X‐Ray Study of Herbig Ae/Be Stars

Abstract

We present the ASCA results of intermediate-mass pre-main-sequence stars\n(PMSs), or Herbig Ae/Be stars (HAeBes). Among the 35 ASCA pointed-sources, we\ndetect 11 plausible X-ray counterparts. X-ray luminosities of the detected\nsources in the 0.5-10 keV band are in the range of log LX ~30-32 ergs s-1,\nwhich is systematically higher than those of low-mass PMSs. This fact suggests\nthat the contribution of a possible low-mass companion is not large. Most of\nthe bright sources show significant time variation, particularly, two HAeBes -\nMWC 297 and TY CrA - exhibit flare-like events with long decay timescales\n(e-folding time ~ 10-60 ksec). These flare shapes are similar to those of\nlow-mass PMSs. The X-ray spectra are successfully reproduced by an absorbed one\nor two-temperature thin-thermal plasma model. The temperatures are in the range\nof kT ~1-5 keV, which are significantly higher than those of main-sequence OB\nstars (kT < 1 keV). These X-ray properties are not explained by wind driven\nshocks, but are more likely due to magnetic activity. On the other hand, the\nplasma temperature rises as absorption column density increases, or as HAeBes\nascend to earlier phases. The X-ray luminosity reduces after stellar age of a\nfew x10^6 years. X-ray activity may be related to stellar evolution. The age of\nthe activity decay is apparently near the termination of jet or outflow\nactivity. We thus hypothesize that magnetic activity originates from the\ninteraction of the large scale magnetic fields coupled to the circumstellar\ndisk. We also discuss differences in X-ray properties between HAeBes and\nmain-sequence OB stars.\n