Speaker
Description
Extreme OH/IR stars are in the final phase of Asymptotic Giant Branch evolution, characterised by high mass-loss rates. This gives rise to the creation of circumstellar envelopes of gas and dust of high optical depth, leading to complete obscuration in the optical wavelength range. These stars are likely descendants from intermediate-mass stars, in which hot-bottom burning prevents their conversion to carbon-rich chemistry. This in turn, allows the formation of O-rich molecules, some of them (OH, H$_2$O and SiO) are sources of luminous maser emission. The masers allow to pinpoint these stars across the Galaxy. We recently completed an OH-1612 MHz monitoring program at the Nancay Radio Telescope of about 114 stars making up the 'Bright OH/IR star sample'. We found 82 Mira-like large-amplitude periodic variables with periods of $1.1-6.8$ years, and 30 'non-variable' OH/IR stars showing at most small-amplitude irregular fluctuations. Among the variable stars, 30 OH/IR stars are 'extreme', i.e. their periods exceed 1100 days ($\sim$3 yr). The 'non-variable' OH/IR stars are thought to have left the AGB and entered the early post-AGB phase, while the central stars are not yet visible again. We will discuss the OH/IR star properties in relation to the period-luminosity relation of Mira variables, and in context with current models of AGB evolution and AGB-post-AGB transition.