Outer atmospheres of cool stars. III - IUE spectra and transition region models for Alpha Centauri A and B
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Ayres, T. R.↗
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Outer atmospheres of cool stars. IV - A discussion of cool stellar wind models
Possible wind models for late-type stars which appear not to have hot coronae and transition regions are investigated; taking Arcturus and the prototypical star, models with T less than approximately 20,000 K are considered, and solutions with mass loss rates of 10 to the -9th solar masses per year are sought. The radiation pressure of L-alpha resonance scattering can exceed the force of gravity in the chromosphere and initiate a net outflow, but is insufficient to sustain a wind, except perhaps in the presence of an additional momentum input term such as Alfven wave pressure. It is concluded that L-alpha radiation-pressure-initiated winds can occur in stars to the right of the Linsky-Haisch dividing line in the H-R diagram between stars with and without transition regions and presumably hot coronae, and that the existence of these winds may explain energetically the absence of hot coronae in these stars.
Haisch, B. M.↗
Outer atmospheres of cool stars. VI - Models for Epsilon Eridani based on IUE spectra of C II, Mg II, Si II, and Si III
Observations of the ultraviolet line spectrum of the active chromosphere star, Epsilon Eridani, obtained with the IUE satellite have been analyzed. The coupled statistical equilibrium and radiative transfer equations for the prominent transitions of C II, Mg II, Si II, and Si III. A satisfactory fit to all of the line strengths can be achieved with a model similar to that recently proposed to explain bright points on the quiet sun. A surface pressure at the base of the transition region of 0.5 dynes/sq cm is derived, which is a factor of 3 higher than the quiet sun value, but a factor of 3 smaller than predicted by scaling laws, assuming a conductively heated stellar transition region. It is found that the surface fluxes of the C II 1334, 1335 and Si III 1892 emission lines are good diagnostics of pressure at the base of the transition region, but line ratio techniques using the 1892 line for estimating electron densities may be valid.
Simon, T.↗
Outer atmospheres of cool stars. V - IUE observations of Capella - The rotation-activity connection
UV spectra of Capella (G6 III + F9 III) obtained with the IUE are analyzed. High-resolution emission-line profiles taken near the elongation at phase 78 days suggest that virtually all of the emission in transition-region lines and most of the emission in chromospheric lines comes from the late-F secondary of the system. It is suggested that the origin of the extraordinary activity levels on these otherwise very similar stars can be traced to the one property that is obviously different, i.e., rotation. The Capella primary is a normal sharp-line slow rotating giant, whereas the secondary has broader lines and is a rapid rotator for a late-type giant.
Ayres, T. R.↗
The outer atmospheres of cool stars. VII - High resolution, absolute flux profiles of the Mg II h and k lines in stars of spectral types F8 to M5
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Stencel, R. E.↗
Outer atmospheres of cool stars. VIII - IUE observations and chromospheric models for the supergiant stars Beta Draconis, Epsilon Geminorum, and Alpha Orionis
A semiempirical modeling of stellar chromospheres is extended to late-type supergiants, where computations match high-resolution, absolute-flux profiles of the Ca II K and Mg II h and k lines. IUE UV spectra of Epsilon Gem and Alpha Ori show no evidence of emission lines formed at temperatures greater than 10,000 K, leading to the computation of chromospheric models extending to 10 to the -6th g/sq cm at temperatures that rise to 6500 K for the former star and 7000 K for the latter. By contrast, the C II-IV, Si IV, and He II and N V strong emission lines of Beta Dra lead to a tentative chromospheric model extension to 16,000 K at 0.012 dynes/sq cm. It is pointed out that the Ca II and Mg II line analyses presented, which assume hydrostatic equilibrium with only thermal and turbulent components to the pressure, imply nearly plane-parallel chromospheres even in the case of Alpha Ori.
