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B. M. Lewis (Arecibo Observatory, HC3 Box53995, Arecibo PR00612)
To understand the short, {~}300 year, duration of the 1612 MHz emission phase of most OH/IR stars we infer that the copious mass-loss rate engendering this phase must abruptly switch on and off. Simulations show this can occur when the density in the dust formation zone rises (or falls) through the threshold that enables dust grains to couple photon-momentum to the gaseous shell. So for a dying OH/IR star, such as FV Boo (ApJ 576, 445), this scenario predicts that the gas density in the innermost shell has recently decreased. Is there any direct evidence for this? While radiative-transfer model fits to the SED of FV Boo suggest its decrease occurred {~}20 years ago, a dramatic change in gas density is more directly implied by the evolution of its mainline OH masers.
Since the 1612 MHz masers of FV Boo have an 11 km/sec expansion velocity, the gas density near its center, when it was a normal shell, would quickly return the OH radical to water, as well as sufficing to quench any water or OH masers. The presence of strong mainline (and water) masers with velocities near the stellar velocity thus shows that the density in the inner shell is no longer sufficient to do this. In 1985 the velocity range of the 1665 MHz emission about the stellar velocity also embraced that of the central water maser. However since monitoring started at Arecibo in 1999, the range of the 1665 MHz masers has contracted so that the previous water maser now lie entirely outside it, while the current central water maser still lies within. These observations are consistent with an initial very abrupt change in expansion velocity (and density), which has been followed by a further factor of {~}2 decrease. Moreover, while its 1667 MHz masers were only present at velocities near those of the 1612 MHz peaks prior to 1999, these have since been replaced by new features in proximity to the stellar velocity.
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.