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 wea k emission levels, with upper limits on the normalized x-ray flux in e ach case of better than log R(x) < - 6.3. The only A-F star we detecte d was 83 Tau, a member of the Hyades that we now belatedly recognize a s 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 attribu te to the presence of strong coronal winds. These new results add furt her weight to earlier suggestions that single A and early-F stars in t he field and in nearby older open clusters generally have intrinsicall y faint coronae.