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Burn out or fade away? X-ray and magnetic death at intermediate stellar masses

posted Apr 24, 2014, 7:13 PM by Jeremy Drake   [ updated Apr 24, 2014, 7:18 PM ]
The Sun was confirmed as a copious source of X-rays by a pinhole camera carried on a rocket flight in the 1940s. The next question was what other stars emit X-rays?  The answer came from the Einstein Observatory X-ray satellite, lead by Nobel Laureate Riccardo Giacconi from SAO in Cambridge Mass., and launched in 1982.  Einstein detected X-rays from nearly all types of stars.  The ones missing were the
stars of 2-4 solar masses or so - the blueish-white late B-types to early A-types.  It was realised that these stars do not have outer convection zones, like the Sun, or strong unstable winds outflowing at 1000 km/s or more, like more massive O and B-type stars.  Solar-like stars generate X-rays from magnetic energy originating in their convection and rotation.  The winds of massive OB stars - millions of times stronger than the solar wind - also generate X-rays through shock-heating, converting about a ten-millionth of their star's energy output into X-rays in the process. Lacking these means of producing X-rays, the intermediate mass stars are X-ray dark.  But not when they are young. 

Young solar-like stars are prominent X-ray sources, producing 1000 or more times the X-ray output of the present day Sun.  Einstein also detected X-rays from very young - million year old or so - intermediate mass stars. How they generate these X-rays is still a mystery.  Some X-rays could be generated from the accretion of gas from their circumstellar disks. X-rays might also originate from so-called internal "shear" - the interior rotates at a different rate to the exterior - producing magnetic fields in the process.  Regardless, by a hundred or so million years the X-rays, and probably the shear, are gone.  We used the Chandra X-ray observatory to try and catch this X-ray decline in an 8 Myr old intermediate mass star, HR 4976A, but failed to detect anything, down to a limit of a couple of hundred millionths of its visible light output. The X-rays burn out rather than fade away, out of the blue and into the black, as Neil Young said.  This work was published in the 24 April 2014 edition of the Astrophysical Journal.