X-ray Study of the Intermediate-Mass Young Stars Herbig Ae/Be Stars

TL;DR

This study investigates the X-ray emissions of Herbig Ae/Be stars, revealing magnetic activity likely linked to star-disk interactions, with properties distinct from main-sequence OB stars and evolving with stellar age.

Contribution

It provides the first detailed X-ray analysis of Herbig Ae/Be stars, highlighting magnetic activity as the primary X-ray source and its evolution over stellar lifetime.

Findings

X-ray luminosities are higher than low-mass PMSs, suggesting minimal contribution from low-mass companions.

Detected X-ray spectra indicate high plasma temperatures (1-5 keV), inconsistent with wind shocks.

X-ray activity decreases with stellar age, correlating with the end of jet or outflow activity.

Abstract

We present the ASCA results of intermediate-mass pre-main-sequence stars (PMSs), or Herbig Ae/Be stars (HAeBes). 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 LX ~30-32 ergs s-1, which is systematically higher than those of low-mass PMSs. This fact suggests that the contribution of a possible low-mass companion is not large. Most of the bright sources show significant time variation, particularly, two HAeBes - MWC 297 and TY CrA - exhibit flare-like events with long decay timescales (e-folding time ~ 10-60 ksec). These flare shapes are similar to those of low-mass PMSs. 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 ~1-5 keV, which are 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 HAeBes ascend to earlier phases. The X-ray luminosity reduces after stellar age of a few x10^6 years. 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 HAeBes and main-sequence OB stars.