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Results for “Stellar Winds”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 145 records · Page 8
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NASA NTRS1980

Synthetic line profiles in early-type stellar winds. I - H and He/+/

Statistical equilibrium calculations for the first 10 levels of H and He(+) were carried out for several model atmospheres of a star with an effective temperature of 40,000 deg K and log g equal to 3.5. Atomic level populations and line profiles were computed for a wind model (Klein and Castor, 1978) with a mass-loss rate of about 1.2 x 10 to the -5th solar mass per yr. A comoving frame method was applied and the radiative transfer problem in the expanding atmosphere is treated. Two variants of the Klein-Castor model produce He II at 4686 A in emission with 3203 A in absorption, as is commonly observed in Of spectra.

Kunasz, P. B.↗
NASA NTRS1981

Search for colliding stellar winds in Plaskett's star (HD 47129)

High dispersion spectra of Plaskett's star (HD 47129) were obtained with the short wavelength spectrograph on IUE at five phases of the binary cycle. The unsaturated wind profiles, particularly those of Si IV lambda 1400, show complex phase dependent structure. Two interpretations for the structure are suggested, neither of which is entirely satisfactory: (1) the structure is a consequence of directed streams, and (2) the structure is a consequence of colliding winds from the primary and secondary.

Heap, S. R.↗
NASA NTRS1980

Stellar winds, fast rotators, and magnetic acceleration

The assumption that observed mass outflow from a star is due to a magnetically driven wind implies an upper bound on the surface magnetic field strength from regions where the wind originates. Evidence is reported that corroborates Rosendhal's observation of an abrupt change in the velocity-gradient-luminosity relationship for B8 and later supergiants. The smallest upper bounds correspond to later spectral types, for which radiation would be a relatively inefficient mechanism for driving wind; these winds may in fact be magnetically driven. Be stars are prime candidates for magnetically accelerated winds due to large rotation rates and small mass loss rates. Observed flows can be driven by 0.1-10 gauss surface fields. Intense field in fast rotators can lead to dramatic mass loss rates and large terminal velocities; sigma Ori E may be such an object. Finally, the effect of magnetic acceleration on the dispersal of solar nebula is considered. The increased velocities allow the wind to disperse a mass of 90 times the initial outflowing mass in the wind.

Nerney, S.↗
NASA NTRS1983

Multiple transonic solutions with a new class of shock transitions in steady isothermal solar and stellar winds

It is shown that a new class of shock transitions arises in the transonic solutions of the steady isothermal solar wind equations when momentum deposition and/or nonradial flow tube divergence give rise to multiple critical points in the flow. These shock transitions between critical solutions occur for a certain range of the parameters which characterize the momentum deposition function. The isothermal wind equations allow multiple transonic solutions in the presence of such shock transitions, yielding a continuous solution passing through an inner critical point and solutions involving a shock transition between critical solutions. It is determined that these multiple transonic solutions have the same flow speed at the base but different supersonic flow speeds at infinity. It is found that the nonradial flow tube divergence and momentum addition are equivalent, which gives rise to multiple critical points and hence to multiple transonic solutions with shock transitions. In addition, the physical relevance of these properties are examined for astrophysical systems such as the inner solar wind, flows in extragalactic jets, and accretion discs.

Habbal, S. R.↗
NASA NTRS1983

The discovery of a hot stellar wind

High-resolution 6 cm observations of two early-type stars have been made with the Very Large Array. Cyg OB2 No. 12 appears to have a normal wind with a temperature of about 5000 K; Cyg OB2 No. 9 has unusual and unexpected radio properties which imply that its radio-emitting circumstellar gas is hotter than 300,000 K.

