Burn Out or Fade Away? On the X-Ray and Magnetic Death of Intermediate Mass Stars
Abstract
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 20 ks 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 LX <= 1.3 × 1027 erg s-1. 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 ionization of helium could host a dynamo that should be effective throughout the main sequence but can only produce X-ray luminosities of the order 1025 erg s-1. 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- May 2014
- DOI:
- 10.1088/0004-637X/786/2/136
- arXiv:
- arXiv:1403.6871
- Bibcode:
- 2014ApJ...786..136D
- Keywords:
-
- stars: activity;
- stars: coronae;
- stars: magnetic field;
- Sun: activity;
- Sun: corona;
- X-rays: stars;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- Accepted for publication in the Astrophysical Journal