ROSAT observations of five chromospherically active stars

Abstract

We present X-ray spectra of three chromospherically active binaries, HR 7428, FF Aqr, and IX Per, and two active giant stars, 29 Dra and HR 9024. The spectra were obtained using the Position Sensitive Proportional Counter (PSPC) on the ROSAT satellite. For coronal plasma models with 1-temperature or 2-temperature components, we find that a range of heavy element abundances, from solar to extreme sub-solar abundances similar to those that have been inferred from analyses of ASCA spectra, provide acceptable fits to the observed PSPC spectra. The highest signal-to-noise spectrum of 29 Dra requires either a 1- or 2-temperature model with greatly sub-solar (photospheric) abundances or else a 3-component model with solar abundances. The degeneracy of these results, i.e., the fact that quite different models can produce acceptable fits to the PSPC spectra, means that it is difficult to infer the temperature structure of a corona from a PSPC spectra analysis without a priori knowledge of the prevailing coronal abundances, and vice versa. We also show that uncertainties in the intervening interstellar column densities can affect the inferred temperatures and/or abundances. Notwithstanding these difficulties, our analysis confirms that models having similar subsolar abundances to those inferred from ASCA and EUVE spectra of active coronal stars do indeed produce good fits to PSPC spectra. This general consistency between different instruments implies that explanations for the subsolar abundances that invoke systematic biases due to instrumental effects can now be ruled out.