Chandra/HETGS Observations of the Capella System: The Primary as a Dominating X-Ray Source

Using the Chandra/High Energy Transmission Grating Spectrometer (HETGS), we have detected the Doppler motion of Capella's X-ray emission lines in the 6-25 Å wave band. The observed motion follows the expected orbital motion of Capella's primary. This finding implies that the primary G8 III star, not the secondary G1 III star in the Hertzsprung gap, has been the dominant source of hot 10^6.8-10⁷ K plasma at least in the last 6 years. In addition, the results demonstrate the long-term stability of the HETGS, and they demonstrate small uncertainties of 25 and 33 km s^-1 in the velocity determination with the High and Medium Energy Gratings (HEG and MEG), respectively.