Computer Science
Scientific paper
Dec 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998phdt.........1r&link_type=abstract
Thesis (PHD). UNIVERSITY OF WYOMING , Source DAI-B 59/06, p. 2804, Dec 1998, 214 pages.
Computer Science
Stellar Atmospheres
Scientific paper
Using spectroscopic observations of the He I λ5876 (D3) absorption line, I have measured the strength of chromospheric activity in 65 late A- and early F-type stars ranging in age from 50 Myr to 3-4 Gyr. The sample includes stars from five open clusters, Alpha Persei, Pleiades, Coma, Praesepe, and Hyades, as well as members of the field population, and represents virtually the entire main sequence lifetimes of the late A- and early F-type stars. For the early F-type stars, the data show that average chromospheric activity levels remain constant to within 20%. This constancy agrees with model calculations of acoustic flux produced within the surface convection zone, thought to be the prodominant mechanism for chromospheric heating in these stars. Each of the samples show large and similar star-to-star ranges in activity, with standard deviations as great as 50% of the mean. This large star-to-star variation remains unexplained. In the late A-type stars, I present the first reported detections of the D3 line. These measurements demonstrate the usefulness of this activity indicator in the late A-type stars, despite the weakness of the line in these stars. The data suggest a possible slow decline in activity with age, but a confirmation of this result will require more extensive observations. I compare the observed activity levels with the supposed 'Bohm-Vitense gap', a low density of stars in the late A and early F spectral types thought to be connected with the onset of convection, and thus the onset of activity. However, the activity data do not support this interpretation of the gap, and I conclude that the gap results from stochastic effects. Finally, I also demonstrate the most persuasive evidence to date for chromospheric activity variability in the early F-type stars. Two cluster stars show variations of the D3 equivalent width of a factor of 2, on time scales of about one day.
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