have generic application to other LBVs, B[e] stars, or SN1987A's nebula. When near-Eddington radiative driving is less influential, our model generalizes to produce bipolar morphologies without disks, as seen in many PNe.Comment: ApJ accepted, 9 page