Determining Parameters of Cool Giant Stars by Modeling Spectrophotometric and Interferometric Observations Using the SAtlas Program

Context: Optical interferometry is a powerful tool for observing the intensity structure and angular diameter of stars. When combined with spectroscopy and/or spectrophotometry, interferometry provides a powerful constraint for model stellar atmospheres. Aims: The purpose of this work is to test the robustness of the spherically symmetric version of the Atlas stellar atmosphere program, SAtlas, using interferometric and spectrophotometric observations. Methods: Cubes (three dimensional grids) of model stellar atmospheres, with dimensions of luminosity, mass, and radius, are computed to fit observations for three evolved giant stars, \psi Phoenicis, \gamma Sagittae, and \alpha Ceti. The best-fit parameters are compared with previous results. Results: The best-fit angular diameters and values of \chi^2 are consistent with predictions using Phoenix and plane-parallel Atlas models. The predicted effective temperatures, using SAtlas, are about 100 to 200 K lower, and the predicted luminosities are also lower due to the differences in effective temperatures. Conclusions: It is shown that the SAtlas program is a robust tool for computing models of extended stellar atmospheres that are consistent with observations. The best-fit parameters are consistent with predictions using Phoenix models, and the fit to the interferometric data for \psi Phe differs slightly, although both agree within the uncertainty of the interferometric observations.Comment: 5 pages, 6 figures, Accepted for publication in A&A as a Research Not

Radio and IR interferometry of SiO maser stars

Radio and infrared interferometry of SiO maser stars provide complementary information on the atmosphere and circumstellar environment at comparable spatial resolution. Here, we present the latest results on the atmospheric structure and the dust condensation region of AGB stars based on our recent infrared spectro-interferometric observations, which represent the environment of SiO masers. We discuss, as an example, new results from simultaneous VLTI and VLBA observations of the Mira variable AGB star R Cnc, including VLTI near- and mid-infrared interferometry, as well as VLBA observations of the SiO maser emission toward this source. We present preliminary results from a monitoring campaign of high-frequency SiO maser emission toward evolved stars obtained with the APEX telescope, which also serves as a precursor of ALMA images of the SiO emitting region. We speculate that large-scale long-period chaotic motion in the extended molecular atmosphere may be the physical reason for observed deviations from point symmetry of atmospheric molecular layers, and for the observed erratic variability of high-frequency SiO maser emissionComment: 8 pages, 4 figures, submitted to Proc. IAU Symp. 287 "Cosmic masers - from OH to H_0", R.S. Booth, E.M.L. Humphreys, W.H.T. Vlemmings (eds.), invited pape

From the atmosphere to the circumstellar environment in cool evolved stars

We discuss and illustrate contributions that optical interferometry has made on our current understanding of cool evolved stars. We include red giant branch (RGB) stars, asymptotic giant branch (AGB) stars, and red supergiants (RSGs). Studies using optical interferometry from visual to mid-infrared wavelengths have greatly increased our knowledge of their atmospheres, extended molecular shells, dust formation, and winds. These processes and the morphology of the circumstellar environment are important for the further evolution of these stars toward planetary nebulae (PNe) and core-collapse supernovae (SNe), and for the return of material to the interstellar medium.Comment: To appear in the Book of the VLTI School 2013, held 9-21 Sep 2013 Barcelonnette (France), "What the highest angular resolution can bring to stellar astrophysics?", Ed. Millour, Chiavassa, Bigot, Chesneau, Meilland, Stee, EAS Publications Series (2015

Extended dynamical density functional theory for colloidal mixtures with temperature gradients

In the past decade, classical dynamical density functional theory (DDFT) has been developed and widely applied to the Brownian dynamics of interacting colloidal particles. One of the possible derivation routes of DDFT from the microscopic dynamics is via the Mori-Zwanzig-Forster projection operator technique with slowly varying variables such as the one-particle density. Here, we use the projection operator approach to extend DDFT into various directions: first, we generalize DDFT toward mixtures of $n$ different species of spherical colloidal particles. We show that there are in general nontrivial cross-coupling terms between the concentration fields and specify them explicitly for colloidal mixtures with pairwise hydrodynamic interactions. Secondly, we treat the energy density as an additional slow variable and derive formal expressions for an extended DDFT containing also the energy density. The latter approach can in principle be applied to colloidal dynamics in a nonzero temperature gradient. For the case without hydrodynamic interactions the diffusion tensor is diagonal, while thermodiffusion -- the dissipative cross-coupling term between energy density and concentration -- is nonzero in this limit. With finite hydrodynamic interactions also cross-diffusion coefficients assume a finite value. We demonstrate that our results for the extended DDFT contain the transport coefficients in the hydrodynamic limit (long wavelengths, low frequencies) as a special case.Comment: 15 pages, 1 figur

Brownian dynamics of a self-propelled particle in shear flow

Brownian dynamics of a self-propelled particle in linear shear flow is studied analytically by solving the Langevin equation and in simulation. The particle has a constant propagation speed along a fluctuating orientation and is additionally subjected to a constant torque. In two spatial dimensions, the mean trajectory and the mean square displacement (MSD) are calculated as functions of time t analytically. In general, the mean trajectories are cycloids that are modified by finite temperature effects. With regard to the MSD different regimes are identified where the MSD scales with t^a with a = 0,1,2,3,4. In particular, an accelerated (a = 4) motion emerges if the particle is self-propelled along the gradient direction of the shear flow.Comment: 6 pages, 4 figure

Stability of liquid crystalline phases in the phase-field-crystal model

The phase-field-crystal model for liquid crystals is solved numerically in two spatial dimensions. This model is formulated with three position-dependent order parameters, namely the reduced translational density, the local nematic order parameter, and the mean local direction of the orientations. The equilibrium free-energy functional involves local powers of the order parameters up to fourth order, gradients of the order parameters up to fourth order, and different couplings between the order parameters. The stable phases of the equilibrium free-energy functional are calculated for various coupling parameters. Among the stable liquid-crystalline states are the isotropic, nematic, columnar, smectic A, and plastic crystalline phases. The plastic crystals can have triangular, square, and honeycomb lattices and exhibit orientational patterns with a complex topology involving a sublattice with topological defects. Phase diagrams were obtained by numerical minimization of the free-energy functional. Their main features are qualitatively in line with much simpler one-mode approximations for the order parameters.Comment: Submitted to Physical Review

Microscopic approach to entropy production

It is a great challenge of nonequilibrium statistical mechanics to calculate entropy production within a microscopic theory. In the framework of linear irreversible thermodynamics, we combine the Mori-Zwanzig-Forster projection operator technique with the first and second law of thermodynamics to obtain microscopic expressions for the entropy production as well as for the transport equations of the entropy density and its time correlation function. We further present a microscopic derivation of a dissipation functional from which the dissipative dynamics of an extended dynamical density functional theory can be obtained in a formally elegant way.Comment: 10 page