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 towa rd the detection of solar-like oscillations in an ensemble of open clu ster stars. Seven groups collaborated in 1992 January to observe twelv e stars in M67 with 4 m class telescopes for a one week period. High q uality time series were collected on 22 telescope nights for a total o f 156 h. The technique of CCD ensemble photometry allowed precisions o f about 250 mumag per minute to be reached in the best cases, and prov ided robust results in conditions that sometimes were far from ''photo metric.'' The longitude-distributed network, coupled with generally lo w noise levels, provided a good window function and yielded detection thresholds of about 20 mumag (five times solar) for solar-like oscilla tions in the best ensemble stars. Sensitivity to solar-like oscillatio ns over our twelve ensemble stars ranges from 30% to a factor of three better than obtained previously by any group. When our simultaneous r esults for 12 stars (prior most sensitive result followed from photoel ectric photometry on a single star) is taken into account this project provides a (multiplexed) factor of 20 to 30 gain over previous experi ments. For two stars we derive interesting upper limits for oscillatio n amplitudes that are near the lower range predicted by theory. Over h alf the stars in the ensemble show suggestive evidence for oscillation s; we develop the evidence for, and the cautions against, claiming det ections in these cases. Given the unique aspects of this project we de scribe in detail the observation planning process, data acquisition, r eductions, and ensuing analyses. We argue that a more aggressive netwo rk campaign could provide a factor of two sensitivity gain with a resu lting high probability of attaining unambiguous oscillation detections on most of the stars in the M67 ensemble.