Determination of an Instability Strip of Long Period Variables

点击翻译按钮获取中文摘要

Infrared photometric investigation of Mira variables in the Large Magellanic Cloud has been extensively carried out since the late 1970s. Since stars in LMC are almost equally distant from us, we can discuss some properties of these stars by dealing with only relative luminosity without accurate knowledge of bolometric correction for absolute magnitudes. In this way, astronomers in SAAO obtained observationally a period-luminosity relation of Mira variables in LMC.In the present research project, we show that there should exist a period-mass-luminosity relation rather than a simple period-luminosity relation for Mira variables. A recent observationally obtained period-luminosity relation for these stars should be considered as a projection of the period-mass-luminosity relation on the (period, luminosity)-plane, and the scatter around an observationally fitted line of the period-luminosity relation should be interpreted partially as a mass distribution of the sample stars. We show that … More from this scatter we can deduce the mass and the position of each of the stars in the HR diagram. An observationally obtained period-luminosity relation of Mira variables implies the existence of an instability strip for these stars.We also propose a possible interpretation for the Blazhko effect in the RR Lyrae variables, which has remained to be solved for a very long time. We assume that these stars have a dipole magnetic field, which is oblique to the rotation axis of the star, and examine the effect of this field on the radial fundamental oscillation mode as a small perturbation. We demonstrate that the observed Blazhko effect is well interpreted by taking account of the perturbation to the eigenfunction. We estimate the magnetic field strength necessary to explain the Blazhko effect. This model will be justified if such magnetic fields will be detected. We also apply this model to the rapidly oscillating Ap stars. Most of Ap stars are believed to have, at least, a quadrupole magnetic field in addition to a main dipole component. In this research, we consider a general poloidal magnetic field and investigate its effect upon oscillations as a perturbation. Comparing the present results with the observational data provides us with asteroseimological information of these stars. We finally investigate the stochastic excitation mechanism of oscillations in the solar-like stars, and obtain the frequency range of the excited oscillation modes in a various stars. We expect that the semi-regular and/or irregular luminosity variation in late type stars may be a manifestation of the oscillations excited by this mechanism. Less