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Results for “Stellar magnetic fields”

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 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Solar and stellar magnetic fields and structures - Observations

A review of stellar magnetic field measurements is presented which includes both a critique of recent spectral diagnostic techniques and a summary of important trends now appearing in the data. Both the Zeeman broadening techniques using unpolarized light and techniques based on circular and linear polarization data are discussed. An agenda for the development of a self-consistent model for the magnetic atmospheres of active stars is presented.

Linsky, Jeffrey L.↗
NASA NTRS1988

Improved methods for the measurement and analysis of stellar magnetic fields

The paper presents several improved methods for the measurement of magnetic fields on cool stars which take into account simple radiative transfer effects and the exact Zeeman patterns. Using these methods, high-resolution, low-noise data can be fitted with theoretical line profiles to determine the mean magnetic field strength in stellar active regions and a model-dependent fraction of the stellar surface (filling factor) covered by these regions. Random errors in the derived field strength and filling factor are parameterized in terms of signal-to-noise ratio, wavelength, spectral resolution, stellar rotation rate, and the magnetic parameters themselves. Weak line blends, if left uncorrected, can have significant systematic effects on the derived magnetic parameters, and thus several methods are developed to compensate partially for them. The magnetic parameters determined by previous methods likely have systematic errors because of such line blends and because of line saturation effects. Other sources of systematic error are explored in detail. These sources of error currently make it difficult to determine the magnetic parameters of individual stars to better than about + or - 20 percent.

Saar, Steven H.↗
NASA NTRS1979

Stellar magnetic fields. 1: The role of a magnetic field in the peculiar M giant, HD 4174

Coronal heating by resonant absorption of Alfvenic surface waves (quiescent), and magnetic tearing instabilities (impulsive), is discussed with emphasis on three principles which may have application to late-type evolved stars. (1) If sq B/8 pi greater than sq. rho V is observed 2 in a stellar atmosphere, then the observed magnetic field must originate in an interior dynamo. (2) Low mass loss rates could imply the presence of closed magnetic flux loops within the outer atmosphere which constrain hydrodynamic flows when the magnetic body forces exceed the driving forces. (3) given that such magnetic loops effect an enhancement of the local heating rate, a positive correlation is predicted between the existence of a corona and low mass loss rates. These principles are applied to the M giant star HD 4174, which is purported to have a kilogauss magnetic field. Several of its spectroscopic peculiarities are shown to be consistent with the above principles, and further observational checks are suggested.

Stencel, R. E.↗
NASA NTRS1979

Stellar magnetic fields - The role of a magnetic field in the peculiar M giant, HD 4174

Principles of coronal heating via basic electrodynamic effects, viz., resonant absorption of Alfven surface waves (quiescent) and magnetic tearing instabilities (impulsive), are detailed to argue three principles which may have application to late-type evolved stars. First, if one observes that B-squared/8 pi is much greater than rho times v-squared in a stellar atmosphere, then the observed magnetic field must originate in an interior dynamo. Second, low mass-loss rates could imply the presence of closed magnetic flux loops within the outer atmosphere, which constrain hydrodynamic flows when the magnetic body forces exceed the driving forces. Third, given that such magnetic loops effect an enhancement of the local heating rate, a positive correlation is predicted between the existence of a corona and low mass-loss rates. Application of these principles is made in the case of the peculiar M giant star HD 4174, which is purported to have a kilogauss magnetic field. Several of its spectroscopic peculiarities are shown to be consistent with the above principles, and further observational checks are suggested. Possible application to dMe and RS CVn objects is sketched.

Stencel, R. E.↗
NASA NTRS1989

Solar and stellar magnetic fields and atmospheric structures - Theory

Theoretical understanding of the role of magnetic fields in the formation of solar atmospheric structure is reviewed. The origin of the solar magnetic field, the dynamical behavior of the azimuthal field in the convective zone, the fibril state of the field in the photosphere, the formation of sunspots and prominences, and the spontaneous formation of current sheets in the bipolar field above the solar surface are addressed. The relation of the latter to coronal heating and flare formation is considered.

