Journal Article

Low frequencies in <i>Kepler</i> δ Scuti stars

L. A. Balona

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 437, issue 2, pages 1476-1484
Published in print January 2014 | ISSN: 0035-8711
Published online November 2013 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1093/mnras/stt1981
Low frequencies in Kepler δ Scuti stars

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We use Kepler photometry to examine the properties of low-frequency oscillations in A stars. We find that multiple peaks in the range 0–5 d−1 are found in nearly all δ Scuti stars, even those well outside the blue edge of the γ Doradus instability strip. All δ Sct stars are essentially δ Sct/γ Dor hybrids. The reason why this fact was not recognized from ground-based observations is due to the fact that the low frequencies reach maximum amplitude close to the blue edge of the γ Dor instability strip. Stars which do not have high frequencies characteristic of δ Sct pulsations also do not have low frequencies. The amplitudes of the low frequencies correlate with the amplitudes of the δ Sct pulsations. The low frequencies have long lifetimes similar to those of the δ Sct modes. They are therefore not excited by a stochastic mechanism. We show that the low frequencies cannot be explained as non-linear combinations of δ Sct modes. By comparing the observed distribution of frequencies with those from δ Sct models, we show that low-frequency peaks cannot be explained as a result of rotational perturbation of high-frequency δ Sct modes. We find that the number of low-frequency peaks increases with rotation, but Rossby and Kelvin modes can be ruled out as possible candidates. The presence of low frequencies in A stars would be explained if the convective blocking mechanism continues to be active even in the hottest A stars.

Keywords: stars: oscillations; stars: rotation; stars: variables: δ Scuti

Journal Article.  7141 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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