scholarly journals Molecular Spectroscopy as Diagnostics of Outer Atmosphere of Cool Luminous Stars: Quasi-Static Turbulent Molecular Formation Zone and Stellar Mass-Loss

1988 ◽  
Vol 108 ◽  
pp. 158-166
Author(s):  
Takashi Tsuji
Keyword(s):  
Mass Loss ◽  
Molecular Spectroscopy ◽  
Recent Theory ◽  
Cool Component ◽  
Giant Stars ◽  
Molecular Formation ◽  
Formation Zone ◽  
Thermally Driven ◽  
Molecular Cooling ◽  
Outer Atmosphere

AbstractThe origin of mass-loss in cool luminous stars is still obscure; several known mechanisms such as thermally driven wind, radiation-driven wind(via dust), wave-driven wind etc all have serious difficulties, if examined in the light of recent observations. At the same time, recent observations in the infrared and radio spectral domains revealed that outer envelope of red (super)giant stars has highly complicated spatial and velocity structures, while inner envelope may have new component that had not been recognized before. For example, recent high resolution infrared spectroscopy revealed a possible presence of a quasi-static turbulent molecular dissociation zone somewhere in the outer atmosphere. This new component may represent a transition zone between the warm chromosphere and the huge expanding molecular envelope, and may be a cool component of chromospheric inhomogeneity or a moleclar condensation in a cool corona extended by turbulent pressure. Such a result can be regarded as observational evidence in support of a recent theory of autocatalytic molecular formation by thermal instability due to molecular cooling. Thus, observation and theory consistently show the presence of a new component - quasi-static turbulent molecular formation zone - in outer atmosphere of cool luminous stars, and a possibility of a unified understanding of outer atmospheric structure and mass-loss, in which turbulence may play important role, can be proposed.

scholarly journals Ultraviolet photometry of stars obtained with the Celescope experiment in the Orbiting Astronomical Observatory

1970 ◽  
Vol 36 ◽  
pp. 109-119
Author(s):  
Robert J. Davis
Keyword(s):  
Astronomical Observatory ◽  
Published Data ◽  
The Sun ◽  
The Moon ◽  
Wavelength Band ◽  
Giant Stars ◽  
Outer Atmosphere ◽  
Consistent Picture ◽  
Far Ultraviolet ◽  
Star Fields

We have used the television photometers in the Celescope OAO experiment to measure the far ultraviolet brightnesses of several thousand stars, including parts of the constellations Draco, Lyra, Puppis, Vela, Taurus, and Orion; and the Moon. As of this date (22 July 1969), three of our four cameras continue to operate satisfactorily, and we are making measurements in additional star fields distributed throughout the sky. Our shortest wavelength band, which includes the Lyman α line of atomic hydrogen, provides information on the Earth's outer atmosphere, as well as on star brightnesses. The intensity of the Lyman α radiation from the geocorona is a maximum when the Sun is near the horizon as seen by the OAO, and a minimum when the Sun is in the nadir. The direction that the telescope points does not much affect the measured intensities.Because of the heavy logistic problems of identification, calibration, and measurement for so many stars and because of the survey character of the program, the scientific interpretation of the results is, as expected, lagging the measurement program. However, one consistent picture beginning to emerge from our data is that our observed stars are about equally divided between those that fall within 0.5 magnitude of the predicted ultraviolet brightnesses and those that are significantly fainter than predicted. Most of the giant stars we observe exhibit these ultraviolet deficiencies. Since some of these giants are stars for which little or no interstellar reddening is predicted, we attribute the observed deficiencies to the stars themselves.Many of the objects we observe do not have accurate ground-based published data regarding magnitude, color, and spectral type; new ground-based observations of these objects are required to ensure satisfactory interpretation of our results.

scholarly journals Red Giant Stars: Comparison of Observations and Theory

1984 ◽  
Vol 108 ◽  
pp. 195-206
Author(s):  
Jeremy Mould
Keyword(s):  
Mass Loss ◽  
Parameter Space ◽  
Loss Rate ◽  
Mass Loss Rate ◽  
Magellanic Clouds ◽  
Asymptotic Giant Branch ◽  
Carbon Stars ◽  
Giant Stars ◽  
Theoretical Investigations ◽  
Red Giant

Recent observations in both the field and the clusters of the Magellanic Clouds suggest a higher mass loss rate during or at the end of the asymptotic giant branch phase than previously supposed. Recent theoretical investigations offer an explanation for the frequency of carbon stars in the Clouds, but a rich parameter space remains to be explored, before detailed agreement can be expected.

