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Linsky, Jeffrey L.

Publications and source records attributed to Linsky, Jeffrey L..

65 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Requirements for theoretical models of outflows

Recent observational and theoretical investigations of astrophysical mass outflows are reviewed, with a focus on the basic physical principles. Specific limitations on the observational data and their interpretation are listed and discussed. Modeling problems considered include the role of the critical point in determining the mass-loss rate and terminal velocity, the physical processes controlling density at the critical point, the possible coexistence of multiple mass-loss mechanisms, time scales, instabilities and phase changes, multiphase atmospheres and winds, the definition of geometries, the role of the environment, explosive transient events, stochastic phenomena, mode-mode coupling and damping processes, departures from ionization equilibrium, and nonthermal phenomena.

Linsky, Jeffrey L.↗
NASA NTRS1988

Active late-type stars

The basic mechanisms and characteristics of stellar activity in evolved stars are discussed, summarizing and analyzing the results of ground-based and space observations obtained in different regions of the electromagnetic spectrum. The types of active stars, the parameters controlling activity, and the underlying astrophysics are reviewed; the specific applications of optical and IR photometry, optical and UV spectroscopy, X-ray photometry and spectroscopy, and radio observations are considered; and particular attention is given to results on RS CVn systems and M dwarfs. Extensive diagrams, graphs, and tables of numerical data are included.

Linsky, Jeffrey L.↗
NASA NTRS1988

Spatially resolved flares in RS CVn systems

Mg II k emission-line profiles arising solely from the flaring region during flares on AR Lac and V 711 Tau have been isolated. From several high-resolution spectra obtained during the lifetime of the flare, the size and position of the flaring regions have been determined, and the decay of the emission line width, radial velocity, and integrated line flux has been studied.

Neff, James E.↗
NASA NTRS1987

A radio-continuum survey of the coolest M and C giants

A VLA continuum survey of 22 cool M and C type giants and supergiants has been conducted in order to investigate the physical properties of the partially ionized, expanding chromospheres of the coolest luminous star. The results indicate small upper limits for the ionized-mass-loss rates and 2-6 cm spectral indices that are much steeper than the 0.6 value predicted by the standard stellar-wind model. It is suggested that hydrogen is predominantly neutral and that carbon and other metals may not be completely ionized in many of the late M giants. For several of the stars the brightness-temperature upper limits imply that the winds and chromospheres are optically thin at 6 cm, setting useful constraints on the chromospheric emission for these stars.

Drake, Stephen A.↗
NASA NTRS1987

A VLA radio-continuum survey of a sample of confirmed and marginal barium stars

Results are reported from a 6-cm VLA survey of five confirmed Ba II stars and eight mild Ba II stars, undertaken to search for evidence of gyrosynchrotron emission or thermal emission from the primary star's wind that is enhanced or photoionized by a white dwarf companion. Of these 13 stars, only Beta UMi was detected as a possible radio source at a flux level of 0.11 mJy (3sigma). The 6-cm radio luminosities (L6) of the other stars are as small as log L6 less than or equal to 14.0 and are an order of magnitude or more lower than the average levels found in RS CVn systems, but are consistent with the L6 upper limits previously found for stars of spectral type similar to the Ba II stars and normal elemental abundances. The upper limit to the radio luminosity for the possible mild Ba II star 56 Peg, when combined with its previously known X-ray luminosity, may provide useful constraints on the various models that have been proposed for this interesting object, once its orbital period is known.

Drake, Stephen A.↗
NASA NTRS1987

Chromospheres and transition regions

The properties of stellar chromospheres are examined, summarizing the results of observations obtained with the IUE satellite since its launch in 1978. Numerous sample spectra, graphs, and diagrams are presented and analyzed in detail. Consideration is given to 120-320-nm spectroscopy of main-sequence stars, giants, and supergiants; the global properties of main-sequence and post-main-sequence chromospheres; dynamic phenomena and structures (systematic flows, atmospheric inhomogeneities, and intrinsic variability); and structural and energy-balance modeling. A number of outstanding problems are listed, and the potential value of data from the Hubble Space Telescope and the Lyman FUV Spectroscopic Explorer in solving them is indicated.

Jordan, Carole↗
NASA NTRS1987

An investigation of stellar coronae with AXAF

Questions concerning the phenomenon of stellar coronae are discussed together with the physical parameters essential to model stellar coronae and the different AXAF instruments assigned for measurements of these parameters. As an illustration of what AXAF will be able to accomplish, three specific examples of studies by AXAF are described. These include coronal modeling for a bright X-ray source such as HR 1099, using spectra obtained by AXAF; time-resolved spectroscopy of AR Lacertae and YY Geminormum during their eclipses, to use the spectra for the identification and modeling of individual geometrical physical structures on these stars; and time-resolved spectroscopy during stellar flares to obtain high-resolution spectra which will make it possible to infer the changes in the flaring plasma temperatures as a function of time.

Linsky, Jeffrey L.↗
NASA NTRS1987

What is the essential physics of mass loss from late-type stars?

A review is given of the data concerning mass loss from late-type stars. The major classes of mass-loss mechanisms (thermally-driven winds, radiatively-driven winds, and wave-driven winds) are discussed, and the empirical mass-loss rates and other data are compared with theoretical predictions for any of these mechanisms acting alone. It is likely that several mechanisms act together to produce the large mass-loss rates in the Mira and nonpulsating M supergiants. Studies of the solar atmosphere suggest that thermal bifurcation driven by molecular condensation instabilities may play a critical role in cooling the atmospheres of luminous cool stars and forming siicate dust. It is possible that several metastable modes of atmospheric structure may exist for a given set of stellar parameters.

Linsky, Jeffrey L.↗
NASA NTRS1987

4 meter FTS observations of photospheric magnetic fields on M dwarfs

Much of the observed activity on M dwarfs (e.g., spots, flares, chromospheric and coronal emission) has been attributed to the interaction of magnetic fields with the stellar atmosphere. Since data on the magnetic field parameters of M dwarfs can potentially reveal much about the physical mechanisms behind these phenomena and, additionally, about stellar dynamos and the evolution of stellar angular momentum, a program has been started to measure the mean magnetic field strength (B) in stellar active regions, and the surface filling factor (f) of these regions for a sample of M dwarfs. In this paper some preliminary results of this survey are discussed.

Saar, Steven H.↗
NASA NTRS1986

Activity in warm stars: IUE observations of early F dwarfs

Deep low dispersion, short wavelength IUE observations of 69 F dwarfs were studied by overexposing the long wavelength end of the SWP camera by up to 100 times in order to bring up the weak chromospheric and transition region (TR) emission lines. All but one of the stars observed shows evidence for stellar activity, as defined by the presence of large C II and C IV surface fluxes, with fluxes greater than 100,000 ergs/sqcm/sec. There is no correlation between surface flux and the stellar rotation rate in the early F dwarfs. The TR surface fluxes increase systematically with decreasing B-V color, with no evidence for any decrease in the activity as the convection zones become very small. It is argued that there is a fundamental difference in the atmospheric heating mechanisms between early F dwarfs and solar-like stars. The heating of the TR in the early F dwarfs may be dominated by acoustic heating.

Walter, Frederick M.↗