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Session 80 - Central Stars of Planetary Nebulae.
Display session, Wednesday, January 17
North Banquet Hall, Convention Center
A central star of a planetary nebula (CSPN) is believed to evolve at a constant luminosity. Typical model calculations for a star of about 0.9 M_ødot show that the CSPN maintains a bolometric magnitude of about -6.0, with the remnant reaching an effective temperature in excess of 300,000 K in about 200 years. As the star becomes hotter, the visual magnitude decreases steeply, falling some 10 magnitudes. On the other hand the UV ionizing luminosity approaches the total stellar luminosity by the time the star has reached 100,000 K. A 0.55 M_ødot evolves at a bolometric magnitude of about -4.0 to a temperature of 100,000 K in some 10,000 years. Again the visual magnitude drops rapidly and the nebula brightens. As a consequence of this evolution at constant luminosity, the CSPN of the highest excitation planetaries are difficult to observe. That is to say, a low-mass star will evolve to slowly to become a PN, while a high-mass star will evolve too quickly to be seen.
Current theory suggests that, while of short duration, most stars in this mass range shed their mass back into the interstellar medium in a similar manner and that the hottest stars are the most massive. It has been difficult to verify this since as described above the hottest are also the faintest. Ground-based observations have met with only limited success in testing this theory. In this light, we present Hubble Space Telescope observations of three such PNs: Me 2-1, NGC 2440, and NGC 7027. The data were obtained with both the WF/PC and WFPC2 instruments. All previous investigations have utilized indirect techniques to determine interstellar extinction. Through the spatial resolution and UV imaging capabilities of HST, we are able to utilize a combination of the 2175 Å\ feature fitting method and the stellar colors technique. The results of our photometric analyses are: Me 2-1 --\,\, V = 18.41 \pm 0.1, E(B-V) = 0.15 NGC 2440 --\,\, V = 17.65 \pm 0.2, E(B-V) = 0.20 NGC 7027 --\,\, V = 16.06 \pm 0.1, E(B-V) = 1.15. We discuss the implications of our results from the framework of stellar evolution theory.
Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under contract NAS5-26555.