Parker, E. N.↗
NASA NTRS2015

Testing Dissipative Magnetosphere Model Light Curves and Spectra with Fermi Pulsars

We explore the emission properties of a dissipative pulsar magnetosphere model introduced by Kalapotharakos et al. comparing its high-energy light curves and spectra, due to curvature radiation, with data collected by the Fermi LAT. The magnetosphere structure is assumed to be near the force-free solution. The accelerating electric field, inside the light cylinder (LC), is assumed to be negligible, while outside the LC it rescales with a finite conductivity (sigma). In our approach we calculate the corresponding high-energy emission by integrating the trajectories of test particles that originate from the stellar surface, taking into account both the accelerating electric field components and the radiation reaction forces. First, we explore the parameter space assuming different value sets for the stellar magnetic field, stellar period, and conductivity. We show that the general properties of the model are in a good agreement with observed emission characteristics of young gamma-ray pulsars, including features of the phase-resolved spectra. Second, we find model parameters that fit each pulsar belonging to a group of eight bright pulsars that have a published phase-resolved spectrum. The sigma values that best describe each of the pulsars in this group show an increase with the spin-down rate (E˙ ) and a decrease with the pulsar age, expected if pair cascades are providing the magnetospheric conductivity. Finally, we explore the limits of our analysis and suggest future directions for improving such models.

acceleration of particles↗
NASA NTRS1983

The role of magnetic fields in stellar chromospheres and transition regions

Consideration is given to the different roles that magnetic fields play in the control of the structure and energy balance in the chromospheres and transition regions of late-type stars, in view of IUE and Einstein satellite observations. The rotational modulation of structures ('plages') that are bright in UV emission lines and overlie dark star spots suggests that magnetic flux tubes are the dominant structural elements in late-type stellar atmospheres, as well. It is proposed that the existence of a boundary between solar-type stars and those with little or no plasma, as well as the different types of G-K giants and supergiants, may be understood in terms of the fractional surface coverage by closed magnetic structures. Models computed for active and quiet regions of the sun are compared with those for active and quiet stars.

Linsky, J. L.↗
NASA NTRS1994

Mass-loss rates, ionization fractions, shock velocities, and magnetic fields of stellar jets

In this paper we calculate emission-line ratios from a series of planar radiative shock models that cover a wide range of shock velocities, preshock densities, and magnetic fields. The models cover the initial conditions relevant to stellar jets, and we show how to estimate the ionization fractions and shock velocities in jets directly from observations of the strong emission lines in these flows. The ionization fractions in the HH 34, HH 47, and HH 111 jets are approximately 2%, considerably smaller than previous estimates, and the shock velocities are approximately 30 km/s. For each jet the ionization fractions were found from five different line ratios, and the estimates agree to within a factor of approximately 2. The scatter in the estimates of the shock velocities is also small (+/- 4 km/s). The low ionization fractions of stellar jets imply that the observed electron densities are much lower than the total densities, so the mass-loss rates in these flows are correspondingly higher (approximately greater than 2 x 10(exp -7) solar mass/yr). The mass-loss rates in jets are a significant fraction (1%-10%) of the disk accretion rates onto young stellar objects that drive the outflows. The momentum and energy supplied by the visible portion of a typical stellar jet are sufficient to drive a weak molecular outflow. Magnetic fields in stellar jets are difficult to measure because the line ratios from a radiative shock with a magnetic field resemble those of a lower velocity shock without a field. The observed line fluxes can in principle indicate the strength of the field if the geometry of the shocks in the jet is well known.

