Theoretical Models of γ Doradus Variables

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The γ Doradus variables are a newly-discovered class of variable stars having spectral types between A7 and F5, luminosity classes of IV, IV -V or V (Kaye et al. 1998). The physical origin of the variations present in these new variables is as yet unknown, although both photometric and spectroscopic observations (Hatzes 1998; Zerbi et al. 1997; Kaye 1998) strongly suggest that high-order (n), low-degree (l), non-radial pulsations are the cause; starspot models were originally proposed, but they fail to explain the multiperiodicity, period stability, and phase information seen in at least some γ Doradus stars. Given that we are observing non-radial pulsations and that the periods range from about 0.5 to 3 days, the pulsations must be g-modes, because p-modes for models corresponding to γ Doradus variables should have periods shorter than about an hour. Given this information, Gautschy and Loffler (1996) computed evolutionary models of stars appropriate to γ Doradus and analyzed them for pulsational stability. All of their models (Z = 0.02) are pulsationally stable and their studies show no suggestions for instability in “normal” models for γ Doradus variables. However, recent observations have better defined the temperatures and luminosities of γ Doradus stars [see, e.g., Villa & Breger (1998); ESA (1997)] and the theoretical work on evolutionary models including diffusion that are appropriate to A and F stars (Turcotte et al. 1998) suggest possible ways that pulsational instability might occur in these otherwise “stable” stars. We use the bona fide γ Doradus variable 9 Aurigae as an example star for our theoretical modeling efforts; in this paper, we present the results of our preliminary modeling and our thoughts on the pulsation mechanism and its physical origins.