Computer Science
Scientific paper
Sep 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004phdt.........9h&link_type=abstract
Thesis (PhD). BOSTON UNIVERSITY, Source DAI-B 65/03, p. 1365, Sep 2004, 268 pages.
Computer Science
Scientific paper
The Sun varies in irradiance, or total flux, by about 0.1%, or 1 millimagnitude (mmag), over a period of days to weeks caused by the passage of active regions (composed of sunspots and associated phenomena) across its disk and over a period of years resulting from the solar cycle. Is the Sun a typical star at this level of activity? The ages of most nearby stars are poorly known, so they may not be suitable for comparison with the Sun. However, solar-type stars in star clusters have well- determined ages. I present results of the search for stellar activity in four old star clusters with ages between 1.6 and 7 billion years. The data span a period of 8 yrs, consisting of more than 5 million individual magnitude measurements. I selected a subset of active stars from the main sequence population by testing for the statistical significance of the photometric variability in each star. In the three youngest clusters, NGC 7789, NGC 6819, and M67, between 4%, and 28% of the stars are significantly active on a rotational timescale with amplitudes of 6 to 10 mmag; no activity is seen on this timescale for the oldest cluster, NGC 188. On a yearly timescale, all of the clusters have a small fraction of active stars (3% to 19%) with amplitudes of 5 to 12 mmag. The lightcurves of individual stars show trends over the 8 year period reminiscent of the solar cycle. The data also show strong evidence that many more stars are active with amplitudes below the formal limits of detection. Stellar activity in solar-type stars on the rotational timescale declines to amplitudes less than 1.5 mmag, the detection limit, by the age of the oldest cluster (7 billion years). The activity in typical stars in the younger clusters has a much higher amplitude than is observed in the Sun, but is in good agreement with previous studies based on observations of younger stars.
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