X-ray study of Herbig Ae/Be stars

We present ASCA results of intermediate- masspre-main-sequence (PMS) stars, or Herbig Ae/Be (HAeBe) stars. Among the 35 ASCA pointed sources, we detect 11 plausible X-ray counterparts. X-ray luminosities of the detected sources in the 0.5-10 keV band are in the range of log L-X similar to30-32 ergs s(-1), which is systematically higher than those of low-mass PMS stars. This fact suggests that the contribution of a possible low-mass companion is not large. Most of the bright sources show significant time variation; in particular, two HAeBe stars MWC 297 and TY CrA-exhibit flarelike events with long decay timescales (e-folding time similar to10-60 ks). These flare shapes are similar to those of low-mass PMS stars. The X-ray spectra are successfully reproduced by an absorbed one- or two-temperature thin-thermal plasma model. The temperatures are in the range of kT similar to1 5 keV, significantly higher than those of main-sequence OB stars (kT < 1 keV). These X-ray properties are not explained by wind-driven shocks, but are more likely due to magnetic activity. On the other hand, the plasma temperature rises as absorption column density increases or as HAeBe stars ascend to earlier phases. The X-ray luminosity reduces after stellar age of a few times 10(6) yr. X-ray activity may be related to stellar evolution. The age of the activity decay is apparently near the termination of jet or outflow activity. We thus hypothesize that magnetic activity originates from the interaction of the large-scale magnetic fields coupled to the circumstellar disk. We also discuss differences in X-ray properties between HAeBe stars and main-sequence OB stars.