Basri, G. S.↗
Outer atmospheres of cool stars. X - HR 1099 at quadrature
IUE high-dispersion, far-UV spectra of the active-chromosphere RS CVn binary HR 1099 are reported. The emission features produced by such high-temperature species as C II and C IV are found to be very bright, to exhibit structure, change significantly in the one-week interval separating the two exposures, and generally follow the radial velocity motion of the K subgiant primary, while the less massive G dwarf secondary appears only weakly in the composite spectrum. It is concluded that chromospheric and transition region emission in RS CVn binaries is a stellar rather than system phenomenon, and the structure evident in some of the emission line shapes is interpreted as a patchy brightness distribution on and above the K star surface that is spread out in velocity by the rapid rotation.
Ayres, T. R.↗
Outer atmospheres of cool stars. XI - High-dispersion IUE spectra of five late-type dwarfs and giants
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Ayres, T. R.↗
Outer atmospheres of cool stars. XIII - Capella at critical phases
The results are presented of a program of high-dispersion spectroscopy of Capella at critical orbital phases utilizing the International Ultraviolet Explorer. The considered program had the objective to establish as carefully as possible the relative ultraviolet emission levels of the Capella primary and secondary in order to obtain an understanding regarding the nature of their chromospheres and coronae. The results of the program confirm the earlier study of the Capella by Ayres and Linsky (1980), which was limited to orbital phases 0.50 and 0.75. Both programs found that the Capella secondary is considerably brighter than the primary, in surface flux, in all of the prominent far-ultraviolet emissions, particularly the high-excitation CII-CIV lines. In addition, the present study has revealed several puzzling aspects of the system. It is concluded that the Capella primary is comparatively bright, for a yellow giant, in far-ultraviolet and perhaps also in soft X-ray normalized fluxes. The implications of this conclusion are evaluated.
Ayres, T. R.↗
Outer atmospheres of cool stars. XIV - A model for the chromosphere and transition region of Beta Ceti (G9.5 III)
In the present chromospheric and transition region model for Beta Ceti, which is consistent with IUE spectra of the Mg II, C II, and C IV resonance lines, the Mg II h and k lines are treated in partial redistribution and the C II and C IV lines in complete redistribution. Computed line fluxes are presented for a range of models to show the range of permitted temperature structures. A comparison of the Beta Ceti model to models previously computed in a similar way for other stars shows a trend of decreasing chromospheric pressures and increasing geometric scales as single stars evolve across the transition region boundary. The present analysis also suggests that transition region pressures drastically decrease and geometric scales rapidly increase as single giant stars evolve to the right, toward the boudnary. Beta Ceti's exceptional X-ray brightness is discussed.
Eriksson, K.↗
Outer atmospheres of cool stars. XV - High-dispersion ultraviolet studies of active chromosphere G-K dwarfs
IUE ultraviolet echelle spectra of three late-type active chromosphere dwarf stars, Chi-1 Orionis, Xi Bootis, and Epsilon Eridani, are compared with previously published observations of the quiet chromosphere dwarfs Alpha Centauri A and Alpha Centauri B. The spectra indicate a gross strengthening of chromospheric and transition-region line fluxes from quiet to active dwarf stars without a fundamental change in their line shapes. Qualitatively the same spectroscopic behavior is seen in detail on the solar surface between regions of strong and weak magnetic fields. It is argued that the correspondence between the stellar and solar behavior is not accidental, but instead affirms that enhanced chromospheric and coronal emission on late-type main sequence stars is a result of increased surface coverage of magnetic active regions.
Ayres, T. R.↗
On the outer atmospheres of hybrid stars
Deep exposures with the IUE satellite have been obtained in order to search for high-temperature emission from stars with cool winds. Iota Aur and Theta Her are confirmed as hybrid stars, and an additional hybrid object, Gamma Aql, has been discovered. The emission line fluxes of the hybrid stars are analyzed to establish the emission measure distribution and, as far as possible, the electron density. The discovery of variable, high-velocity Mg II circumstellar absorption in Gamma Aql, Theta Her, and Alpha TrA is reported. Very long exposure of the latter show that cool material is being accelerated to velocities of at least 180 km/s. These observations suggest that high-velocity mass loss is more common than previously thought.
