X‐Ray Spectroscopic Signatures of the Extended Corona of FK Comae

Abstract

High-resolution Chandra X-ray spectra and surface Doppler images obtained from optical spectra of the rapidly rotating giant FK Com have been analyzed in order to investigate links between coronal and surface magnetic structures. Net redshifts were detected at the 3 σ level in the light of Ne λ12.13 amounting to ~140 km s−1. Smaller shifts of ~60 km s−1 at the ~2 σ level are seen in the X-ray spectrum as a whole, while the observed position of O VIII λ18.97, the second strongest line in the spectrum after Ne X, is also consistent with its rest wavelength. There is no statistical difference between redshifts during the first and second halves of the observation. Spectral line widths are most consistent with thermal broadening combined with rotational broadening at a scale height of ~1R, although they are also statistically consistent with surface rotational broadening. We interpret the results as indicative of hot plasma predominantly residing in extended structures centered at phase = 0.75 with a size similar to that of the star itself. The contemporaneous Doppler images of the surface of FK Com reveal active longitudes at phases ~ 0.6 and 0.9. We speculate that extended coronal structures correspond to magnetic fields joining the two active longitudes, which theoretical models predict are of opposite magnetic polarity. Such structures are supported by coronal potential field extrapolations of typical theoretical model surface magnetic field distributions. This study was based on a relatively short 40 ks Chandra observation. A much longer observation of FK Com with the HETGS, combined with contemporaneous Zeeman-Doppler imaging, would be of great value for constraining magnetospheric structure and dynamo models of rapidly rotating stars.