Kepler observations of rapidly oscillating Ap, δ Scuti and γ Doradus pulsations in Ap stars

L. A. Balona 2 M. S. Cunha 1 D. W. Kurtz 0 I. M. Brand ao 1 M. Gruberbauer 6 H. Saio 5 R. stensen 4 V. G. Elkin 0 W. J. Borucki 3 J. Christensen-Dalsgaard 7 H. Kjeldsen 7 D. G. Koch 3 S. T. Bryson 3 0 Jeremiah Horrocks Institute of Astrophysics, University of Central Lancashire , Preston PR1 2HE 1 Centro de Astrofisica e Faculdade de Ciencias, Universidade do Porto , 4150 Porto , Portugal 2 South African Astronomical Observatory , PO Box 9, Observatory 7935, Cape Town , South Africa 3 NASA Ames Research Center , MS 244-30, Moffett Field, CA 94035 , USA 4 Instituut voor Sterrenkunde, KULeuven , Celestijnenlaan 200D, 3001 Leuven , Belgium 5 Astronomical Institute, Graduate School of Science, Tohoku University , Sendai 980-8578 , Japan 6 Department of Astronomy & Physics, Saint Mary's University , Halifax, NS B3H 3C3 , Canada 7 Department of Physics and Astronomy , Building 1520 , Aarhus University , 8000 Aarhus C , Denmark A B S T R A C T Observations of the A5p star KIC 8677585 obtained during the Kepler 10-d commissioning run with 1-min time resolution show that it is a rapidly oscillating Ap (roAp) star with several frequencies with periods near 10 min. In addition, a low frequency at 3.142 d1 is also clearly present. Multiperiodic Doradus ( Dor) and Scuti ( Sct) pulsations, never before seen in any Ap star, are present in Kepler observations of at least three other Ap stars. Since Dor pulsations are seen in Ap stars, it is likely that the low frequency in KIC 8677585 is also a Dor pulsation. The simultaneous presence of both Dor and roAp pulsations and the unexpected detection of Sct and Dor pulsations in Ap stars present new opportunities and challenges for the interpretation of these stars. Since it is easy to confuse Am and Ap stars at classification dispersions, the nature of these Ap stars in the Kepler field needs to be confirmed. 1 I N T R O D U C T I O N The Kepler Mission is designed to detect Earth-like planets around solar-type stars (Koch et al. 2010). To achieve that goal, Kepler will continuously monitor the brightness of over 150 000 stars for at least 3.5 yr in a 105-deg2 fixed field of view. Photometric results from the 10-d commissioning run show that micromagnitude precision in amplitude can be attained for the brighter stars (810 mag) for long-cadence (29.4-min) exposures. With this level of precision, interesting pulsational behaviour never seen before is being found in many stars (Gilliland et al. 2010). In addition, Kepler has a small allocation for short-cadence (1-min) exposures. In this mode it is possible to detect and study the light variations of short-period pulsating stars such as solar-like pulsators, Sct stars and rapidly oscillating Ap (roAp) stars. The roAp stars, discovered by Kurtz (1982), are found amongst the coolest subgroup of Ap stars, namely the SrCrEu group (6400 10 000 K). About 40 roAp stars are known at present, with temperatures in the range 6400 Teff 8400 K, and exhibiting either single or multiperiodic pulsations with periods in the range of 5.621 min. These oscillations are interpreted as acoustic modes of low degree and high radial order (typically n > 15), which are modified at the surface layers by strong, large-scale magnetic fields. In that respect they are quite different from the Sct stars which, having similar mass and effective temperature, tend to have oscillations with radial order not exceeding 4 or 5. The difference in the radial orders of the oscillations found in these two classes of pulsators is thought to result from the difference in the regions where the modes are excited. While in roAp stars the oscillations are believed to be excited in the region of hydrogen ionization (Balmforth et al. 2001; Cunha 2002; Saio 2005; Theado et al. 2009), which is partially or fully stabilized against convection by the presence of strong magnetic fields, in Sct stars the excitation takes place in the region of second helium ionization (Dupret et al. 2005). Despite the general understanding of the driving of oscillations in roAp stars, there are still a number of puzzling questions to be answered. Whereas models of hot roAp stars show that driving of pulsations is possible when convection is suppressed, this is not the case for cooler roAp stars. For example, Saio, Ryabchikova & Sachkov (2010) find that all roAp-like pulsations are stable in magnetic models of HD 24712 (HR 1217; DO Eri) in which convection is suppressed. A second puzzling aspect is that apparently nonoscillating Ap stars called noAp stars occupy a similar part of the HertzsprungRussell (HR) diagram as the roAp stars. At present it is not clear what determines which of these peculiar magnetic stars show high radial overtone p-mode pulsation, and which are apparently non-variable. This may either be the result of a selective driving mechanism or simply an observational bias. It is hoped that the Kepler Mission, with its ability to detect much lower amplitude roAp stars than has previously been possible, will illuminate these problems. The global magnetic field which is present in all Ap stars plays an essential role in roAp oscillations, influencing their geometry, frequencies and energy balance. The presence in some roAp stars of frequency multiplets with spacings exactly equal to the angular frequency of rotation led Kurtz (1982) to introduce the oblique pulsator model, according to which the observed pulsations are axisymmetric about the magnetic axis, which is tilted with respect to the rotational axis. This simple picture is challenged by the fact that in some stars the peaks lying symmetrically about the central peak in the multiplet do not have identical amplitudes. This could potentially be explained by the combined effect of rotation and magnetic field on the pulsations, which may break the alignment between the pulsation and magnetic axis (Bigot & Dziembowski 2002; Gough 2005). However, that requires the magnetic and centrifugal effects on the oscillation frequencies to be comparable, something that is not expected except, possibly, for particular combinations of the magnetic field strength and oscillation frequencies (Cunha 2007). Most roAp stars do not show the rotational multiplets predicted by the oblique pulsator model either because the rotational period is too long (and the multiplets unresolved) or because they have amplitudes below the threshold of detectability. Many do, however, show multiple frequencies which in some cases can be interpreted in terms of modes of alternating spherical harmonic degree, , and of consecutive radial orders, n. In the case of linear, adiabatic oscillations, in a spherically symmetric star, at high radial order the frequencies of modes of the same degree are repeated at approximately regular intervals (Tassoul 1980). This interval, defined as the frequency difference between modes of the same but successive radial orders n, = n n1, , is called the large separation, and is a measure (...truncated)