The SOHO-Stellar Connection

Abstract

The’ solar-stellar connection’ bridges the daytime and nighttime communities; an essential link between the singular, but detailed, views of our Sun, and the broad, but coarse, glimpses of the distant stars. One area in particular — magnetic activity — has profited greatly from the two way traffic in ideas. In that spirit, I present an evolutionary context for coronal activity, focusing on the very different circumstances of low-mass main-sequence stars like the Sun, compared with more massive stars. The former are active mainly very early in their lives, whereas the latter become coronal only near the end of theirs, during the brief incursion into the cool half of the Hertzsprung-Russell diagram as yellow, then red, giants. I describe tools at the disposal of the stellar astronomer; especially spectroscopy in the ultraviolet and X-ray bands where coronae leave their most obvious imprints. I compare HST STIS spectra of solar-type dwarfs — ζ Dor (F7 V), an active coronal source, and α Cen A (G2 V), near twin of the Sun — to the SOHO SUMER UV solar atlas. I also compare the STIS line profiles of the active coronal dwarf to the corresponding features in the mixed activity ‘hybrid chromosphere’ bright giant α TrA (K2II) and the archetype ‘non-coronal’ red giant Arcturus (α Boo; K2 III). The latter shows dramatic evidence for a ‘cool absorber’ in its outer atmosphere that is extinguishing the ‘hot lines’ (like SiIV λ1393 and Nv λ1238) below about 1500 Å; the corona of the red giant seems to lie beneath its extended chromosphere, rather than outside as in the Sun. I present an early taste of the moderate resolution spectra we can expect from the recently launched Chandra X-ray Observatory (CXO), and contemporaneous STIS high resolution UV measurements of the CXO calibration star Capella (α Aur; G8 III + Gl III). Last, I describe preliminary results from a May 1999 observing campaign involving SOHO SUMER, TRACE, and the Kitt Peak Infrared Imaging Spectrometer (IRIS). The purpose was to explore the dynamics of the quiet solar atmosphere through the key ‘magnetic transition zone’ that separates the kinetically dominated deep photosphere from the magnetically dominated coronal regime. Linking spatially and temporally resolved solar phenomena to properties of the average line shapes (widths, asymmetries, intensity ratios, and Doppler shifts) is a crucial step in carrying physical insights from the solar setting to the realm of the distant stars.