A comparison of the outer atmosphere of the "flat activity" star τ CETI (G8 V) with the sun (G2 V) and α centauri A (G2 V)

Driven by the desire to understand the roles of acoustic and magnetic mechanisms in heating the outer atmospheres of Sun-like stars, we compare solar UV spectra with archival STIS spectra from the Hubble Space Telescope of α Cen A (G2 V) and new STIS spectra of the extremely inactive dwarf star τ Cet (G8 V, V = 3.5). The activity of τ Cet shows occasional rotational modulations but no long-term cyclic variation. It may be in a phase analogous to the solar Maunder minimum. Solar disk center intensities from both the HRTS instrument and the SUMER instrument on SOHO were converted to Sun-as-a-star fluxes by using center-to-limb data from Dammasch and colleagues. The derived solar flux spectrum represents conditions near the minimum of the solar magnetic activity cycle. We find that the τ Cet line profiles differ systematically from those of the Sun and α Cen A. While lines formed in the middle chromospheres appear similar, the entire emission from the upper chromosphere to the middle transition region of τ Cet has lower flux densities by factors of ≈2, the line widths are significantly narrower, and, uniquely, the transition region lines are not significantly redshifted. The soft X-ray surface flux of τ Cet, measured between 0.1 and 2.4 keV, is ≈9 × 10 ³ ergs cm^-2 s^-1, several times smaller than the median solar value. We also find that the UV spectrum of α Cen serves as a proxy for a Sun-as-a-star spectrum when the Sun is in an intermediate phase of its activity cycle but that its coronal emission may be somewhat smaller. We discuss the implications of these results for magnetic fields and heating mechanisms in the stars and speculate that τ Cet's UV spectrum might represent the solar spectrum during a grand minimum phase.