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B. M. Lewis (Arecibo Observatory)
IRAS 19479+2111 exhibited an idiosyncratic set of 1612 MHz masers with peak intensities of 134 to 271 mJy & velocities of 2 -10 km/s when first searched in May 1987. These had completely disappeared when the object was revisited in June 2000 after the completion of the Arecibo Gregorian upgrade, and were not seen at any of the 10 epochs checked during the next 2.5 yr. Our observations set a <2 mJy limit on the intensity of any 1612 MHz emission during 2000-2002, so the masers had faded by a factor >100 for an interval much longer than the likely 350-450 d pulsation period. This OH/IR star, which only has a 12% IRAS probability of being a MIR variable, was thus considered to be ``dead" (ApJ 576, 445). Nevertheless we now have to report the reappearance since June 2005 of 1612 MHz masers in the same velocity range with evolving intensities of 20 - 90 mJy. Hence the intensity of 1612 MHz masers in OH/IR stars, besides being subjected to a regular factor of 2-3 change with pulsation period, may also exhibit a much larger and longer-period modulation.
The likely explanation for this new phenomenon is the existence of an interaction between the dust formation and mass-loss processes, which was modelled for carbon stars by Simis (A&A 371, 205) to reproduce the ring-like optical-intensity enhancements seen in PPN & PN shells. When this occurs in the circumstellar shell (CS) of a low-mass star, the change in dM/dt can be large enough to switch the maser-pump ON or OFF. A recent time-dependent modelling study of 1612 MHz masers (astro-ph/0509218) shows they disappear <10 yr after mass-loss stops, as a result of changes in the reprocessing of the SED by the dense innermost portion of a CS, which changes the 53 {\mu }m pump. Our observation therefore points to the applicability of Simis’s process to O-rich stars.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.