Gemini South Adaptive Optics Imager (GSAOI)
Missing Mass in Magellanic Cloud Planetary Nebulae:
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Despite vast increase in our understanding of the evolutionary nature
of planetary nebulae (PNe) during the last three decades, a central
question remains: where is the missing mass? Theoretical evolutionary
tracks predict that all stars with masses in the range 1.4-8
MSun should become PNe. According to the models,
much of the mass of these stars is lost during a series of thermal
pulses in the asymptotic giant branch (AGB) phase of
evolution. Observations reveal that the central star mass is close to
0.6 MSun, except for a few Peimbert Type I nebulae
(e.g., NGC 6302 with 0.8 MSun, or a handful of
objects in the LMC). However, the ionized nebular mass of the PN is
typically of order 0.1 MSun, and the derived mass is
found to depend very strongly on the radius of the nebula. Clearly,
much of the mass of the planetary nebula shell must remain un-ionized,
and much of it may be in molecular form. This molecular gas has been
mapped in the 110.2 GHz (1-0) transition of 12CO in a
number of nearby PNe. These observations confirm the much greater
extent of the nebula in molecular gas. In the infrared, the nebula can
be mapped in the 2.122 µm 1-0 S(1) and 2.248 µm 2-1 S(1) lines
of molecular hydrogen, which in most objects seem to be fluorescently
excited by UV radiation from the central star. Generally speaking, the
size of the observed nebula is larger, and the estimated molecular
hydrogen masses much greater, than the ionized gas component. The
nebula of NGC 7027 is a splendid example of this.
For Galactic objects, accurate mass inventories are bedeviled by uncertainties in the PN distance scale, which can only be resolved by the study of a population of PN at a known distance and having low field reddening. The Magellanic Cloud PNe are ideal for this. At optical wavelengths, the Hubble Space Telescope (HST) has been used to systematically investigate the morphologies and ionization of the ionized component. These data typically have a spatial resolution of a little better than 0.1", which corresponds to a linear resolution of ~ 0.02 pc at the distance of the LMC. This is quite sufficient to reveal the internal morphology, given that the typical diameter of a PNe is about 0.1 pc and some objects are as large as 1.0 pc across including the faint outer structure. With its superb spatial resolution, the GSAOI instrument will be ideally suited to perform a systematic study of both the PNe and the proto-planetary nebulae in the Magellanic Clouds at a spatial resolution that matches the observations that have been made by HST of the ionized gas components. The quality of the images that could be obtained would be comparable with the NICMOS images that HST has obtained of PNe towards the Galactic center. Not only can the extent and distribution of the molecular hydrogen be determined, but the PNe can also be mapped in the [Fe II] 1.644 µm line, which in some PNe reaches an intensity in excess of 2×10-14 erg cm-2 s-1 arcsec-1 and which traces the positions of shocks driven into the molecular shell by the high pressure of the ionized zone and fast winds that have shaped the PNe morphology. For the Magellanic Cloud PNe, these data will enable us to derive quantitative data that cast direct light on the evolution of PNe and on their AGB precursors. The positions of the central stars on the Hertzsprung-Russell diagram are known, we can distinguish between H-burning and He-burning stars, and the dynamical ages of the nebulae can be determined for these PNe. |