The Impact of Large-Scale Surveys on Pulsating Star Research ASP Conference Series, Vol. 203, 2000 L. Szabados & D. W. Kurtz, eds. Driving g-mode Pulsations in 7 Doradus Variables J. A. Guzik, A. B. Kaye, P. A. Bradley, A. N. Cox, C. Neuforge Applied Physics Division, X-2, MS B220, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Abstract. We suggest that the modulation of radiative flux by convec- tion at the base of a deep envelope convection zone during the pulsation cycle is responsible for driving high-order g modes with periods 0.4 to 3 d observed in 7 Doradus variables. 1. Introduction The 7 Doradus variables are a new class of pulsating main sequence stars (Kaye et al. 1999, 2000) located near or beyond the cool edge of the 8 Scuti instability strip. The observed pulsations, with periods of order one day, are believed to be high-order g modes. To explore possible pulsation driving mechanisms, we calculated nonadiabatic pulsation properties of 1.62-M0 evolution models with Y = 0.28 and Z — 0.03, chosen to lie near the 7 Doradus instability region in the HR diagram. We find g-mode pulsation driving consistent with the observations. 2. Results and Conclusions The models considered have relatively deep envelope convection zones, with a temperature at the convection zone base of 177,000 K on the ZAMS. The convec- tion zone deepens during the evolution, reaching a base temperature of 297,300 K at an age of 0.765 Gyr. These evolution models do not include diffusive element settling or radiative levitation. We use the Pesnell (1990) nonadia- batic pulsation code to calculate the i = 0, 1, and 2 pulsation frequencies. We find that the models are unstable to high-order g modes with frequencies be- tween 4 and 25 /LtHz (~ 0.4 to 3 d), coinciding with the observed range of 7 Dor periods. The growth rates (fractional change in mode kinetic energy) for the un- stable modes are between 10~ 4 and 10~ 8 per period. The mode kinetic energy varies by three orders of magnitude over this frequency range, and interestingly reaches a minimum at a frequency of ~ 11 /zHz, or about one day, near the most commonly observed 7 Dor period. The models are stable to pulsations with frequencies between 30 and 150 pHz (0.08 to 0.4 d), but become unsta- ble to 6 Scuti-like p modes at frequencies higher than the radial fundamental. When diffusive settling of helium and heavier elements is included, but radiative levitation neglected, both the envelope convection zone and the helium ioniza- tion zone rapidly disappear, and the models become pulsationally stable. For models without diffusion, all of the pulsation driving occurs at the base of the envelope convection zone, where the opacity is increasing rapidly 445 terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0252921100058358 Downloaded from https://www.cambridge.org/core. IP address: 198.252.38.74, on 14 Jun 2019 at 02:30:09, subject to the Cambridge Core