Physics
Scientific paper
Apr 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995aj....109.1797e&link_type=abstract
Astronomical Journal (ISSN 0004-6256), vol. 109, no. 4, p. 1797-1783
Physics
23
Chromosphere, Cool Stars, Giant Stars, H Alpha Line, Stellar Luminosity, Stellar Mass, Stellar Temperature, Astronomical Models, Atmospheric Models, Light Scattering, Spectrum Analysis, Stellar Spectra, Telluric Lines, Velocity
Scientific paper
The H-alpha line in a cool star is usually an indication of the conditions in its chromosphere. I have collected H-alpha spectra of many northern G-M stars, which show how the strength and shape of the H-alpha line change with spectral type. These observations detect surprising little variation in absoption-line depth (Rc approximately 0.23 +/- 0.08), linewidth (FWHD approximately 1.44 +/- 0.22 A), or equivalent width (EW approximately 1.12 +/- 0.17 A) among G5-M5 III giants. Lines in the more luminous stars tend to be broader and stronger by 30%-40% than in the Class III giants, while the H-alpha absorption tends to weaken among the cooler M giants. Velocities of H-alpha and nearby photospheric lines are the same to within 1.4 +/- 4.4 km/s for the whole group. To interpret these observations, I have calculated H-alpha profiles, Ly-alpha strengths, and (C II) strengths for a series of model chromospheres representing a cool giant star like alpha Tau. Results are sensitive to the mass of the chromosphere, to chromospheric temperature, to clumping of the gas, and to the assumed physics of line formation. The ubiquitous nature of H-alpha in cool giants and the great depth of observed lines argue that chromospheres of giants cover their stellar disks uniformly and are homogeneous on a large scale. This is quite different from conditions on a small scale: To obtain a high enough electron density with the theoretical models, both to explain the exitation of hydrogen and possibly also to give the observed C II multiplet ratios, the gas is probably clumped. The 6540-6580 A spectra of 240 stars are plotted in an Appendix, which identifies the date of observation and marks positions of strong telluric lines on each spectrum. I assess the effects of telluric lines and estimates that the strength of scattered light is approximately 5% of the continuum in these spectra. I give the measurements of H-alpha as well as equivalent widths of two prominent photospheric lines, Fe I lambda 6546 and Ca I lambda 6572, which strengthen with advancing spectral type.
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