Planetary nebulae (PNs) that evolve from relatively high mass progenitor stars can "masquerade" as low mass objects. We simulate the evolution of PNs and their central stars based on simple models, using various wind speeds and mass loss rates. Even when our nebulae become ionized beyond the characteristic dense inner shell, a faint halo can comprise most of ionized matter while contributing little luminosity. For such PNs, standard techniques severely underestimate ionized mass. Curiously, ionized masses that would be observationally derived for our model nebulae ("Shklovsky masses") are insensitive to variations in the model's input parameters. For evolved PNs, the Shklovsky mass remains a few tenths of a solar mass, despite the total ionized mass varying over two orders of magnitude in our simulations. We show that this is consistent with the range of masses determined for PNs with independent distance estimates. This small mass variance should produce only $\sim$30% distance errors using Shklovsky's constant mass method and may explain why this method is successful despite the incorrect assumption of low ionized mass. We describe a new distance method for PNs based on a theoretical/empirical relationship between their radii and radio surface brightnesses. This method requires only readily available radio flux and angular size measurements. We use Galactic bulge PNs along with PNs with independent distances to establish, calibrate, and test this method. Our distance method appears to yield errors of only $\sim$20% using the best available data. We also find that the Shklovsky method predicts the distances of large, low surface brightness PNs well, but overestimates distances of smaller PNs. We have also made deep radio observations of two PNs, NGC 6804 and NGC 6826, to examine their halo masses. Despite large dynamic ranges, we detect inner halos of both nebulae. Derived halo-to-shell mass ratios demonstrate that the halos contain $>$60% of the total ionized mass while contributing $<$25% of the emission. We further test our distance method by comparison with kinematic distances derived using measured radial velocities of a sample of PNs. Our method agrees with Galactic kinematics within limits of measurement uncertainties and velocity dispersion.
| Identifer | oai:union.ndltd.org:UMASS/oai:scholarworks.umass.edu:dissertations-8963 |
| Date | 01 January 1994 |
| Creators | Buckley, David |
| Publisher | ScholarWorks@UMass Amherst |
| Source Sets | University of Massachusetts, Amherst |
| Language | English |
| Detected Language | English |
| Type | text |
| Source | Doctoral Dissertations Available from Proquest |
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