The X-Ray Spectrum of a Metal-poor Corona

Abstract

We present an ASCA SIS spectrum of the Population II binary star HD 89499. This is the most metal poor star ([Fe/H] = -2.1) known to have a corona. The ASCA data confirm initial results from ROSAT which suggested a very hot, single-component plasma temperature, with the temperature derived from the ASCA data being slightly higher (2.6 × 107 K) than the one determined by ROSAT. The ASCA spectrum clearly shows the absence of emission lines and can be best modeled as thermal bremsstrahlung, although strong absorption at photon energies less than 0.5 keV is indicated. We suggest that such an extremely high coronal temperature results from the lack of an efficient cooling mechanism due to the absence of metallic (mostly Fe) emission lines.