scholarly journals Stellar Winds and Coronae in Cool Stars

1980 ◽  
Vol 51 ◽  
pp. 279-291 ◽  
Author(s):  
A.K. Dupree ◽  
L. Hartmann
Keyword(s):  
Theoretical Models ◽  
Stellar Winds ◽  
Late Type ◽  
Dwarf Stars ◽  
Giant Stars ◽  
Cool Stars ◽  
Thermally Driven ◽  
Effective Temperatures ◽  
Hot Corona ◽  
Theoretical Results

AbstractRecent observational and theoretical results are reviewed that pertain to the presence and characteristics of stellar coronae and winds in late-type stars. It is found that stars - principally dwarfs - exist with “hot” coronae similar to the Sun with thermally driven winds. For stars, at the lowest effective temperatures, and gravities characteristic of supergiant and giant stars, high temperature (~105K) atmospheres are absent (or if present are substantially weaker than in the dwarf stars), and massive winds are present. There also exist “hybrid” examples - luminous stars possessing both a “hot” corona and a supersonic stellar wind. Constraints for theoretical models are discussed.

scholarly journals Fluorescence in the Outer Atmospheres of Red Giant Stars

1989 ◽  
Vol 106 ◽  
pp. 372-372
Author(s):  
K.G. Carpenter
Keyword(s):  
Radiative Decay ◽  
Energy Levels ◽  
Uv Spectrum ◽  
Line Radiation ◽  
Giant Star ◽  
Lyman Alpha ◽  
Giant Stars ◽  
Chromospheric Emission ◽  
Outer Atmosphere ◽  
Red Giant

The outer atmosphere of a cool red giant star is an ideal locale for the operation of line fluorescence processes. Low plasma densities imply low rates of collisional de-excitation and thus allow radiative decay of levels populated by selective radiative pumping. There are many strong sources of line radiation (i.e. possible pumps) and numerous possible upward transitions from highly populated low-lying levels of abundant elements such as Fe II, thus providing many chance coincidences between potential pumps and lines to be pumped. These conditions ensure that many of the chromospheric emission features observed in the UV spectrum of such a star are affected by fluorescence. Many of the observed emission features originate from energy levels populated solely by radiative fluorescent excitation, including strong lines of S I, O I, CO, Ni II, Si I, Fe I and Fe II, as well as weaker lines from Cr II and Co II. Important pumps active in these atmospheres include hydrogen Lyman alpha, and individual lines of 0 I, C I, Si II, Fe II, and Mg II. In the case of Fe II, there are many additional features arising from upper levels whose populations, although primarily maintained by collisions, are also significantly affected by radiative fluorescent excitation. In fact, there may be virtually no level in Fe II not affected to one degree or another by direct decays or cascades down from levels populated by fluorescence, driven either by Lyman alpha or, in some cases, by lines of Fe II itself (“self-fluorescence“).

Evolution and Problems of Red Long Period Variables

1979 ◽  
Vol 46 ◽  
pp. 163-176
Author(s):  
P. R. Wood
Keyword(s):  
Mass Loss ◽  
Atomic Mass ◽  
Surface Enrichment ◽  
Giant Stars ◽  
Long Period ◽  
Wide Range ◽  
Comprehensive Picture ◽  
Period Variable ◽  
Red Giant ◽  
Process Elements

Variability on the red giant branch (RGB) occurs at an interesting and important stage in the life of a star. This phase of evolution is characterized by a substantial amount of mass loss (as a continuous stellar wind or as an abrupt planetary nebula ejection), by flashing of the helium burning shell (at least in stars less massive than ~ 8 M) and by surface enrichment of elements heavier than hydrogen. The galactic importance of these processes is illustrated by the calculations of Cahn and Wyatt (1978) who estimate that material is presently being lost by stars on the RGB at a rate which is roughly equal to the rate at which material is being locked up in stars by star formation. A complementary calculation by Iben and Truran (1978) shows that shell flashing in red giant stars, coupled with mass loss, is a major source of enrichment of the interstellar medium over a wide range of atomic mass from C to heavy s-process elements. Hopefully, studies of the red long period variable (RLPV) stars can assist in the investigation of some of the above processes so that a comprehensive picture of the later phases of stellar evolution can be obtained.

Chromospheres and mass loss in metal-deficient giant stars

1984 ◽  
Vol 281 ◽  
pp. L37 ◽  
Author(s):  
A. K. Dupree ◽  
L. Hartmann ◽  
E. H. Avrett
Keyword(s):  
Mass Loss ◽  
Giant Stars
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