Magnetic structure in cool stars. I. The CA II H and K emission from giants.
Abstract
Estimates of the intensities of the Ca II H and K emission lines of 500 subgiants, giants and supergiants are reanalyzed in an investigation of the emission mechanisms in evolved cool stars. The Ca II emission intensity classes estimated by Wilson (1976) are compared with B-V colors of the stars in the sample. The majority of K- and M-type giants and many late G-type giants and subgiants are found to constitute a band of intensity class increasing with B-V, while G-type giants show a large spread in intensity class. Enhanced H and K emission is found in two groups of evolved stars: some of the G-type giants, and short-period binaries among the giants and subgiants with orbital periods shorter than a critical value, and the emission is explained by a relatively large stellar rotation rate. Data are consistent with the hypothesis that Ca II H and K emission depends on dynamo action in the convective envelope, with dynamo efficiency decreasing with decreasing rotation rate.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- August 1981
- Bibcode:
- 1981A&A...101...26M
- Keywords:
-
- Binary Stars;
- Chromosphere;
- Cool Stars;
- Giant Stars;
- Magnetic Stars;
- Stellar Rotation;
- Stellar Spectra;
- Calcium;
- Emission Spectra;
- G Stars;
- H Lines;
- Histograms;
- K Lines;
- Stellar Evolution;
- Stellar Temperature;
- Supergiant Stars;
- Astrophysics