A SEARCH FOR SOLAR-LIKE OSCILLATIONS IN THE STARS OF M67 WITH CCD ENSEMBLE PHOTOMETRY ON A NETWORK OF 4-M TELESCOPES

Results are presented from a large observational project directed toward the detection of solar-like oscillations in an ensemble of open cluster stars. Seven groups collaborated in 1992 January to observe twelve stars in M67 with 4 m class telescopes for a one week period. High quality time series were collected on 22 telescope nights for a total of 156 h. The technique of CCD ensemble photometry allowed precisions of about 250 mumag per minute to be reached in the best cases, and provided robust results in conditions that sometimes were far from ''photometric.'' The longitude-distributed network, coupled with generally low noise levels, provided a good window function and yielded detection thresholds of about 20 mumag (five times solar) for solar-like oscillations in the best ensemble stars. Sensitivity to solar-like oscillations over our twelve ensemble stars ranges from 30% to a factor of three better than obtained previously by any group. When our simultaneous results for 12 stars (prior most sensitive result followed from photoelectric photometry on a single star) is taken into account this project provides a (multiplexed) factor of 20 to 30 gain over previous experiments. For two stars we derive interesting upper limits for oscillation amplitudes that are near the lower range predicted by theory. Over half the stars in the ensemble show suggestive evidence for oscillations; we develop the evidence for, and the cautions against, claiming detections in these cases. Given the unique aspects of this project we describe in detail the observation planning process, data acquisition, reductions, and ensuing analyses. We argue that a more aggressive network campaign could provide a factor of two sensitivity gain with a resulting high probability of attaining unambiguous oscillation detections on most of the stars in the M67 ensemble.