Burn Out or Fade Away? On the X-ray and Magnetic Death of Intermediate Mass Stars

Kavli Affiliate: Moritz Guenther

| First 5 Authors: Jeremy J. Drake, Jonathan Braithwaite, Vinay Kashyap, H. Moritz Guenther, Nicholas J. Wright

| Summary:

The nature of the mechanisms apparently driving X-rays from intermediate mass
stars lacking strong convection zones or massive winds remains poorly
understood, and the possible role of hidden, lower mass close companions is
still unclear. A 20ks Chandra HRC-I observation of HR 4796A, an 8 Myr old main
sequence A0 star devoid of close stellar companions, has been used to search
for a signature or remnant of magnetic activity from the Herbig Ae phase.
X-rays were not detected and the X-ray luminosity upper limit was L_X =<
1.3×10^27 erg/s. The result is discussed in the context of various scenarios
for generating magnetic activity, including rotational shear and subsurface
convection. A dynamo driven by natal differential rotation is unlikely to
produce observable X rays, chiefly because of the difficulty in getting the
dissipated energy up to the surface of the star. A subsurface convection layer
produced by the ionisation of helium could host a dynamo that should be
effective throughout the main-sequence but can only produce X-ray luminosities
of order 10^25 erg/s. This luminosity lies only moderately below the current
detection limit for Vega. Our study supports the idea that X-ray production in
Herbig Ae/Be stars is linked largely to the accretion process rather than the
properties of the underlying star, and that early A stars generally decline in
X-ray luminosity at least 100,000 fold in only a few million years.

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