THE CORONAE OF MAIN-SEQUENCE A-STARS

We have undertaken a deep x-ray imaging survey of single, normal, mid-to-late-A- and early-F-type main-sequence stars using the ROSAT PSPC. Eight of the nine such stars in our fields went undetected at very weak emission levels, with upper limits on the normalized x-ray flux in each case of better than log R(x) < - 6.3. The only A-F star we detected was 83 Tau, a member of the Hyades that we now belatedly recognize as a possible binary star, whose emission may come from an unidentified companion. Our observations reveal a systematic decline in coronal x-ray brightness with increasing effective temperature, which we attribute to the presence of strong coronal winds. These new results add further weight to earlier suggestions that single A and early-F stars in the field and in nearby older open clusters generally have intrinsically faint coronae.