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Chromospheric lines in red dwarf flare stars. I - AD Leonis and GX Andromedae
Pettersen, B. R.; Coleman, L. A.
AA(McDonald Observatory; Texas, University, Austin, TX), AB(McDonald Observatory; Texas, University, Austin, TX)
Astrophysical Journal, Part 1, vol. 251, Dec. 15, 1981, p. 571-582. Research supported by the Norges Almenvitenskapelige Forskningsrad. (ApJ Homepage)
Publication Date:
NASA/STI Keywords:
Chromosphere, Dwarf Stars, Flare Stars, H Alpha Line, Line Spectra, Stellar Spectrophotometry, Absorption Spectra, Calcium, Deuterium, Emission Spectra, M Stars, Optical Thickness, Sodium, Solar Atmosphere, Spectral Resolution, Stellar Temperature
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Line profiles with a spectral resolution of 0.45 A have been obtained for Halpha(lambda6563), the Na D lines (lambdalambda5890, 5896), He I lines (lambdalambda5876, 6678), and the Ca II infrared triplet lines (lambdalambda8498, 8542, 8662) in the flare stars AD Leonis and GX Andromedae. The spectroscopic Reticon diode array observations of AD Leo were obtained simultaneously with high speed photometry, and major influences in the spectra due to flares are avoided by this technique. GX And has photospheric parameters very similar to AD Leo but shows a flare activity level more than one order of magnitude smaller. Simultaneous photometry was not done during spectroscopic observations of GX And because of the small probability that a flare should contaminate the data.

Despite apparently equal physical parameters at the photospheric level, the chromospheric lines observed in this program appear very different in AD Leo and GX And. Halpha is a prominent emission line in AD Leo with a central absorption, for which the two peaks are separated by 0.6 Å. The FWHM of the line itself is 1.35 Å in AD Leo. In contrast, the Ha line in GX And is an absorption feature with FWHM =0.75 Å. The strength of this line is too large to be formed purely in the photosphere because temperatures are so low in this region that it is transparent to Halpha and consequently forms only a very weak Ha absorption line. A collisionally dominated line may strengthen its absorption feature in a chromosphere before turning into emission (Cram and Mullan), and this is probably what is seen in GX And.

An emission feature in the blue wing of Halpha in AD Leo, 20% above the continuum, is found to be present also in two nonflaring M dwarfs with Ha in absorption, and is associated with less TiO blanketing at that wavelength.

The Na D lines are broad absorption features in both stars with equal intensity distributions in the outer wings, reflecting the similarity of photospheric conditions in these stars. In AD Leo, however, the cores of the lines are seen in emission, the D2 feature being stronger than D1. These emission cores are unresolved at our resolution, but have a stellar origin. Also, in AD Leo the He I lambda5876 line is detected clearly in emission, but the lambda6678 line is much weaker. The filling-in of a Fe I absorption line by emission in this He I line is discussed, and the flux ratio of the lambda5876 to lambda6678 lines is not believed to give a correct triplet-to-singlet ratio for He I unless correction for the filling-in of the Fe I line is performed. In GX And none of the He I lines are detected, neither in emission nor absorption.

The Ca II infrared triplet lines are resolved for the first time in flare stars. In GX And, all three lines are seen in absorption. In the flare active AD Leo, however, the lines are found to be heavily filled in by emission, and only weak remnants of the absorption features remain. A central emission is present in the cores of the lines, but these features are unresolved at our spectral resolution. The central intensity is largest at the lambda8498 line, which is the least opaque of the triplet lines. This sharply contradicts an optically thin formation of the lines and leaves an intriguing modeling problem.

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