The Experts below are selected from a list of 267 Experts worldwide ranked by ideXlab platform

K N Nagendra - One of the best experts on this subject based on the ideXlab platform.

  • resonance line polarization in spherically symmetric moving media a parametric study
    arXiv: Solar and Stellar Astrophysics, 2020
    Co-Authors: A Megha, M Sampoorna, K N Nagendra, L S Anusha, K Sankarasubramanian
    Abstract:

    In the present paper we consider the problem of resonance line polarization formed in the spherically symmetric expanding atmospheres. For the solution of the concerned polarized transfer equation we use the Comoving Frame formulation, and apply the Accelerated Lambda Iteration (ALI) method. We restrict ourselves to the non-relativistic regime of velocities wherein mainly Doppler shift effects are significant. For our studies, we consider the scattering on a two-level atom, including the effects of partial frequency redistribution (PFR). We present the dependence of linearly polarized profiles on different atmospheric and atomic parameters.

  • POLARIZED LINE FORMATION IN MOVING ATMOSPHERES WITH PARTIAL FREQUENCY REDISTRIBUTION AND A WEAK MAGNETIC FIELD
    The Astrophysical Journal, 2015
    Co-Authors: M Sampoorna, K N Nagendra
    Abstract:

    The dynamical state of the solar and stellar atmospheres depends on the macroscopic velocity fields prevailing within them. The presence of such velocity fields in the line formation regions strongly affects the polarized radiation field emerging from these atmospheres. Thus it becomes necessary to solve the radiative transfer equation for polarized lines in moving atmospheres. Solutions based on the "observer's Frame method" are computationally expensive to obtain, especially when partial frequency redistribution (PRD) in line scattering and large-amplitude velocity fields are taken into account. In this paper we present an efficient alternative method of solution, namely, the Comoving Frame technique, to solve the polarized PRD line formation problems in the presence of velocity fields. We consider one-dimensional planar isothermal atmospheres with vertical velocity fields. We present a study of the effect of velocity fields on the emergent linear polarization profiles formed in optically thick moving atmospheres. We show that the Comoving Frame method is far superior when compared to the observer's Frame method in terms of the computational speed and memory requirements.

  • Partial redistribution effects on line polarization in the presence of velocity fields
    Solar Physics, 1996
    Co-Authors: K N Nagendra
    Abstract:

    Velocity fields in line formation regions strongly affect the line polarization. The conventionally used observer's Frame method of solving the polarized transfer equation becomes expensive and inaccurate for partial redistribution problems, when large amplitude velocity fields have to be considered in the observer's Frame. An alternative method of solution is the Comoving Frame method. Partial redistribution problems are solved using Comoving Frame formalism for line polarization caused by resonance scattering.

Tomer Shenar - One of the best experts on this subject based on the ideXlab platform.

  • Coupling hydrodynamics with Comoving Frame radiative transfer: I. A unified approach for OB and WR stars
    Astronomy & Astrophysics, 2017
    Co-Authors: Andreas Sander, Wolf-rainer Hamann, Helge Todt, Rainer Hainich, Tomer Shenar
    Abstract:

    CONTEXT: For more than two decades, stellar atmosphere codes have been used to derive the stellar and wind parameters of massive stars. Although they have become a powerful tool and sufficiently reproduce the observed spectral appearance, they can hardly be used for more than measuring parameters. One major obstacle is their inconsistency between the calculated radiation field and the wind stratification due to the usage of prescribed mass-loss rates and wind-velocity fields. AIMS: We present the concepts for a new generation of hydrodynamically consistent non-local thermodynamical equilibrium (non-LTE) stellar atmosphere models that allow for detailed studies of radiation-driven stellar winds. As a first demonstration, this new kind of model is applied to a massive O star. METHODS: Based on earlier works, the PoWR code has been extended with the option to consistently solve the hydrodynamic equation together with the statistical equations and the radiative transfer in order to obtain a hydrodynamically consistent atmosphere stratification. In these models, the whole velocity field is iteratively updated together with an adjustment of the mass-loss rate. RESULTS: The concepts for obtaining hydrodynamically consistent models using a Comoving-Frame radiative transfer are outlined. To provide a useful benchmark, we present a demonstration model, which was motivated to describe the well-studied O4 supergiant Zeta Pup. The obtained stellar and wind parameters are within the current range of literature values. CONCLUSIONS: For the first time, the PoWR code has been used to obtain a hydrodynamically consistent model for a massive O star. This has been achieved by a profound revision of earlier concepts used for Wolf-Rayet stars. The velocity field is shaped by various elements contributing to the radiative acceleration, especially in the outer wind. (...)

