Prospects for Asteroseismology of Rapidly Rotating B-Type Stars
In rapidly rotating stars, Coriolis forces and centrifugal deformations modify the properties of oscillations; the Coriolis force is important for low-frequency modes, while the centrifugal deformation affects mainly p-modes. Here, we discuss properties o
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Prospects for Asteroseismology of Rapidly Rotating B-Type Stars Hideyuki Saio
Abstract In rapidly rotating stars, Coriolis forces and centrifugal deformations modify the properties of oscillations; the Coriolis force is important for lowfrequency modes, while the centrifugal deformation affects mainly p-modes. Here, we discuss properties of g- and r-mode oscillations in rotating stars. Predicted frequency spectra of high-order g-modes (and r-modes) excited in rapidly rotating stars show frequency groupings associated with azimuthal order m. We compare such properties with observations in rapidly rotating Be stars and discuss what is learnt from such comparisons.
8.1 Oscillations in Main-Sequence B-Stars Thanks to OPAL and OP opacity tables [5, 27], we now understand that radial and nonradial oscillations found in main-sequence B-stars; i.e., β Cephei and SPB (Slowly pulsating B) stars, are excited by the kappa-mechanism associated with the Fe opacity bump at T ∼ 2 × 105 K [21, 23, 28, 36]. Low-order p- and g-modes are excited in β Cephei stars, while high-order g-modes are excited in SPB stars. Figure 8.1 shows their positions in the HR diagram and predicted instability regions; solid and dotted lines are for models without and with core overshooting, respectively. The instability regions bounded by solid lines are roughly consistent with β Cephei (inverted triangles) and SPB (triangles) stars, which are mostly slow rotators. Stellar oscillations give us useful information on the stellar interior that is hard to obtain by other means. For β Cephei stars having low-order p- and g-modes, mode identifications are less ambiguous, so that detailed asteroseismic studies are possible. Comparing observed frequencies with theoretical ones yields best estimates of physical parameters as well as the extent of core-overshooting for each star. In addition, rotational m-splittings of p- and g-modes, which have different depth sensitivity, can be used to measure the strength of differential rotation in the stellar interior. Such asteroseismic analyses have been done for some β Cephei stars, e.g.,
H. Saio (B) Astronomical Institute, Graduate School of Science, Tohoku University, Sendai, Japan e-mail: [email protected] M. Goupil et al. (eds.), Studying Stellar Rotation and Convection, Lecture Notes in Physics 865, DOI 10.1007/978-3-642-33380-4_8, © Springer-Verlag Berlin Heidelberg 2013
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Fig. 8.1 Positions of rapidly rotating Be stars (filled circles) in the HR diagram are shown along with the well-known B-type main-sequence variables SPB (triangles) and β Cep stars (inverted triangles). Big filled circles indicate the Be stars with nonradial pulsations detected by space photometry from the MOST and CoRoT satellites. Parameters of Be stars are taken mainly from [22, 53] and those of SPB stars from [15, 39]. Also shown are theoretical instability boundaries for p- and g-modes. Solid lines for evolutionary tracks and instability boundaries are from models without core overshooting, while dotted lines come
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