Pulsation of the lambda Bootis stars HD 111786 and HD 142994

Abstract

The detection of possible nonradial pulsation modes among some lambda Bootis stars offers the prospect of using asteroseismology to determine their masses and ages, thereby testing competing theories for the origin of their chemical peculiarities. As a step toward this goal, we conducted multi-site photometric campaigns spanning two weeks each for two lambda Bootis stars already known to show oscillations with periods from 0.75 to 4 hr: HD 111786} \def\2{HD 142994 and \2. Comparison of the observed eigenfrequency spectrum with model eigenmode spectra can constrain the age of the star. Since two of the main proposed mechanisms for the lambda Bootis phenomenon (mass loss coupled with diffusion, and accretion) predict widely different stellar ages, such an analysis could distinguish between these options. Frequency analysis of campaign data yielded four frequencies for each program star. The best matches of our observed eigenfrequencies to Main Sequence stellar models (which include the effects of rotation on the stellar structure and the pulsation modes) suggest that both stars are too far from the Zero-Age Main Sequence, to be consistent with the widely favoured accretion scenario. However, we caution that the results are preliminary and suggest future observing strategies to confirm/deny this finding. Based on observations obtained at CTIO, SAAO and with the Hipparcos satellite