  • coupling hydrodynamics with Comoving Frame radiative transfer i a unified approach for ob and wr stars
    Astronomy and Astrophysics, 2017
    Co-Authors: Andreas Sander, Wolf-rainer Hamann, Helge Todt, Rainer Hainich, Tomer Shenar
    Abstract:

    Context. For more than two decades, stellar atmosphere codes have been used to derive the stellar and wind parameters of massive stars. Although they have become a powerful tool and sufficiently reproduce the observed spectral appearance, they can hardly be used for more than measuring parameters. One major obstacle is their inconsistency between the calculated radiation field and the wind stratification due to the usage of prescribed mass-loss rates and wind-velocity fields. Aims. We present the concepts for a new generation of hydrodynamically consistent non-local thermodynamical equilibrium (non-LTE) stellar atmosphere models that allow for detailed studies of radiation-driven stellar winds. As a first demonstration, this new kind of model is applied to a massive O star. Methods. Based on earlier works, the PoWR code has been extended with the option to consistently solve the hydrodynamic equation together with the statistical equations and the radiative transfer in order to obtain a hydrodynamically consistent atmosphere stratification. In these models, the whole velocity field is iteratively updated together with an adjustment of the mass-loss rate. Results. The concepts for obtaining hydrodynamically consistent models using a Comoving-Frame radiative transfer are outlined. To provide a useful benchmark, we present a demonstration model, which was motivated to describe the well-studied O4 supergiant ζ Pup. The obtained stellar and wind parameters are within the current range of literature values. Conclusions. For the first time, the PoWR code has been used to obtain a hydrodynamically consistent model for a massive O star. This has been achieved by a profound revision of earlier concepts used for Wolf-Rayet stars. The velocity field is shaped by various elements contributing to the radiative acceleration, especially in the outer wind. The results further indicate that for more dense winds deviations from a standard β -law occur.

E. Baron - One of the best experts on this subject based on the ideXlab platform.

  • a 3d radiative transfer Framework x arbitrary velocity fields in the Comoving Frame
    Astronomy and Astrophysics, 2012
    Co-Authors: E. Baron, Peter H. Hauschildt, Bin Chen, S. Knop
    Abstract:

    Aims. General 3D astrophysical atmospheres will have random velocity fields. We seek to combine the methods we have developed for solving the 1D problem with arbitrary flows to those that we have developed for solving the fully 3D relativistic radiative transfer problem for monotonic flows. Methods. The methods developed for 3D atmospheres with monotonic flows, solving the fully relativistic problem along curves defined by an affine parameter, are very flexible and can be extended to the case of arbitrary velocity fields in 3D. Simultaneously, the techniques we developed for treating the 1D problem with arbitrary velocity fields are easily adapted to the 3D problem. Results. The algorithm we present can be used to solve 3D radiative transfer problems that include arbitrary wavelength couplings. We use a quasi-analytic formal solution of the radiative transfer equation that significantly improves the overall computation speed. We show that the approximate lambda operator developed in previous work gives good convergence, even neglecting wavelength coupling. Ng acceleration also gives good results. We present tests that are of similar resolution to what has been presented using Monte-Carlo techniques, thus our methods will be applicable to problems outside of our test setup. Additional domain decomposition parallelization strategies will be explored in future work.

  • Comoving Frame radiative transfer in arbitrary velocity fields ii large scale applications
    arXiv: Solar and Stellar Astrophysics, 2009
    Co-Authors: S. Knop, Peter H. Hauschildt, E. Baron
    Abstract:

    A solution of the radiative-transfer problem in arbitrary velocity fields introduced in a previous paper, has limitations in its applicability. For large-scale applications, the methods described also require large memory sets that are commonly not available to state-of-the-art computing hardware. In this work, we modify the algorithm to allow the computation of large-scale problems. We reduce the memory footprint via a domain decomposition. By introducing iterative Gauss-Seidel type solvers, we improve the speed of the overall computation. Because of the domain decomposition, the new algorithm requires the use of parallel-computing systems. The algorithm that we present permits large-scale solutions of radiative-transfer problems that include arbitrary wavelength couplings. In addition, we discover a quasi-analytic formal solution of the radiative transfer that significantly improves the overall computation speed. More importantly, this method ensures that our algorithm can be applied to multi-dimensional Lagrangian radiative-transfer calculations. In multi-dimensional atmospheres, velocity fields are in general chaotic ensuring that the inclusion of arbitrary wavelength couplings are mandatory.

  • Comoving-Frame radiative transfer in arbitrary velocity fields - II. Large scale applications
    Astronomy & Astrophysics, 2009
    Co-Authors: S. Knop, Peter H. Hauschildt, E. Baron
    Abstract:

    Aims. A solution of the radiative-transfer problem in arbitrary velocity fields introduced in a previous paper, has limitations in its applicability. For large-scale applications, the methods described also require large memory sets that are commonly not available to state-of-the-art computing hardware. In this work, we modify the algorithm to allow the computation of large-scale problems. Methods. We reduce the memory footprint via a domain decomposition. By introducing iterative Gauss-Seidel type solvers, we improve the speed of the overall computation. Because of the domain decomposition, the new algorithm requires the use of parallelcomputing systems. Results. The algorithm that we present permits large-scale solutions of radiative-transfer problems that include arbitrary wavelength couplings. In addition, we discover a quasi-analytic formal solution of the radiative transfer that significantly improves the overall computation speed. More importantly, this method ensures that our algorithm can be applied to multi-dimensional Lagrangian radiative-transfer calculations. In multi-dimensional atmospheres, velocity fields are in general chaotic ensuring that the inclusion of arbitrary wavelength couplings are mandatory.

