Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Astronomy

Supervisor

Gray, David F.

Abstract

The four K0IIIHyades red clump giants, γ , δ , ϵ , and θ 1Tauri, are ideal candidates for cool star research. They are easily observable at a distance of only 47 pc and one star, ϵ Tauri, is a suspected Maunder minimum star. These stars also provide a useful opportunity to investigate the nature of the photospheric velocity field, and assess the effectiveness of a range of spectroscopic tools including the merits of the microturbulence-macroturbulence approach to the study of cool star spectra. High resolution (λ /Δ λ ∼ 100,000) exposures, taken from January 2001 to October 2008 in the 6250 Å region, are used to study photospheric properties of the four Hyades giants, focusing on the three signatures of stellar granulation (Gray 2009): line broadening, line asymmetry, and the variation of line core velocities with depth. Values of projected rotational velocity and macroturbulence are obtained. Microturbulence shows a clear tendency to increase with mean photospheric height of formation. Also included are estimates of temperature differences between granular and intergranular regions using flux deficit, and temperature differences between the program stars using line-depth ratios. Measures of convective overshoot velocities, such as the velocity scale are used together with the rotational velocity to estimate the Rossby number for each program star. The results for δ Tauri are inconclusive, however, ϵ Tauri shows a larger Ro than the more active γ and θ 1Tauri, a value which is similar to that of the Sun, and consistent with its previously reported low level of magnetic activity. ϵ Tauri appears to be more massive the other Hyades giants and may have evolved beyond the coronal boundary into an inactive dynamo stage. The contrast of this scenario with conditions in the Sun is discussed.

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