Spitzer-IRS's view of Halo PNe

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We propose to observe a sample of 4 Halo planetary nebulae (PNe) using the IRS spectrograph on board Spitzer. The planetary nebula phase is ideal to obtain accurate abundances and the study of halo planetary nebulae offers a great opportunity to understand the chemical evolution of the Galactic halo which is still in debate. Halo PNe are very rare, out of 2000 known PNe in the Galaxy only 10 belong to the halo. They presumably have low progenitor masses and seem to show subsolar oxygen abundance, as well as low abundance of other heavier elements. However, several studies have clearly shown that the oxygen abundance can be strongly affected by nucleosynthesis, rendering the Ar/O and S/O ratios (often used in the literature) not useful. In addition, the study of the abundance of sulfur and argon (elements not suppose to change in the course of evolution of low- and intermediate-mass stars) is hampered by the fact that these elements emit most of their energy in the infrared. For these reasons, The abundances variations from source to source haven't been able to be confirmed. Ground based efforts have been made to measure lines of these elements in the infrared, but only in one halo PN this has been successful. The IRS spectrograph on board the Spitzer Space Telescope covers the ideal wavelength range where most of the lines from these elements are emitted, plus it has the superb sensitivity to study these objects. Because of this, Spitzer provides an ideal opportunity to obtain accurate abundances and give important insight in the halo composition. Moreover, the infrared data offer a good opportunity to study the dust, and the halo PNe provides a perfect laboratory to study the properties of the dust in a low metallicity environment in our own Galaxy.

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