Variability in Old Open Cluster Stars on the Solar Cycle Timescale

Details Variability in Old Open Cluster Stars on the Solar Cycle Timescale Variability in Old Open Cluster Stars on the Solar Cycle Timescale

Dec 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003aas...20314709h&link_type=abstract

American Astronomical Society Meeting 203, #147.09; Bulletin of the American Astronomical Society, Vol. 36, p.593

Astronomy and Astrophysics

Astronomy

Scientific paper

We present the results of six seasons of observations of the old open clusters NGC 7789, NGC 6819, M67, and NGC 188 (age range: 1.6 to 7 Gyr) for variability on a range of timescales due to stellar activity. Using broad-band CCD photometry, we have observed brightness fluctuations due to starspots and plages passing across the faces of the cluster stars. As part of our project we have examined the seasonal, or solar cycle, timescale using our photometry measured through the V filter. The Sun's irradiance varies by approximately 1 millimagnitude (mmag) over the 11-year solar cycle (Frohlich & Lean, 1998, Geophysical Review Letters, 25, 4377). A subset of the Mt. Wilson HK Project stars have been monitored in narrow-band photometry and show similar variability (Lockwood, Skiff & Radick, 1997, ApJ, 485, 789). We searched for analogous variability in the cluster stars by comparing the RMS V magnitude fluctuations to the RMS photometric errors. We find that a large fraction of stars in all four clusters are variable with amplitudes > 1 mmag over six seasons of observations. We also find little decline in seasonal activity with age. The lightcurves of individual stars are similar to those of the Mt. Wilson stars. Our work confirms that the Sun is rather quiet for its age.

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