Time-resolved CCD photometry of an ensemble of stars in the open cluster M67

Abstract

Stars in the central region of the old open cluster M67 were monitored for stability using CCD, time-resolved photometry. Five sites with 1 m class telescopes participated in an intensive monitoring effort, yielding contiguous 14 night coverage with 1 min temporal resolution. Noise levels to 0.0008 mag variation relative to an ensemble mean were attained at 13th mag over 9 hr time series. Limits to detection of coherent oscillations with periods of 3-20 min, assuming a solarlike spectrum of frequencies, are 100 micromag (30 times solar amplitude) for the best cases. Evidence suggestive of p-mode oscillations is shown for a few stars, but an unambiguous detection is not claimed. Serendipitous detection was made of two new W UMa stars in M67, two oscillating blue stragglers (with multiple modes) a likely AM Her cataclysmic variable with a 2.091 hr period, and several stars with low-amplitude intensity changes over timescales of 1-10 days, that presumably result from rotation or orbital motion.