White, R. L.↗
NASA NTRS1983

Multiple transonic solutions and a new class of shock transitions in solar and stellar winds

The steady isothermal solar wind equations are shown to admit, under certain circumstances, mutliple transonic solutions when, for example, momentum deposition gives rise to multiplee critical points in the flow. These multiple solutions consist of a continuous solution and solutions which involve shock transitions between critical solutions. The ambiguity arising from the multiplicity of the solutions can be resolved by following the time evolution of a wind profile with one critical point. Results of the numerical integration of the time-dependent equations with momentum addition show that each of these multiple solutions is physically accessible and depends on the rate of change of momentum deposition. These results suggest that standing shocks are likely to be present in the inner solar wind flow.

Habbal, S. R.↗
NASA NTRS1985

Synchrotron Emission from Chaotic Stellar Winds

A model is presented for the radio emission from hot stars. Electrons are accelerated to relativistic energies by shocks in the wind near the star and emit radio radiation through the synchrotron mechanism. The particle spectrum and radio spectrum for this model are derived. The model accounts for many of the observed characteristics of some recently discovered stars which have peculiar radio emission.

White, R. L.↗
NASA NTRS1985

On the Role Played by Lines in Radiatively Driven Stellar Winds Depending on the Position of the Stars in the HR Diagram

The radiative force due to transfer in ultraviolet lines is always an important mechanism in hot star wind dynamics. However, it is not clear when it is the dominant mechanism and which are the noise parameters. To investigate the efficiency of purely radiative momentum/energy transfer in hot star winds and in various regions of the HR diagram, the Leroy and Lafon model was improved and put to its limits; correlations between the mass loss rate, the luminosity and other parameters and the theoretical and the observational results, looking for observed stars violating the model were compared. It is concluded that in widespread region of the HR diagram, line driven models are consistent with observations, the radiative equilibrium physics is relevant throughout the expanding atmospheres and the mass loss rate is quasilinearly correlated with the luminosity.

Migozzi, M. C.↗
NASA NTRS1986

Doppler tomography of the stellar wind of the Wolf-Rayet binary star V444 Cygni

High dispersion IUE observations of V444 Cyg around its orbit, emphasizing the C IV, N V, N IV, and He II profile variations are presented. The density and velocity structure of the wind, and details of the interaction between the O star and WR wind are discussed. Absorption becomes strongest and most nearly symmetric during primary eclipse, when the line of sight passes through the densest and most sluggish part of the wind. As the orbit progresses through quadrature, the terminal velocity does not change substantially, although the line of sight to the O star intersects more distant portions of the wind at lower velocity, indicating that there is still a considerable absorption in the line being seen toward the WR core. At secondary minimum, the core is partially occulted and the presence of the cavity eliminates the absorption component, while at third quadrature, a profile almost identical in appearance to that at first quadrature is observed.

Brown, Douglas N.↗
NASA NTRS1986

IUE observations of magnetically controlled stellar winds in the helium peculiar stars

Dramatic periodic variations in the C IV resonance lines of magnetic helium-weak sn stars HD 5737 = alpha Scl, HD 21699 = HR 1063, and HD 79158 = 36 Lyn are discussed. In all three cases, the 1548,50 doublet is the only non-negligibly variable UV spectral feature. The line profiles are consistent with outflow in a jet-like structure. In HD 21699 this outflow arises from one of the magnetic polar regions. Observations of two additional He-wk sn stars do not reveal strong C IV absorption, implying that the UV characteristics of these stars are less uniform than the optical phenomenology.

Shore, Steven N.↗
NASA NTRS1987

Stellar winds and magnetic fields in the helium peculiar stars

This note discusses the consequences of the interaction of radiatively driven mass loss with magnetic fields in upper-main-sequence helium-peculiar stars. The field constrains mass loss to occur in narrow cones above the magnetic poles. These jets can explain the observed behavior of UV resonance lines in the helium-peculiar stars, especially the sn and helium-rich stars. Suppression of mass outflow at the magnetic equator produces a region in which radiatively driven diffusive mass transport can levitate helium in the hottest stars, producing helium enrichment of the atmosphere. In the cooler stars, the surface becomes progressively more helium poor.

