Study on Evolution of OH/IR Star by Modelling

AGB phase is the last evolutionary stage in the lifetime of a nuclear energy supported star.Many interesting processes such as thermal pulse,periodically varibility caused by pulsating,large mass lose rate,strong infrared excess,molecular maser etc.,occur on the AGB stars.Astronomers have casted a great amount of concerning on the evolutionary property of AGB stars in the past decades.As we have known,AGB stars will experience a set of so called Thermal Pulses and its mean luminosity and mean mass loss rate both increase,then,after the stopping of Thermal Pulse and mass loss,they will begin to evolve towards Planetary Nebulae.But we are still not aware much of the details of AGB evolution.In the past,astronomers used many indirect methods to study the AGB evolution,and succeeded to find some overall properties of the evolution process.Now we have accomplished a huge amount of model calculations to study the property of the radiation from the envelope of OH/IR stars,a special evolutionary stage of AGB.And more importantly,we have collected 21 OH/IR stars with good-quality phase-lag distance to study the evolutionary property of their luminosity and mass loss rate.In our study,besides the comformity of double shell stage of OH/IR star evolution,we also find that a OH/IR star well appear as a non-thermal-pulse star in the early part of the OH/IR stage,and then it will step into the Thermal Pulse phase at a period of about 600 days accompanying with a sudden dramatic drop of its luminosity and mass loss rate at the very beginning of the first thermal pulse.We have tried to attribute the sudden drop of luminosity and mass loss rate to the destroy of the H burning layer in the center star by the thermal pulse and the destroyed H burning layer can never recover to its thermal pulse free status. Our study may have opened a new gate into the research work on the detailed evolution process of OH/IR stars.