Hartmann, L.↗
Relations between the photospheric magnetic field and the emission from the outer atmospheres of cool stars. I - The solar Ca II K line core emission
Observations of a solar active region complex and its surroundings are used to establish a quantitative relation between the Ca II K line core intensity and magnetic flux density. The Ca II K line core intensity is transformed to a Ca II H + K line core flux density to facilitate a comparison of solar and stellar data. A new absolute calibration for the Mount Wilson Ca II H + K fluxes for G-type dwarfs is derived. The minimum Ca II K flux, found in the centers of supergranulation cells in quiet regions on the sun, is identical to the minimum flux that is observed for solar-type stars. An expression is presented for the nonlinear trend between the Ca II H + K line core excess flux density and the absolute value of the magnetic flux density. Models that explain the nonlinearity of the mean trend and the large intrinsic scatter about it are discussed. The solar data define a relation that is similar to the relation between stellar hemisphere-average magnetic flux densities and Ca II H + K excess flux densities.
Schrijver, C. J.↗
Relations between the photospheric magnetic field and the emission from the outer atmospheres of cool stars. II - The C IV 1550 A doublet
Solar and stellar data are used to determine the relationship between C IV 1548 + 1551 A and magnetic flux densities. C IV SMM-UVSP spectroheliograms of solar quiet regions and plages are analyzed. Typical distribution functions of C IV flux densities over both quiet and active solar regions are presented. The C IV emission is shown to be dependent on magnetic flux density.
Schrijver, C. J.↗
Three Dimensional Structures in the Atmospheres of Cool Stars
This grant has supported my GHRS-related activities since 1990. This included both instrumental calibration activities and independent scientific research using the Goddard High Resolution Spectrograph on the Hubble Space Telescope. The activities under this grant are essentially complete. Publications to date which have resulted in whole or in part from this grant are included.
Walter, Frederick M.↗
Summary of the Origin of Nonradiative Heating/momentum in Hot Stars
Nonradiative heating in the atmospheres of stars and the deposit of nonradiative momentum in the stellar atmosphere were examined. Observations made from space have been very important in changing the concept of what a star is like. In order to interpret the spectroscopic information gained in the wavelength region inaccessible from the surface of the Earth, chiefly in the domain of X-rays and of ultraviolet radiation. The assumptions which underlie the theory of stellar atmospheres will have to be reused. Improved observation facilities are major reasons to consider new ideas for interpreting the spectra of stars. The photometric precision and sensitivity for observing at high and at moderate spectral resolutions have increased greatly. The development of high dispersion, high resolution spectrographs for observing is noted.
Underhill, A. B.↗
The nature of RV Tauri variables. II- Properties of the peculiar F stars
Basic atmospheric parameters for six RV Tauri variables are considered. The Fe/H abundance ratios for the stars are found to be between -1 and - 1.7, inclusively. Stars of the Preston (1962) type B and C subclasses do not appear segregated by metallicity. In comparison to the cooler Preston type A stars, types B and C are shown to be more metal poor by a factor of 10, on average. The results are consistent with the theory that RV Tau type B and C variables are a direct evolutionary analog to the cluster type II Cepheids.
Cardelli, Jason A.↗
Chromospheres and mass loss in metal-deficient giant stars
Semiempirical atmospheric models indicate that the characteristic emission in the wings of the H-alpha line observed in Population II giant stars can arise naturally within static chromospheres. Radial expansion gives an asymmetric, blueshifted H-alpha core accompanied by greater emission in the red line wing than in the blue wing. Wind models with extended atmospheres suggest mass loss rates much smaller than 2 x 10 to the -9th solar mass per yr. Thus H-alpha provides no evidence that steady mass loss can significantly affect the evolution of stars on the red giant branch of globular clusters.
Dupree, A. K.↗