Hartigan, Patrick↗
NASA NTRS1988

Semi-empirical models of the wind in cool supergiant stars

A self-consistent semi-empirical model for the wind of the supergiant in zeta Aurigae type systems is proposed. The damping of the Alfven waves which are assumed to drive the wind is derived from the observed velocity profile. Solution of the ionization balance and energy equation gives the temperature structure for given stellar magnetic field and wave flux. Physically acceptable solutions of the temperature structure place limits on the stellar magnetic field. A crude formula for a critical mass loss rate is derived. For a mass loss rate below the critical value the wind cannot be cool. Comparison between the observed and the critical mass loss rate suggests that the proposed theory may provide an explanation for the coronal dividing line in the Hertzsprung-Russell diagram. The physical explanation may be that the atmosphere has a cool wind, unless it is physically impossible to have one. Stars which cannot have a cool wind release their nonthermal energy in an outer atmosphere at coronal temperatures. It is possible that in the absence of a substantial stellar wind the magnetic field has less incentive to extend radially outward, and coronal loop structures may become more dominant.

Kuin, N. P. M.↗
NASA NTRS1974

The instability of strong magnetic fields in stellar interiors

There has been discussion of the possibility of resolving the solar neutrino dilemma with a sufficiently strong magnetic field (500 MG) in the solar interior to relieve the gas pressure by some 10% or more. The time in which magnetic buoyancy will bring a strong field to the surface is examined and is found to be less than 100 m.y. No possibility is seen for retaining a suitably strong magnetic field in the solar interior.

Parker, E. N.↗
NASA NTRS1979

Accretion by rotating magnetic neutron stars. II - Radial and vertical structure of the transition zone in disk accretion

The radial and vertical structure of the transition zone at the magnetospheric boundary of an aligned rotating neutron star accreting matter from a Keplerian disk are calculated. The results obtained indicate that: (1) the inner edge of the disk is located where the integrated magnetic stress acting on the disk plasma becomes comparable to the integrated material stress associated with its inward radial drift and orbital motion; (2) the stellar magnetic field threads the disk near its inner edge via the Kelvin-Helmholtz instability, turbulent diffusion, and reconnection, producing a broad transition zone between the unperturbed disk flow and corotating magnetosphere; (3) the transition zone consists of two qualitatively different regions, viz., a broad outer transition zone where the motion is Keplerian and a narrow inner zone, or boundary layer, where the departure from Keplerian motion is substantial; (4) the stellar magnetic field is largely but not entirely screened by currents flowing in the boundary layer; and (5) there are no steady-flow solutions for sufficiently fast stellar rotation.

Ghosh, P.↗
NASA NTRS1983

Magnetic fields and activity of the sun and stars - An overview

Recent work on the observation and theory of solar and stellar magnetic field activity and its relation to stellar activity is reviewed, emphasizing those aspects relevant to the problem of activity of red dwarf stars. New observational facts relevant to understanding the root cause of stellar surface activity are summarized and theoretical questions concerning the underlying physical basis for the observed correlations between stellar activity, rotation, and magnetic fields are addressed. These include dyanamo theory and the rotation-activity connection as well as flux tube dynamics and plasma heating.

Rosner, R.↗
NASA NTRS1983

Closed and open magnetic fields in stellar winds

A numerical study of the interaction between a thermal wind and a global dipole field in the sun and in a giant star is reported. In order for closed field lines to persist near the equator (where a helmet-streamer-like configuration appears), the coronal temperature must be less than a critical value Tc, which scales as M/R. This condition is found to be equivalent to the following: for a static helmet streamer to persist, the sonic point above the helmet must not approach closer to the star than 2.2-2.6 stellar radii. Implications for rapid mass loss and X-ray emission from cool giants are pointed out. The results strengthen the case for identifying empirical dividing lines in the H-R diagram with a magnetic topology transition locus (MTTL). Support for the MTTL concept is also provided by considerations of the breakdown of magnetostatic equilibrium.

