Real-time difference imaging analysis of MOA Galactic bulge observations during 2000

We describe observations carried out by the MOA group of the Galactic bulge during 2000 that were designed to detect efficiently gravitational microlensing of faint stars in which the magnification is high and/or of short duration. These events are particularly useful for studies of extrasolar planets and faint stars. Approximately 17 deg² were monitored at a sampling rate of up to six times per night. The images were analysed in real time using a difference imaging technique. 20 microlensing candidates were detected, of which eight were alerted to the microlensing community whilst in progress. Approximately half of the candidates had high magnifications (≳10), at least one had very high magnification (≳50), and one exhibited a clear parallax effect. The details of these events are reported here, together with details of the on-line difference imaging technique. Some nova-like events were also observed and these are described, together with one asteroid.     

Introduction

A historical sketch of the nonlinear theory of variable stars is outlined briefly. The main break-through came from the hydrodynamic study of stellar pulsation. From a theoretical point of view, coupling needs to be discussed more carefully. The impact of new opacities on the astrophysical problems is also discussed.     

Oscillations de l'atmosphère de l'étoile pulsante

Résumé On étudie les oscillations non-adiabatiques d'une atmosphère stellaire sous l'approximation du premier ordre, en déduisant les solutions générales des équations différentielles. On confirme les conditions aux limites de la pulsation d'étoile. Pour les étoiles variables des supergéantes rouges l'onde progressive est possible dans l'atmosphère, avec l'oscillation stationnaire.

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Non-adiabatic linear oscillations of the stellar atmosphere are studied by deriving the general solution of the equation of the problem. Boundary conditions which we obtained at the stellar surface are confirmed for the pulsation of the cepheid variables. It is shown that the progressive wave coupling with the stellar pulsation is possible in the atmospheres of red supergiants.

     

The effect of synchronization on the modal selection in classical cepheids

The coupled self-exciting oscillator model is investigated in the non-resonant case and applied to classical cepheids. The modal selection in these models is explained as the result of the synchronization caused by the mutual interaction between different modes. By using linear adiabatic coupling coefficients, it is shown that the fundamental mode suppresses the first overtone mode marginally in short-period classical cepheids. The double periodicity of several cepheids is expected as the result of a small change of physical state in the outer envelopes of these stars.     

Study of time-evolving hydrodynamic cepheid models

For studying modal coupling, we ran the hydrodynamic models of cepheids and observed the growth and disappearance of various modes in their pulsation. The existence of modal coupling has been confirmed.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

The coupling coefficients of pulsation for radiative stellar models

The second order theory of coupling is discussed regarding the radial pulsation of stellar models which are constructed ignoring convection. The formula including the nonadiabatic effect is presented. Numerical values given for model classical cepheids are considerably greater than the adiabatic values.     

A discrete-dynamic study of pulsating stars

A sequence of successive kinetic energy maxima at expanding phases of a pulsating star is considered. Transition from strictly periodic stellar pulsation to irregular one can be explained by the change in the rate of dissipation.     

The cause of intermittent outbursts in radiative hydrodynamic stellar models

The cause of intermittent outbursts found in the pulsations of radiative hydrodynamic stellar models is discussed. The effect of radiation pressure, which will not yield the outbursts in convective models, can be a candidate process for the phenomena.     

An accretion disc surrounding a component of epsilon aurigae

The low-mass model of Epsilon Aurigae consisting of an F supergiant star a Main-Sequence B star which is surrounded by a cool accretion disc has been investigated. The disc requires a high accretion rate if we assume the disc to be completely heated up by friction alone. On the other hand, discs with low accretion rates are possible when the disc are heated up from the radiation of both stars. A swollen crescent portion produced by the radiation of the F star may form a bulky body sufficient to obscure the radiation of the F star.Astronomical Institute, Tohoku University, Sendai, Japan.