Stars earlier than about F5 (Teff ~ 8000 K) don’t have convective
envelopes and don’t have any X-ray emission…
Except that O and early B
stars do have X-ray emission - they are strong
sources of soft X-rays. And they have strong
stellar winds.
Wind broadened and blueshifted UV absorption lines of an O and a B star.
HST image of h Car; an extreme example of a hot star wind.