We study the x-ray property of intermediate mass pre-main-sequence stars (PMSs) Herbig Ae/Be stars (HAeBes), mainly using the observational and archival data of the ASCA and ROSAT satellites. Among 39 HAeBes covered with 42 ASCA pointings, we detect 15 plausible x-ray counterparts. X-ray luminosities of the detected sources are in a high range of log Lx of about 30-32 ergs/s compared with those of low mass stars. This fact excludes the possibility that HAeBe x-ray emission comes from a low mass companion. Most sources with moderate photon counts (> 1000 cts) show time variations. In particular, three HAeBes - MWC 297, TY CrA and VY Mon - exhibit flare-like time variations with long decay timescales (e-folding time of about 10-50 ksec). These flare shapes are quite similar to the flares of low mass protostars. In contrast, HR 6000 show a rapid variation with fast rise and fast decay (tau < 2 ksec). Each spectrum is successfully reproduced with an absorbed one or two temperature thin-thermal plasma (MeKaL) model. The temperature ranges between 1-5 keV, which is significantly higher than that of main-sequence OB stars (kT < 1 keV).
For their x-ray origin, we test a few models for emission mechanisms of stellar x-rays - wind driven shock, wind-fed magnetosphere, accretion shock and magnetic activity. These models except for the magnetic activity cannot explain the plasma temperature, x-ray luminosity or time variability. Nevertheless, HAeBes are not thought to have solar type dynamo mechanism to amplify the magnetic field because they lack the convection zone on the stellar surface. We cannot find the Lx dependence on v sin i and omega sin i, that are indicators of the solar type dynamo. To find out the generating mechanism of the magnetic field, we investigate the evolutional variation of plasma temperature and x-ray luminosity. We then find that the plasma temperatures have a clear rising trend with increasing the absorption column density (NH), which traces the surrounding materials and hence the evolutional status of young stars. This indicates that younger stars have more violent high-energy activity. Whereas, the x-ray luminosity seems to depend more on stellar masses. We therefore plot the x-ray luminosity in the HR diagram together with additional samples in previous papers. We then find Lx drops after the age a few times 1e6 yrs. Considering the circumstellar structure, our results favor the mechanism of large scale magnetic fields linking a star and its circumstellar disk.
Stars earlier than spectral type B reach to the main-sequence phase before a few times 1e6 year-old. This indicates that young massive main-sequence stars still operate the magnetic activity. From several pieces of supporting evidence, we propose that the x-ray emission from B1-9 stars and hard tails seen in O stars originate in this magnetic activity. On the other hand, we find the hard x-ray emission from the giant molecular cloud core Monoceros R2. We suggest that it might come from progenitors of HAeBes. Finally, we draw a unified picture of x-ray activity on young stellar objects in view of the magnetic activity.