ROSAT PSPC observations of 27 near-main-sequence B stars

In this paper, we report on ROSAT Position Sensitive Proportional Counter (PSPC) observations of 27 near-main-sequence B stars made with unprecedented sensitivity. Contrary to the results of previous surveys, it 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 from 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 approximate 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 luminosity, corrected for this attenuation, actually exceeds 10(-7)L(Bol) in some cases. Presumably, this situation is even more severe for O stars; 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 observations of most of the B stars are very soft, with the notable exception 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 line-force instability, wind-shock mechanism if the mass-loss rates of these stars are a few times higher than theory currently predicts. However, the X-rays from stars later than B2 require filling factors greater than unity and thus cannot be produced by any radiation-driven wind-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 represent some kind of transition to, or hybrid of, wind and coronal X-ray mechanisms.