Problems with the Pulsation Mode Selection Mechanism in the Lower Instability Strip (Observations and Theory)


ABSTRACT We examine the severe disagreement between the number of predicted and
observed pulsation modes for Delta Scuti stars. The selection of
nonradial modes trapped in the outer envelope is considered on the basis
of kinetic energy arguments. The trapped l=1 modes for the star 4 CVn
are in good, but not perfect agreement with the observations. The
trapping of the l=2 modes is weaker, so that this simple rule of mode
selection may apply to l=1, and possibly not to l=2 modes.

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    ABSTRACT: The Extreme Helium stars are hot luminous stars of about one solar mass that are believed to be shell He-burning. We modeled the pulsations of the hottest ExHe star V2076 Oph, for which the 2000 multisite campaign of Wright et al. detected as many as eight pulsation modes with periods of 0.4 to 2.5 days. Our shallower envelope-only models predict such periods for l=0 and 1. However, if the radiative core is included in the models, a large number of closely-spaced modes are predicted that are not observed, since a large number of g-type nodes are present in the eigenfunctions in the deeper interior. This problem occurs also for models of evolved shell H-burning delta Scuti stars such as 4 CVn.
    Memorie della Societa Astronomica Italiana. 01/2006;
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    ABSTRACT: Mode identification in pulsating stars is challenging because the modes that are predicted to be excited and visible are not all observed, and because sometimes modes that are not expected are observed. In principle, finding rotationally split multiplets can assist mode identification, but often not all of the components are observed, and rapid and differential rotation complicates the interpretation. Other challenges include distinguishing pulsations from star spots, identifying frequencies that are linear combinations of other (perhaps invisible) intrinsic modes, mode coupling, and variable mode amplitudes and frequencies. For brighter stars and modes with high signal-to-noise, spectroscopic and photometric techniques have had some success in separating l = 0, 1 and 2 modes and in identifying the azimuthal orders. The nearly equal frequency (period) spacings for high order p- (g-) mode pulsators expected from asymptotic theory can guide mode identifications. We review theoretical expectations for pulsation mode driving and damping, focusing on main-sequence variables, and compare with observational examples. Insights into mode selection and amplitudes may be possible by examining the energy partition between various processes in these stars and their contributions to driving and damping of the oscillation modes. Future progress will require two- and three-dimensional stellar models and nonadiabatic, nonlinear, and nonradial pulsation modeling.
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    ABSTRACT: Aims:This paper investigates the pulsational behavior of the exceptionally slow rotating delta Scuti star 44 Tau. Methods: During 2000 and 2003 we carried out a photometric observing campaign and obtained 470 h of Strömgren photometry (v and y). Results: We observed 29 pulsational frequencies, from which 16 linear combinations were detected. Amplitude variations were found to be significant for three frequencies. Furthermore a close frequency pair was derived. The pulsational constant Q has been calculated for all frequencies. The radial fundamental and the first overtone were identified based on Q and the positive phase shifts in all three observing seasons. Additionally we found evidence for the presence of g modes. The newly derived value of the projected rotational velocity of v sin i = 2 ± 1 km s-1 is discussed. Conclusions: .Fast rotation can be excluded, but there are no evident proofs to favor slow rotation or pole-on view.
    Astronomy and Astrophysics 01/2007; 463:225-232. · 5.08 Impact Factor

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