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At least 289 records · Page 16

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NASA NTRS1984

The appearance of magnetic flux on the surfaces of the early main-sequence F stars

Available chromospheric, transition region, and coronal observations of the early main-sequence F stars are examined in order to find that while these objects exhibit enhanced levels of magnetic field-related radiative emissions, significant inhomogeneities in surface activity are not present. This phenomenon is discussed within the context of the calculations published by Schmitt and Rosner (1983) for the production of flux ropes of various spatial scales at a given rotation rate at the bottom of a stellar convection zone. It is found that the spatial scales and area contrast of emergent magnetic flux in these stars that, as a class, are characterized by rapid rotation and thin convection zones are substantially reduced relative to that of the sun.

Giampapa, M. S.↗

NASA NTRS1973

Mechanical heating in stellar chromospheres using the sun as a test case

The shock dissipation hypothesis for solar atmospheric heating is extended to stellar chromospheres of late type stars which have significant convective envelopes. Principle wave modes are studied for their generation, propagation, and heating properties in the presence of a magnetic field, and various approximations on when a sound wave becomes strong enough to produce heating in the atmosphere are presented.

Jordan, S. D.↗

NASA NTRS1986

A flare-induced cascade model of gamma-ray bursts

An analytical model is developed for the source of gamma ray bursts as a stellar flare in the magnetosphere of a neutron star. It is suggested that the loss of energy through synchrotron radiation experienced by electrons moving through a sufficiently strong magnetic field at a large pitch angle may not be regained. Instead, pulsar theory is applied to show that the acceleration of electrons in an electric field parallel to the magnetic field will rapidly be inhibited by curvature radiation as the loop experiences a reconnection. It is shown that electrons passing through a curvature with a radius of one million with an electric field strength of 10 billion e.s.u. will emit photons with energies of up to 10 to the 12.6 eV by curvature radiation. The photons, gamma rays, would annihilate in the magnetosphere, which they cannot escape. The resulting cascade of electron-positron particles would eventually produce photons of sufficiently low energy to escape. Upper and low bounds are estimated for the resulting emission spectrum, which would vary according to the magnetic field geometry. The model explains the observed 511 keV annihilation line and the optical radiation which at times accompanies gamma-ray bursts.

Sturrock, P. A.↗

NASA NTRS1978

Comments on the 'minimum flux corona' concept

Hearn's (1975) models of the energy balance and mass loss of stellar coronae, based on a 'minimum flux corona' concept, are critically examined. First, it is shown that the neglect of the relevant length scales for coronal temperature variation leads to an inconsistent computation of the total energy flux F. The stability arguments upon which the minimum flux concept is based are shown to be fallacious. Errors in the computation of the stellar wind contribution to the energy budget are identified. Finally we criticize Hearn's (1977) suggestion that the model, with a value of the thermal conductivity modified by the magnetic field, can explain the difference between solar coronal holes and quiet coronal regions.

Antiochos, S. K.↗

NASA NTRS1988

Stellar radio emission

This paper presents an overview of the various radiation mechanisms believed to play a role in stellar radio emission. The radio emission from most stars is nonthermal and is generally due to mildly relativistic electrons with energies from a few keV to over 10 MeV. Magnetic fields play a crucial role both in accelerating the electrons to the requisite energies and in mediating the emission mechanism. They also play a fundamental role in creating the velocity anisotropies that are necessary for the operation of some of the coherent emission mechanisms. Coherent emission is seen most commonly on the M dwarfs, rarely on the RS CVns, and has yet to be detected for any other class of star. These coherent processes are best studied by means of their dynamic spectra; such studies are now just getting underway.

Bookbinder, Jay A.↗

NASA NTRSDate not supplied

Atmospheric Escape Processes and Planetary Atmospheric Evolution: From Misconceptions to Challenges

The recent discoveries of telluric exoplanets in the habitable zone of different stars have led to questioning the nature of their atmosphere, which is required to determine their habitability. Atmospheric escape is one of the challenging problems to be solved: simply adapting what is currently observed in the solar system is doomed to fail due to the large variations in the conditions encountered around other stars. A better strategy is to evaluate the different processes that shaped planetary atmosphere and to evaluate their importance depending upon the stellar conditions. We reviewed the different escape mechanisms and their magnitude in function of different conditions [Gronoff et al. 2020]. This led us to discuss the importance of a magnetic field in protecting an atmosphere. The importance of the thermal escape, of polar wind, and of the transport of plasma within the magnetosphere are typically forgotten when claiming that magnetic fields are protecting planetary atmospheres and leading to their habitability. Overall, the habitability of a planet should not be claimed only on by its location in the habitable zone but also after careful analysis of the interaction between its atmosphere and its parent star . Gronoff, G., Arras, P., Baraka, S., Bell, J. M., Cessateur, G., Cohen, O., et al. ( 2020). Atmospheric Escape Processes and Planetary Atmospheric Evolution. Journal of Geophysical Research: Space Physics, 125, e2019JA027639. https://doi.org/10.1029/2019JA027639

