ASCA OBSERVATIONS OF HD 104237 (A4e) AND THE ORIGIN OF X-RAY EMISSION FROM HERBIG Ae STARS

摘要

ASCA observed the bright Herbig star HD 104237 (A4e) for 30 ks in 1995 April, providing the best X-ray spectrum yet obtained of an intermediate mass pre-main-sequence star. The objective was to identify the physical mechanism responsible for the X-ray emission, with emphasis on discriminating between the softer emission that is characteristic of shocks and the harder emission (≥ 1 keV) that is normally associated with magnetic activity. Spectral fits using optically thin plasma models show that most of the emission comes from a cool component at 0.2-0.4 keV (～2-4 MK) and a hotter component whose temperature is not tightly constrained but is above 1.6 keV (～18 MK). We consider several possible emission mechanisms, including wind shocks, accretion shocks, a wind-fed magnetosphere, and a corona. Our main conclusion is that the X-ray emission most likely arises in a corona. However, coronal X-ray emission is unanticipated since Herbig stars are thought to lack the convection zones needed to sustain magnetic activity via a solar-like dynamo. We examine two possible solutions to this apparent paradox, namely (1) a corona around the Herbig star itself, sustained by a nonsolar shear-induced dynamo (Tout & Pringle 1995), and (2) a corona around a faint late-type companion whose presence is suspected on the basis of recent infrared observations.