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Articles 601 - 613 of 613

Full-Text Articles in Physical Sciences and Mathematics

Scattering Polarization Due To Light Source Anisotropy I. Large Spherical Envelope., M. B. Al-Malki, J. F. L. Simmons, R. Ignace, J. C. Brown, D. Clarke Jun 1999

Scattering Polarization Due To Light Source Anisotropy I. Large Spherical Envelope., M. B. Al-Malki, J. F. L. Simmons, R. Ignace, J. C. Brown, D. Clarke

Richard Ignace

Expressions are developed for the flux and polarization of radiation scattered by a spherically symmetric envelope for a central point stellar light source that radiates anisotropically. These are obtained in terms of the components of the spherical harmonics of the flux anisotropy from the source. Such anisotropy can arise from stellar spots, or from distortion of the star through rotation, pulsation, or magnetic effects. Explicit expressions for the Stokes parameters are obtained in the case of an ellipsoidal star of uniform surface brightness. It is thus shown that even when the scattering envelope is spherical, observationally significant polarization can arise …

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Microlensing Of Circumstellar Envelopes I. Simplified Considerations For Diagnosing Radial And Azimuthal Flow., R. Ignace, M. A. Hendry Dec 1998

Microlensing Of Circumstellar Envelopes I. Simplified Considerations For Diagnosing Radial And Azimuthal Flow., R. Ignace, M. A. Hendry

Richard Ignace

This paper presents first results on the line profile shapes from a circumstellar envelope in bulk motion as modified by a microlensing event. Only geometrically and optically thin spherical shells in uniform expansion or rotation are considered here so as to emphasise the information content available in the profile shapes. In particular it is demonstrated that for the case of expansion, the line emission can increase by significant factors and the time variation of the profile shape is symmetric about line centre. For uniform rotation the line emission also increases significantly, but the time evolution of the profile shape is …

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"Wcfields": A Magnetic Rotating Stellar Wind Model From Wind Compression Theory., R. Ignace, J. P. Cassinelli, J. E. Bjorkman Sep 1998

"Wcfields": A Magnetic Rotating Stellar Wind Model From Wind Compression Theory., R. Ignace, J. P. Cassinelli, J. E. Bjorkman

Richard Ignace

A stellar wind model for a magnetic rotating star is presented. We use the semianalytic wind compression model that predicts the two-dimensional geometry of outflows from rotating stars and consider the addition of a magnetic field. In the limit of weak magnetic fields, in such a way that the fields are unimportant in accelerating the flow, the wind compression model can be used to predict the magnetic field distribution throughout the wind, which is shown to follow the mass flux distribution. A compression of field lines near the equator results as the flow of material from higher latitudes brings magnetic …

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Emission Line Profile Shapes From Anisotropic Scattering In Planar Equatorial Disks., R. Ignace Aug 1998

Emission Line Profile Shapes From Anisotropic Scattering In Planar Equatorial Disks., R. Ignace

Richard Ignace

The consequences of anisotropic resonance line scattering for the emission profiles of equatorial disks are considered. In particular the opportunity to infer the disk velocity field owing to the anisotropic scattering is discussed. Analytic expressions for the profile shapes are derived for the cases of constant expansion and rotation, and numerical results are given for more realistic disk velocity fields of linear expansion and Keplerian rotation. The essential result is that the anisotropic line scattering produces a different profile signature in expanding disks as compared to rotating disks, owing to the difference in the isovelocity pattern of the two cases …

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Inference Of Steady Stellar Wind V(R) Laws From Optically Thin Emission Lines Iii. Inversion Of Total Line Intensity Distributions., R. Ignace, J. C. Brown, J. E. Milne, J. P. Cassinelli Aug 1998

Inference Of Steady Stellar Wind V(R) Laws From Optically Thin Emission Lines Iii. Inversion Of Total Line Intensity Distributions., R. Ignace, J. C. Brown, J. E. Milne, J. P. Cassinelli

Richard Ignace

The variation with wavelength for a sequence of total intensities of stellar wind lines is considered as a basis for deriving the wind velocity law v(r). In particular, we focus on the case where the continuum formation in the wind is dominated by the free-free opacity so that the inner radius increases with wavelength, as is realized in some massive winds like those of the Wolf-Rayet stars. The line emission in the wind occurs exterior to the continuum photosphere, hence lines observed at different wavelengths probe different regions of the wind acceleration. A major consequence of these physical conditions is …

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A Re-Evaluation Of Profile Shapes From Resonance Line Scattering In Spherical Stellar Winds., R. Ignace Mar 1998

A Re-Evaluation Of Profile Shapes From Resonance Line Scattering In Spherical Stellar Winds., R. Ignace

Richard Ignace

It is common to treat the scattering of light by resonance lines as isotropic, but in fact it has been known for some time that general resonance line scattering is partially isotropic and partially dipolar, the relative strength of the two components depending on the specific transition. As a result, the profile shapes of lines that scatter with strong dipole distributions could in principle differ markedly from those that scatter isotropically. This paper explores the consequences of general resonance line scattering in spherically symmetric stellar envelopes. As a simplified example, a resonance line profile arising in a constant expansion wind …