G Gronoff↗

NASA NTRS1982

Wave-driven winds from cool stars. I - Some effects of magnetic field geometry

The wave-driven wind theory of Hartmann and MacGregor (1980) is extended to include effects due to non-radial divergence of the flow. Specifically, isothermal expansion within a flow tube whose cross-sectional area increases outward faster than the square of the radius near the stellar surface is considered. It is found that the qualitative conclusions of Hartmann and MacGregor concerning the physical properties of Alfven wave-driven winds are largely unaffected. In particular, mass fluxes of similar magnitude are obtained, and wave dissipation is still necessary to produce acceptably small terminal velocities. Increasingly divergent flow geometries generally lead to higher initial wind speeds and slightly lower terminal velocities. For some cases of extremely rapid flow tube divergence, steady supersonic wind solutions which extend to infinity with vanishing gas pressure cannot be obtained. In addition, departures from spherical symmetry can cause the relative Alfven wave amplitude delta-B/B to become approximately greater than 1 within several stellar radii of the base of the wind, suggesting that nonlinear processes may contribute to the wave dissipation required by the theory.

Hartmann, L.↗

NASA NTRS1990

Gamma-ray bursts from magnetospheric plasma oscillations. II - Model spectra

Several mechanisms for the primary release of energy in gamma-ray bursts (GRBs) may result in the excitation of relativistic, magnetospheric plasma oscillations above the polar cap of a neutron star. This paper presents a survey of detailed calculations of the inverse Compton scattering interaction between the sinusoidally accelerated particles in relativistic, magnetospheric plasma oscillations and the self-consistently determined thermal radiation from the stellar surface. The upscattered photons are boosted to gamma-ray energies and a Monte Carlo simulation is used to obtain the spectrum for different viewing angles relative to the magnetic field in the oscillating region. It is shown that several GRB spectral characteristics may be understood in the context of a model wherein the overall spectrum changes with aspect angle as a result of the superposition of four components with different angular distributions.

Melia, Fulvio↗

NASA NTRS1977

Stellar rotation and the thermomagnetic torque

It is noted that any reasonable assumption for the rotation rates of the interstellar clouds which must collapse to produce stars will result in stars which rotate much faster than suggested by observations if angular momentum is conserved throughout the collapse. The contribution of thermomagnetic torque, which can exist when a rarefied gas of polyatomic molecules is subjected to a magnetic field and a nonuniform temperature gradient, to the angular momentum of a collapsing protostellar object is examined in order to determine whether the magnitude of this torque is sufficient to slow down contracting protostellar clouds. Calculations are performed for two times during protostellar collapse when conditions may be favorable for thermomagnetic torque to play a role in angular-momentum considerations. The results indicate that this torque is apparently of no importance in determining or modifying stellar rotation during the early stages of stellar evolution.

