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We are carrying out a survey of magnetic fields in Ap stars in open clustersin order to obtain the first sample of magnetic upper main sequence stars withprecisely known ages. These data will constrain theories of field evolution inthese stars. Using the new spectropolarimeter ESPaDOnS at CFHT, we haveobtained 44 measurements of the mean longitudinal fields of 23 B6 - A2 starsthat have been identified as possible Ap stars and that are possible members ofopen clusters, with a median uncertainty of about 45 G. Of these stars, 10 havedefinite field detections. Nine stars of our sample are found not to bemagnetic Ap stars. The ESPaDOnS data contain a large amount of usefulinformation not readily obtained from lower resolution spectropolarimetry. Withthe new observations we are able to expand the available data on fields oflow-mass, relatively evolved Ap stars, and identify more robustly whichobserved stars are actually magnetic Ap stars and cluster members. Re-analysisof the enlarged data set of cluster Ap stars indicates that such stars withmasses in the range of 2 -- 5 \mo show RMS fields larger than about 1 kG onlywhen they are near the ZAMS. The time scale on which these large fieldsdisappear varies strongly with mass, ranging from about 250 Myr for stars of 2- 3 solar mass to 15 Myr for stars of 4 - 5 solar mass. Our data are consistenteither with emergent flux conservation for most (but not all) Ap stars, or withmodest decline in flux with age.

Searching for links between magnetic fields and stellar evolution III. Measurement of magnetic fields in open cluster Ap stars with ESPaDOnS