Goddard High‐Resolution Spectrograph Observations of Cool Low‐Gravity Stars. IV. A Comparison of the K5 III stars α Tauri and γ Draconis

Abstract

The Goddard High-Resolution Spectrograph (GHRS) has observed the K5 III star a Tau in the 2330 Angstrom region on three separate occasions. These spectra show marked changes with time, with the UV continuum varying by a factor of 2, and with the emission lines changing in flux by 30% or more, with the amount of change dependent upon the opacity of the line. The variations suggests a restructuring of the atmosphere rather than simply a change in the surface area covered by chromospheric material. Surprisingly, there was no detectable change in the chromospheric turbulence on timescales of hours or years. On average, the lower part of the atmosphere was found to be fairly static, with a slight infall of 1-2 km s(-1). At higher altitudes, probed by observation of the stronger Fe rr lines as well as of the O I (UV 2) and Mg II (UV 1) resonance lines, there is evidence for the acceleration of a slow wind, similar to that seen in the M giants gamma Cru and mu Gem. This wind is much less massive than for the later type giants, however, since its effects are seen in only the most optically thick of the Fe II lines. Comparison of the alpha Tau observations with similar data for the K5 VT hybrid star gamma Dra shows remarkable similarity in the photosphere and lower chromosphere. Both stars have pronounced UV continua, identical turbulences and chromospheric densities, and very similar line fluxes and profiles for all lines formed in the lower chromosphere, including C II], Co I, Si II], and Fe II. A deep exposure near 1500 Angstrom also shows the first evidence for hot plasma in the atmosphere of alpha Tau through the detection of the C Iv (UV 1) doublet with a surface flux about 30% of that observed in gamma Dra. Most of the evidence for the stellar wind is in the Mg II (UV 1) and O I (UV 2) resonance Lines. Modeling these lines using the Sobolev with Exact Integration (SEI) radiative transfer code shows that the wind in gamma Dra accelerates faster and reaches a higher terminal velocity than does the wind in alpha Tau. However, the wind turbulent velocity in gamma Dra is only about one-third of the value seen in alpha Tau. We conclude that the observations support the suggestion by Judge & Stencel that the processes that heat the chromosphere are distinct from those that drive the stellar winds.