X-RAY-EMISSION FROM NEAR-MAIN-SEQUENCE B-STARS

Results of a ROSAT X-ray survey of 12 nearby near-main-sequence B star s are presented. Objects with very low interstellar hydrogen column de nsity were chosen to study the soft X-ray emission properties of the s tars. All of the stars were detected at the 3 sigma level. Spectral fi ts to seven of the stars are presented, and temperatures and source em ission measures are derived. The spectra and characterized by emission from gas at a temperature of about 2 x 10(6) K, which is lower than t hat typically observed for O stars. The ratio L(X)/L(Bol) decreases sh arply with spectral types later than B1, reaching values of about 10(- 9) at B3 V; much less than the value of 10(-7) that holds for O stars and early-B stars. The survey includes four beta Cephei stars and two Be stars. Our observations are consistent with the finding that beta C ephei variables have softer spectra than other stars of the same spect ral type. The X-ray emission measures for the stars are compared with upper and lower limits derived using mass loss rates and terminal velo cities from line-driven wind theory. It is concluded that if shocks em bedded in the winds are the source of the X-ray emission then a signif icant fraction of the winds in B stars must be hot. One to a few shock s can account for the observed emission measures. As a function of spe ctral type, there appears to be a transition in X-ray emission propert ies at about B1 to B1.5, with later type stars having a smaller L(X)/L (Bol) ratio. Coronal and shock models are discussed in regards to the B0 V star tau Sco which has an anomalously high source temperature.