Spear, G. G.↗

NASA NTRS1991

Basic research in solar physics

This grant, dating back more than 20 years has supported a variety of investigations of the chromospheres and coronae of the Sun and related cool stars by the Principal Investigator, his postdocs and graduate students, and colleagues at other institutions. This work involved studies of radiative transfer and spectral line formation theory, and the application of these techniques to the analysis of spectra obtained from space and ground-based observatories in the optical, ultraviolet, x-ray and radio portions of the spectrum. Space observations have included the analysis of spectra from OSO-7, Skylab, SMM, and the HRTS rocket experiments. Recent work has concentrated on the interaction of magnetic fields, plasma and radiation in the outer atmospheres of the Sun and other magnetically active stars with different fundamental parameters. Our study of phenomena common to the Sun and stars, the 'solar-stellar connection', can elucidate the fundamental physics, because spatially-resolved observations of the Sun provide us with the 'groundtruth,' while interpretation of stellar data permit us to isolate those parameters critical to stellar activity. Recently, we have studied the differences in physical properties between solar regions of high magnetic flux density and the surrounding plasma. High-resolution CN and CO spectroheliograms have been used to model the thermal inhomogeneities driven by unstable CO cooling, and we have analyzed spatially resolved UV spectra from HRTS to model the thermal structure and energy balance of small-scale structures. The study of nonlinear relations between atmospheric radiative losses and the photospheric magnetic flux density has been continued. We have also proposed a new model for the decay of plages by random walk diffusion of magnetic flux. Our analysis of phenomena common to the Sun and stars included the application of available spectroscopic diagnostics, establishing evidence that the atmospheres of the least active stars are heated at a 'basal' rate that is also found in the centers of solar supergranules, and using the Doppler-imaging technique to measure the position, size, and brightness of stellar active regions. We are computing multi-component models for solar and stellar atmospheres, and models for coronal loops and for the transition-region down flows. The study of solar and stellar flares permits us to assess the role of turbulent energy transport, to pinpoint the mechanism behind Type I radio bursts, to determine whether plasma radiation or cyclotron maser is responsible for microwave flares on M dwarfs, and to extend our knowledge of the basic physics pertinent to cyclotron-maser processes operating on the Sun.

Linsky, Jeffrey L.↗

NASA NTRS1999

Airborne Astronomy Program

Our understanding about the inter-relationship between the collapsing cloud envelope and the disk has been greatly altered. While the dominant star formation models invoke free fall collapse and r(sup -1.5) density profile, other star formation models are possible. These models invoke either different cloud starting conditions or the mediating effects of magnetic fields to alter the cloud geometry during collapse. To test these models, it is necessary to understand the envelope's physical structure. The discovery of disks, based on millimeter observations around young stellar objects, however makes a simple interpretation of the emission complicated. Depending on the wavelength, the disk or the envelope could dominate emission from a star. In addition, the discovery of planets around other stars has made understanding the disks in their own right quite important. Many star formation models predict disks should form naturally as the star is forming. In many cases, the information we derive about disk properties depends implicitly on the assumed envelope properties. How to understand the two components and their interaction with each other is a key problem of current star formation.

Butner, Harold M.↗

NASA NTRS1992

Constraints on the surface magnetic fields of hot stars with winds

The present study discusses several constraints on the surface magnetic fields of rotating stars with winds. It is shown that there are two allowed ranges for the strengths of surface radial magnetic fields, which are called the 'strong field' and 'weak field' ranges. Attention is given to rotating hot stars with winds and weaker surface magnetic fields. Constraints on rotation and magnetic fields are derived in relation to the extreme case when the equatorial rotation speed approaches the critical rotation. For the O-type main-sequence star 9 Sgr and the B supergiant star Zeta-1 Sco, the upper bound for a weak field is found to be of order 1 G, which is consistent with recent interpretations of radio observations. For a Wolf-Rayet star, with stellar parameters similar to those of CV Ser, it is found to be about 20 G, at faster rotation speeds. From the requirement of hydrostatic equilibrium in the interior, a condition is derived for a star to be an 'extreme magnetic rotator', which is similar to the Eddington limit for stars with radiative forces.

Maheswaran, Murugesapillai↗

NASA NTRS1973

On the theory of group generation of stars

The hypothesis proposed is that topology of a rotating gaseous cloud can be variable in the contraction process. Due to rotation an originally spherical cloud is transformed into a toroidal body. The contraction of a thin torus is considered with different suppositions on cooling the gas. In the determined time the torus will become gravitationally unstable. The excitation of Jeans' waves is shown to result in the disintegration of the torus into fragments. The number of the fragments and their mass distributions are calculated. The proposed hypothesis on toroidal stages in stellar evolution can remove some difficulties in the theory of structure and evolution of stars, such as absence of limitary stars, distribution of rotation velocities of early-type stars, origin of poloidal magnetic fields and decline rotators with the magnetic axis orthogonal to the axis of rotation.

Zhilyayev, B. Y.↗

NASA NTRS1979

Einstein observations of the X-ray structure of Centaurus A - Evidence for the radio-lobe energy source

The X-ray source at the center of the radio galaxy Centaurus A has been resolved into the following components with the imaging detectors on board the Einstein X-ray Observatory: (1) a point source coincident with the infrared nucleus; (2) diffuse X-ray emission coinciding with the inner radio lobes; (3) a 4-arcmin extended region of emission about the nucleus; and (4) an X-ray jet between the nucleus and the NE inner radio lobe. The 2 x 10 to the 39th ergs/s detected from the radio lobes probably arises from inverse Compton scattering of the microwave background. The average magnetic field in the SW lobe is determined to be not less than 4 microgauss. The extended region may be due to emission by a cloud of hot gas, cosmic-ray scattering, or stellar sources. The jet provides strong evidence for the continuous resupply of energy to the lobes from the nucleus.

