Steve V. W. Beckwith

Simultaneous far-infrared, near-infrared, and radio observations of OH/IR stars

Simultaneous far-infrared, near-infrared, and radio observations have been made of five infrared stars which show OH maser emission at 1612 MHz. These stars have very thick circumstellar dust shells and are not seen optically. The data permit a direct comparison of the far-infrared and maser emission from these sources, which strongly supports the hypothesis that the maser emission is pumped by 35 micron photons. A comparison with data obtained at earlier epochs suggests that the maser emission is saturated. The infrared and radio data are used together with estimates of the source distances to determine the luminosities and mass loss rates for these objects. The luminosities lie in the range 2000-30,000 solar luminosities and are consistent with either Mira variable or M supergiant classifications for the underlying stars. The estimated mass loss rates lie between 0.000005-0.00007 solar mass/year.

H_2 morphology of young planetary nebulae

The distributions of H_2 1-0 S(l) emission in the young planetary nebulae BD +30°3639 and NGC 7027 show striking similarities: both have limb-brightened arcs of H_2 emission with radii that are about twice those of their H II regions. The extended H_2 emission in both nebulae is attributed to a photodissociation region. This implies that the neutral envelopes of these young planetaries extend well beyond the edge of the H II region, in contrast to older nebulae where the ionized and molecular gas are more nearly coextensive. The contrast between young and old planetaries can only be explained if the molecular envelope is inhomogeneous. We endorse a scenario for the evolution of a planetary nebula in which a photodissociation front propagates through the clumpy molecular envelope, leaving the ionized core embedded in an envelope of partially ionized atomic gas and dense molecular knots. In an evolved planetary, the H II region has expanded to engulf some of the dense molecular knots, which can be identified with bright [O I] and H_2 1-0 S(l) condensations, while the remnant of the photodissociated envelope may be detected as a faint optical halo.