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Inference Of Steady Stellar Wind V(R) Laws From Optically Thin Emission Lines Ii. Occultation Effects And The Determination Of Intrinsic Stellar Properties., R. Ignace, J. C. Brown, L. L. Richardson, J. P. Cassinelli Jan 1998

Inference Of Steady Stellar Wind V(R) Laws From Optically Thin Emission Lines Ii. Occultation Effects And The Determination Of Intrinsic Stellar Properties., R. Ignace, J. C. Brown, L. L. Richardson, J. P. Cassinelli

Richard Ignace

This paper extends previous work on the inversion of line profiles to obtain wind velocity laws to a case that includes the occultation of light from the far side of the star. The velocity law v(r) is assumed to be from a wind that is steady and spherically symmetric. The wind is also assumed to be optically thin in the emission line profile. The major result here is the derivation of an analytic inversion formula. The effects of stellar occultation are shown to produce a significant change in the analysis from paper I, and by accounting for the occultation, the …

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The Hanle Effect As A Diagnostic Of Magnetic Fields In Stellar Envelopes I. Theoretical Results For Integrated Line Profiles., R. Ignace, K. H. Nordsieck, J. P. Cassinelli Aug 1997

The Hanle Effect As A Diagnostic Of Magnetic Fields In Stellar Envelopes I. Theoretical Results For Integrated Line Profiles., R. Ignace, K. H. Nordsieck, J. P. Cassinelli

Richard Ignace

The Hanle effect concerns the modification of polarized resonance-line scattering by magnetic fields; thus, it can be used as a diagnostic of stellar magnetic fields. The Hanle effect has been used to determine the field strength and distribution of magnetic structures present in prominences of the Sun. To investigate its potential use in stellar astronomy, the simplified case of an optically thin axisymmetric ring illuminated by a stellar point source is considered. The results are then used to derive the polarization from polar plumes, equatorial disks, and spherical shells. The integrated line polarization is calculated for axisymmetric rings with a …

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Inference Of Steady Stellar Wind V(R) Laws From Optically Thin Emission Lines I. Idealised Analysis For The Profile Of A Single Line., J. C. Brown, L. L. Richardson, J. P. Cassinelli, R. Ignace Aug 1997

Inference Of Steady Stellar Wind V(R) Laws From Optically Thin Emission Lines I. Idealised Analysis For The Profile Of A Single Line., J. C. Brown, L. L. Richardson, J. P. Cassinelli, R. Ignace

Richard Ignace

Abstract The form of the profile Flambda_0__({DELTA}λ) of an emission line from a steady spherical wind of velocity profile v(r) is derived for the case when optical depths are small, when stellar occultation of the wind is neglected, and when v(r) is highly supersonic. It is shown how the resulting integral equation for v(r), given Flambda_0__ ({DELTA}λ), can be inverted to yield v(r) if the line emissivity function j(r) is known. Solutions are demonstrated for simulated data in the case of a recombination line (j{prop.to}n^2^) for various trial forms of v(r). The solution is unique provided dv/dr does not change …

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Equatorial Wind Compression Effects Across The H-R Diagram., R. Ignace, J. P. Cassinelli Mar 1996

Equatorial Wind Compression Effects Across The H-R Diagram., R. Ignace, J. P. Cassinelli

Richard Ignace

We investigate the degree to which moderate stellar rotation rates can influence the two-dimensional density structure in the winds of four classes of stars: WoIf-Rayet, B[e], asymptotic giant branch (AGB), and novae. These classes are distributed across the H-R diagram and have a wide range of escape speeds and wind acceleration. Furthermore, all have members which possess asymmetric circumstellar nebulae. It has been suggested that these asymmetries could result from stellar winds which have moderate equatorial density enhancements. Large enhancements may arise as the result of stellar rotation as demonstrated by the wind-compressed disk (WCD) model of Bjorkman & Cassinelli. …

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Evidence For A Disk In The Wind Of Hd 93521: Uv Line Profiles From An Axisymmetric Model., Richard Ignace Oct 1994

Evidence For A Disk In The Wind Of Hd 93521: Uv Line Profiles From An Axisymmetric Model., Richard Ignace

Richard Ignace

Recently it has been suggested (Massa 1992; Howarth & Reid 1993), from the C IV ultraviolet resonance line profile of HD 93521, that there is a high-speed component in the polar outflow from the star as well as a low-speed component in the equatorial regions. In this paper we present theoretical calculations of the line profiles that would be produced by such a model. We find from Hubble Space Telescope (HST) Goddard High Resolution Spectrograph (GHRS) observations of HD 93521 that the observed C IV profile can be produced if the star has an inclination angle very close to 90 …

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Multi-Ball Collisions, Terry L. Smith, Jay S. Huebner Dec 1991

Multi-Ball Collisions, Terry L. Smith, Jay S. Huebner

Jay S Huebner

Explanation to use the "double-ball" demonstration as a model for the supernova core bounce.

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An Assessment Of Potential Hazards To The Cassini Spacecraft From Debris Along Satellite Orbits, Robert A. Kolvoord Dec 1988

An Assessment Of Potential Hazards To The Cassini Spacecraft From Debris Along Satellite Orbits, Robert A. Kolvoord

Robert A Kolvoord

No abstract provided.

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