Mullan, D. J.↗
NASA NTRS2005

Accretion Flows in Magnetic White Dwarf Systems

We received Type A and B funding under the NASA Astrophysics Data Program for the analysis and interpretation of hard x-ray data obtained by the Rossi X-ray Timing Explorer and other NASA sponsored missions for Intermediate Polars (IPS) and Polars. For some targets, optical data was available. We reduced and analyzed the X-ray spectra and the X-ray and optical (obtained at the Cerro Tololo Inter-American Observatory) timing data using detailed shock models (which we constructed) to place constraints on the properties of the accreting white dwarfs, the high energy emission mechanisms of white dwarfs, and the large-scale accretion flows of Polars and IPS. IPS and Polars are white dwarf mass-transfer binaries, members of the larger class of cata,clysmic variables. They differ from the bulk of the cataclysmic variables in that they contain strongly magnetic white dwarfs; the white dwarfs in Polars have B, = 7 to 230 MG and those in IPS have B, less than 10 MG. The IPS and Polars are both examples of funneled accretion flows in strong magnetic field systems. The IPS are similar to x-ray pulsars in that accretion disks form in the systems which are disrupted by the strong stellar magnetic fields of the white dwarfs near the stellar surface from where the plasma is funneled to the surface of the white dwarf. The localized hot spots formed at the footpoints of the funnels coupled with the rotation of the white dwarf leads to coherent pulsed x-ray emission. The Polars offer an example of a different accretion topology; the magnetic field of the white dwarf controls the accretion flow from near the inner Lagrangian point of the system directly to the stellar surface. Accretion disks do not form. The strong magnetic coupling generally leads to synchronous orbital/rotational motion in the Polars. The physical system in this sense resembles the Io/Jupiter system. In both IPS and Polars, pulsed emission from the infrared to x-rays is produced as the funneled flows merge onto the white dwarfs through the formation of strong radiating shock waves. A comparative study of the IPS and Polars can elucidate the primary effects of the magnetic fields on the dynamics and thermodynamics in accreting white dwarf systems.

Imamura, James N.↗
NASA NTRS1975

The escape of magnetic flux from a turbulent body of gas

Topological conditions on the surface of a turbulent body of highly conductive fluid under which escape of magnetic force lines would be impossible (Drobyshevski, Yuferev 1974) are studied with exact calculation of intermediate field configurations as a possible explanation of the decay of stellar magnetic fields via turbulent mixing. It is shown that upward escape of magnetic force lines in the regions between convective cells is possible through rapid reconnection of the force lines (studied as neutral point annihilation) such as occurs continually in rapidly evolving sunspot groups. It is concluded that there exists no topological barrier to the escape of magnetic flux from the sun or from the local galaxy.

Parker, E. N.↗
NASA NTRS1981

Evidence for chromospheres and coronae in stars - Recent observations, some unanswered theoretical questions, and a speculative scenario

The paper discusses recent UV and X-ray evidence for the existence of solar-type chromospheres, transition regions and coronae in stars other than the sun. The stellar spectral types associated with emission lines indicative of chromospheres and transition regions and X-ray fluxes indicative of coronae are examined, and the possibility of chromospheres and transition regions existing in stars hotter or cooler than the late-type stars is considered. The stellar parameters determining the properties of stellar chromospheres, transition regions and coronae are discussed, including varying magnetic fields, stellar rotation and convection zone parameters. A speculative scenario for the source of chromospheres, transition regions and coronae in O, B and A stars, late-type dwarfs, and G-M giants and supergiants is then presented which is based on the predominant role of magnetic fields in the nonradiative heating of outer atmospheres.

Linsky, J. L.↗
NASA NTRS1991

The time evolution of magnetic fields on solar-like stars

Magnetic fields play a crucial role in governing the rotational evolution of solarlike stars. Techniques for measuring stellar magnetic fields are reviewed, and the most recent determinations of magnetic-field strengths and surface-area coverages on cool stars are analyzed for correlations with age. Tentative evidence for a decrease in magnetic flux with age is found, primarily caused by a reduction in the magnetic area filling factor with time.

Saar, Steven H.↗