  • Improved discretization of the wavelength derivative term in CMF operator splitting numerical radiative transfer
    Astronomy & Astrophysics, 2004
    Co-Authors: Peter H. Hauschildt, E. Baron
    Abstract:

    We describe two separate wavelength discretization schemes that can be used in the numerical solution of the Comoving Frame radiative transfer equation. We present an improved second order discretization scheme and show that it leads to significantly less numerical diffusion than the previous scheme. We also show that due to the nature of the second order term in some extreme cases it can become numerically unstable. We stabilize the scheme by introducing a mixed discretization scheme and present the results from several test calculations.

Andreas Sander - One of the best experts on this subject based on the ideXlab platform.

  • Coupling hydrodynamics with Comoving Frame radiative transfer: I. A unified approach for OB and WR stars
    Astronomy & Astrophysics, 2017
    Co-Authors: Andreas Sander, Wolf-rainer Hamann, Helge Todt, Rainer Hainich, Tomer Shenar
    Abstract:

    CONTEXT: For more than two decades, stellar atmosphere codes have been used to derive the stellar and wind parameters of massive stars. Although they have become a powerful tool and sufficiently reproduce the observed spectral appearance, they can hardly be used for more than measuring parameters. One major obstacle is their inconsistency between the calculated radiation field and the wind stratification due to the usage of prescribed mass-loss rates and wind-velocity fields. AIMS: We present the concepts for a new generation of hydrodynamically consistent non-local thermodynamical equilibrium (non-LTE) stellar atmosphere models that allow for detailed studies of radiation-driven stellar winds. As a first demonstration, this new kind of model is applied to a massive O star. METHODS: Based on earlier works, the PoWR code has been extended with the option to consistently solve the hydrodynamic equation together with the statistical equations and the radiative transfer in order to obtain a hydrodynamically consistent atmosphere stratification. In these models, the whole velocity field is iteratively updated together with an adjustment of the mass-loss rate. RESULTS: The concepts for obtaining hydrodynamically consistent models using a Comoving-Frame radiative transfer are outlined. To provide a useful benchmark, we present a demonstration model, which was motivated to describe the well-studied O4 supergiant Zeta Pup. The obtained stellar and wind parameters are within the current range of literature values. CONCLUSIONS: For the first time, the PoWR code has been used to obtain a hydrodynamically consistent model for a massive O star. This has been achieved by a profound revision of earlier concepts used for Wolf-Rayet stars. The velocity field is shaped by various elements contributing to the radiative acceleration, especially in the outer wind. (...)

  • coupling hydrodynamics with Comoving Frame radiative transfer i a unified approach for ob and wr stars
    Astronomy and Astrophysics, 2017
    Co-Authors: Andreas Sander, Wolf-rainer Hamann, Helge Todt, Rainer Hainich, Tomer Shenar
    Abstract:

    Context. For more than two decades, stellar atmosphere codes have been used to derive the stellar and wind parameters of massive stars. Although they have become a powerful tool and sufficiently reproduce the observed spectral appearance, they can hardly be used for more than measuring parameters. One major obstacle is their inconsistency between the calculated radiation field and the wind stratification due to the usage of prescribed mass-loss rates and wind-velocity fields. Aims. We present the concepts for a new generation of hydrodynamically consistent non-local thermodynamical equilibrium (non-LTE) stellar atmosphere models that allow for detailed studies of radiation-driven stellar winds. As a first demonstration, this new kind of model is applied to a massive O star. Methods. Based on earlier works, the PoWR code has been extended with the option to consistently solve the hydrodynamic equation together with the statistical equations and the radiative transfer in order to obtain a hydrodynamically consistent atmosphere stratification. In these models, the whole velocity field is iteratively updated together with an adjustment of the mass-loss rate. Results. The concepts for obtaining hydrodynamically consistent models using a Comoving-Frame radiative transfer are outlined. To provide a useful benchmark, we present a demonstration model, which was motivated to describe the well-studied O4 supergiant ζ Pup. The obtained stellar and wind parameters are within the current range of literature values. Conclusions. For the first time, the PoWR code has been used to obtain a hydrodynamically consistent model for a massive O star. This has been achieved by a profound revision of earlier concepts used for Wolf-Rayet stars. The velocity field is shaped by various elements contributing to the radiative acceleration, especially in the outer wind. The results further indicate that for more dense winds deviations from a standard β -law occur.

Adela Emilia Ringuelet - One of the best experts on this subject based on the ideXlab platform.