Dissecting Capella's Corona: GHRS Spectra of the Fe XXI λ1354 and He II λ1640 Lines from Each of the Capella Stars
Abstract
We report on moderate (λ/Δλ = 20,000) and high (λ/Δλ = 90,000) resolution spectra of the 104-day period Capella binary system (HD 34029) obtained with Hubble Space Telescope's Goddard High-Resolution Spectrometer (GHRS) on 1995 September 9 and 1996 April 9. The observations include a long-duration, moderate-resolution spectrum of the coronal Fe XXI λ1354 line and both moderate- and high-resolution spectra of the He II λ1640 multiplet.
Our objective in observing the Fe XXI line formed at T = 1 × 107 K is to determine for the first time the line shape parameters and the contribution of each star's corona. This is feasible because the GHRS can resolve the 53 km s-1 radial velocity separation of the stars. Our analysis led to four surprising results: (1) The contribution of the slowly rotating G8 III star to the total Fe XXI λ1354 flux is similar to that of the more active rapidly rotating G1 III star, in contrast to other UV lines formed at lower temperatures. (2) The centroid velocities of the Fe XXI lines from both stars are near their respective photospheric radial velocities. Thus, there is no evidence for downflows or winds, and the hot coronal plasma must be confined, presumably by strong, closed magnetic fields. This is the first direct kinematic evidence for magnetic confinement in the corona of a giant star. (3) The line widths are thermal, indicating very low turbulence (ξ < 23 km s-1) compared with the 54 km s-1 thermal speed. (4) Our analysis of Extreme-Ultraviolet Explorer (EUVE) spectra that include four Fe XXI lines shows that the volume emission measures of these lines are about a factor of 3 smaller than for the λ1354 line, which was observed several months after the closest EUVE observation. We consider possible explanations for this discrepancy and conclude that variability of the high-temperature coronal emission is the most likely explanation. We observed the He II λ1640 line twice with a time separation of 7 months, corresponding to about 2 orbital periods. The profiles are nearly identical, except for a large difference in flux near the expected radial velocity of the G8 III star. We believe that the broad He II emission produced by the G1 star is formed mostly by collisional excitation in its transition region, while the emission from the G8 star is produced predominantly by a photoionization/recombination process driven by EUV radiation from the G8 star's corona. The He II λ1640 flux that we measure from the G8 star is consistent with predictions based on EUVE flux measurements. The decrease in the λ1640 emission from the G8 star between the phase 0.73 and 0.78 measurements indicates that its EUV radiation is variable, as is observed by EUVE. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 1998
- DOI:
- 10.1086/305075
- Bibcode:
- 1998ApJ...492..767L
- Keywords:
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- STARS: BINARIES: SPECTROSCOPIC;
- STARS: CORONAE;
- STARS: CHROMOSPHERES;
- STARS: INDIVIDUAL NAME: CAPELLA;
- STARS: INDIVIDUAL CONSTELLATION NAME: ALPHA AURIGAE;
- ULTRAVIOLET: STARS;
- Stars: Binaries: Spectroscopic;
- Stars: Chromospheres;
- Stars: Coronae;
- Stars: Individual: Name: Capella;
- Stars: Individual: Constellation Name: α Aurigae;
- Ultraviolet: Stars