ROSAT PSPC OBSERVATIONS OF 27 NEAR-MAIN-SEQUENCE B-STARS

In this paper, we report on ROSAT Position Sensitive Proportional Coun ter (PSPC) observations of 27 near-main-sequence B stars made with unp recedented sensitivity. Contrary to the results of previous surveys, i t is found that 75% of the sample stars are X-ray sources, albeit most at modest levels. The X-ray luminosities of the program stars range f rom 5.6 x 10(27) up to 2.2, 10(32) ergs s(-1). We find that L-X/L-Bol decreases abruptly beyond about B0 and stabilizes at L-X/L-Bol approxi mate to 10(-8.5) by about B2, with seven nondetections at B2 and later . For the B0 and B1 stars, our modeling suggests that wind attenuation of the X-ray photons is significant, so that the emitted X-ray lumino sity, corrected for this attenuation, actually exceeds 10(-7)L(Bol) in some cases. Presumably, this situation is even more severe for O star s; thus, the well-known L-X/L-Bol approximate to 10(-7) law simply may be an artifact of the neglect of wind attenuation. The ROSAT PSPC obs ervations of most of the B stars are very soft, with the notable excep tion of tau Sco (B0 V). The wind emission measure filling factors that we find for the very early B stars are rather large (roughly 0.1-1). This could be brought into line with theoretical calculations of the l ine-force instability, wind-shock mechanism if the mass-loss rates of these stars are a few times higher than theory currently predicts. How ever, the X-rays from stars later than B2 require filling factors grea ter than unity and thus cannot be produced by any radiation-driven win d-shock mechanism because there is simply not enough wind material to produce the observed X-rays. It is possible that mid-to late-B stars r epresent some kind of transition to, or hybrid of, wind and coronal X- ray mechanisms.