THE TRANSITION REGIONS OF CAPELLA

We have used the Goddard High Resolution Spectrometer (GHRS) to observ e the spectroscopic binary system Capella (G8 III + G1 III). Exposures with the G140L, G140M, G160M, G200M, and echelle gratings provide emi ssion line profiles with unprecedented signal-to-noise and spectral re solving power (lambda/Delta lambda) up to 92,000. Multi-Gaussian fits to the line profiles show that the hotter star contributes 60%-70% of the total flux in the chromospheric O I and Mg II resonance lines, but about 90% of the flux in the Si III, Si IV, and C IV lines formed in the transition region at T less than or equal to 10(5) K. We find clea r evidence that the emission lines from the hotter star are systematic ally redshifted relative to the photosphere with Doppler shifts of +5 +/- 1 km s(-1) and +9 +/- 3 km s(-1) in the chromospheric Mg II and O I lines, respectively, increasing to +24 +/- 5 km s(-1) for the transi tion region Si IV 1393.8 Angstrom line. The multi-Gaussian fits to per mitted transition region lines of Si III, Si IV, C IV, and N V indicat e the presence of three components: moderately broad lines formed in t he transition region of the hotter star (component H), narrow lines fo rmed in the transition region of the cooler star (component C), and ve ry broad lines that we think are formed in microflares on the hotter s tar (component B). The He II 1640.4 Angstrom feature has an broad prof ile, which indicates that it is formed by collisional excitation prima rily from the hotter star, and a weak narrow component that we interpr et as due to radiative recombination on the cooler star. We observed s pin-forbidden emission lines of C III], O III], Si III], O IV], O V], and S IV] that are sensitive to electron density. Fainter members of t he O IV] multiplet and all of the S IV] lines have never before been s een in any star other than the Sun. We determine electron densities in the transition regions of the Capella stars using line ratios of O IV ] lines and emission measure analysis. The emission measures are self- consistent only when the fluxes from each emitting component are consi dered separately. In particular, the transition region abundance distr ibutions appear to be different on the two stars, and the spin-forbidd en:lines were not detected in the B component. The emission measures f or component H (the G1 star) are consistent with a constant electron p ressure (or hydrostatic equilibrium) transition region with P-e = 10(1 5+0.1) cm(-3) K and possibly solar ''coronal'' abundances.