The evolution of chromospheric activity of cool giant and subgiant stars

Details The evolution of chromospheric activity of cool giant and subgiant stars The evolution of chromospheric activity of cool giant and subgiant stars

Nov 1989

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989apj...346..303s&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 346, Nov. 1, 1989, p. 303-329.

Astronomy and Astrophysics

Astrophysics

148

Chromosphere, Cool Stars, Giant Stars, Late Stars, Stellar Activity, Stellar Evolution, Subgiant Stars, Expansion, F Stars, Iue, Stellar Coronas, Stellar Rotation, Ultraviolet Spectra, X Ray Stars

Scientific paper

IUE spectra for a large sample of cool subgiant stars are examined, and evidence is found that subgiants in the mass range 1.2-1.6 solar masses undergo a sudden decline in UV transition region emission near B - V = 0.6, which corresponds to spectral type G0 IV. The decline in UV emission coincides with a sharp decrease in stellar rotation rates, and it is suggested that this decay in activity and rotation marks a transformation from acoustic heating in the early F stars to magnetic dynamo-driven activity in the cooler stars, resulting in a strong rotational braking action by stellar wind. For more massive giant stars, there is a similar transformation in the nature of chromospheric activity near B - V = 0.7, or spectral type G0 III, from acoustic heating in the F-type giants to a solarlike dynamo mechanism in the cooler giants. No sign of an abrupt drop in activity near spectral type G5 III at the location of Gray's proposed rotational boundary line is seen.

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