Schreier, E. J.↗

NASA NTRS1976

Accretion onto magnetized neutron stars - Structure and interchange instability of a model magnetosphere

A self-consistent model is analyzed for the spherical infall of weakly magnetized plasma into the magnetosphere of a slowly rotating, strongly magnetized neutron star. It is shown that spherical infall is probably a good approximation for X-ray sources which accrete from a stellar wind. The location of the standoff shock which halts the hypersonic infall is estimated along with the emission from the shocked layer. The location of the equilibrium magnetopause and the structure of the magnetic field within it are calculated; it is found that the magnetic poles are true cusps and that the entry of gas due to equilibrium flow across a cusp is almost certainly dominated by the interchange instability near the magnetic equator. The energy principle is applied to derive necessary conditions for the occurrence of this instability. The results indicate that the strong magnetic-pressure gradient stabilizes the gas unless moderately strong radiative cooling takes place and that the cooled plasma enters the magnetosphere as long filaments capable of moving between field lines. The rate at which the equilibrium magnetopause can 'absorb' mass and momentum is derived, the validity of the approximations employed is discussed, and the likely evolution of the sinking filaments is outlined to show that the spatial distribution of the plasma is determined mainly by the dynamics and thermodynamics of the filaments rather than the magnetic-field structure.

Arons, J.↗

NASA NTRS1983

The magnetic field on the RS Canum Venaticorum star Lambda Andromedae

A program to detect and measure magnetic flux on the surfaces of late-type stars is discussed. A technique is adopted to deconvolve magnetically insensitive lines from similar, magnetically sensitive lines to infer the degree of Zeeman splitting in the latter lines. These measurements yield values for the magnetic field strength and filling factor (flux). To illustrate the approach, observations of the RS CVn star Lambda And are presented. At the epoch of observation, 1981 April 26, a field strength of 1290 + or - 320 gauss covering 48 + or - 7 percent of this star's surface is found. This measurement compares with an estimate of coronal magnetic flux in the cooler component of the stellar corona of 1110 gauss with a coronal volume filling factor of 75 percent, based on X-ray data for Lambda And.

Golub, L.↗

NASA NTRS1993

Observations of entrainment and time variability in the HH 47 jet

We present new Fabry-Perot images of the HH 47 jet that show the first clear evidence for entrainment in a jet from a young star. The material in the jet moves faster down the axis of the flow and slower at the edges, similar to viscous flow in a pipe. The higher excitation lines occur along the edges of the jet, as expected if entrainment accelerates and heats the ambient material. We confirm previous observations of multiple bow shocks in this system. Together, time variability and entrainment produce much of the observed shock-excited gas in this object. Our data show that the 'wiggles' along the jet are not caused by jet material tied to a spiraling magnetic field, but instead result from time variability, variable ejection angles, or inhomogeneities in the flow. The gas entrained in the HH 47 jet may be atomic; our results do not provide direct evidence that stellar jets drive molecular outflows.

Hartigan, Patrick↗

NASA NTRS1994

Observations of accretion and angular momentum regulation in young circumstellar disks and the implications for planetary formation

Accretion disks around young stars produce excess infrared continuum associated with the disk and excess optical and ultraviolet continua associated with the boundary layer or 'hot spot' as material falls from the disk onto the stellar photosphere. When we subtract the excess continuum and photospheric contributions to the total spectrum, we can obtain high-quality emission line profiles of the Balmer lines as well as permitted lines from other elements. These emission lines often exhibit redshifted absorption, indicative of infalling material. Remarkably, objects with large accretion rates tend to rotate slower than their counterparts that lack accretion disks. Hence, there must be some process, probably involving magnetic fields, that allows the star to accrete large amounts of material from the disk without increasing its rotational velocity. Young stars typically do not have optically thick inner disks that do not accrete. Hence, either planets form within accretion disks, or the timescale for planetary formation is considerably shorter than approx. 3 x 10(exp 6) yr, the duration of the classical T Tauri star phase of young stellar evolution.

Hartigan, P.↗