Shore, Steven N.↗
NASA NTRS1988

Colliding stellar winds in the eclipsing Wolf-Rayet binary V444 Cygni

High resolution spectra of V444 Cygni have been obtained using the International Ultraviolet Explorer Satellite. These spectra span both eclipses and include one observation at third quadrature. Together with seven archival spectra, they provide reasonably complete phase coverage for the system. The variations in the P Cygni profiles of the He(II) and N(IV) lines, imply the existence of a low density region in the WR wind. This region occupies a relatively narrow range of orbital phase coinciding with the highest terminal velocities observed in C IV. These data are interpreted to be evidence of an interaction region separating the winds of the O-star and Wolf-Rayet star.

Brown, Douglas N.↗
NASA NTRS1988

Stellar wind variations in HD 45166: The continuing story

High resolution SWP IUE spectra of HD 45166 (qWR+B8V) obtained over a 36 hr continuous run, together with earlier observations, reveal 2 distinct modes of UV variability in this object. Gross, epoch-linked changes are seen in the strengths of the qWR emission lines, accompanied by large changes in its highly ionized photospheric absorption spectrum. Rapid (hours) variability in strong, multiple, high velocity, wind discrete absorption components (DAC), in the CIV lambda 1550 resonance lines, which superpose to give the appearance of a broad P Cygni absorption profile at many epochs is also observed. These multiple DAC's (often at least 3 are seen) propagate in velocity, from 0.6 to 1.0 v inf, on a timescale of 1 day, implying an acceleration of 180 cm/s comparable to that seen in O-type stars.

Willis, Allan J.↗
NASA NTRS1988

Colliding stellar winds in the eclipsing Wolf-Rayet binary V444 Cygni

High resolution spectra of V444 Cygni have been obtained using the International Ultraviolet Explorer Satellite. These spectra span both eclipses and include one observation at third quadrature. Together with seven archival spectra, they provide reasonably complete phase coverage for the system. The variations in the P Cygni profiles of the He(II) and N(IV) lines, imply the existence of a low density region in the WR wind. This region occupies a relatively narrow range of orbital phase coinciding with the highest terminal velocities observed in C IV. These data are interpreted to be evidence of an interaction region separating the winds of the O-star and Wolf-Rayet star.

Shore, Steven N.↗
NASA NTRS1990

Cometary compact H II regions are stellar-wind bow shocks

Comet-shaped H II regions, like G34.3 + 0.2, are easily explained as bow shocks created by wind-blowing massive stars moving supersonically through molecular clouds. The required velocities of the stars through dense clumps are less than about 10 km/s, comparable to the velocity dispersion of stars in OB associations. An analytic model of bow shocks matches the gross characteristics seen in the radio continuum and the velocity structure inferred from hydrogen recombination and molecular line observations. The champagne flow model cannot account for these structures. VLBI observations of masers associated with the shells of cometary compact H II regions should reveal tailward proper motions predominantly parallel to the shell, rather than perpendicular. It is predicted that over a decade baseline, high signal-to-noise VLA observations of this class of objects will show headward pattern motion in the direction of the symmetry axis, but not expansion. Finally, shock-generated and coronal infrared lines are also predicted.

Van Buren, Dave↗
NASA NTRS1990

Hydrodynamic simulations of stellar wind disruption by a compact X-ray source

This paper presents two-dimensional numerical simulations of the gas flow in the orbital plane of a massive X-ray binary system, in which the mass accretion is fueled by a radiation-driven wind from an early-type companion star. These simulations are used to examine the role of the compact object (either a neutron star or a black hole) in disturbing the radiatively accelerating wind of the OB companion, with an emphasis on understanding the origin of the observed soft X-ray photoelectric absorption seen at late orbital phases in these systems. On the basis of these simulations, it is suggested that the phase-dependent photoelectric absorption seen in several of these systems can be explained by dense filaments of compressend gas formed in the nonsteady accreation bow shock and wake of the compact object.

Blondin, John M.↗