Planetary Nebulae As A Laboratory For Molecular Hydrogen in the Early Universe

Details Planetary Nebulae As A Laboratory For Molecular Hydrogen in the Early Universe Planetary Nebulae As A Laboratory For Molecular Hydrogen in the Early Universe

Mar 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008sptz.prop50179s&link_type=abstract

Spitzer Proposal ID #50179

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We propose to obtain Spitzer IRS observations of the mid-infrared rotational lines of H2 in planetary nebulae (PNe) with very hot central stars, T > 100,000 K. Our primary motivation is to investigate the excitation and cooling of H2 exposed to UV radiation near very hot stars, which can serve as a proxy for conditions in the early universe. Cosmological simulations show that the first stellar generation (Pop. III) had high masses, > 100 Msun, and hot photospheres. The UV radiation they produced and its effect on the thermal state of the ambient H2 is relevant to subsequent star formation because stellar masses are determined by accretion processes which depend on temperature, and the metal-free primordial gas cooled primarily through excited H2. Yet the effects of this radiative feedback are uncertain: for example, whether it triggers or suppresses further star formation, and the resultant characteristic masses of second generation stars, which are key to cosmic reionization. PNe with hot central stars may be the only place where we can study the relevant microphysics. We therefore propose to obtain Spitzer spectra of such nebulae, sampling regions with a range of gas densities and radiation field dilution factors. We will use the results to derive an improved H2 cooling function to be incorporated into state of the art cosmological models. Our targets have been previously observed in the near-infrared H2 lines, so we have confidence that the lower excitation rotational lines are detectable. Evidence already exists that in some PNe the excited rotational states are overpopulated relative to standard fluorescence models, and that this may be related to the presence of Lyman-continuum photons. The observations proposed here will enable us to verify and quantify this phenomenon, and improve our understanding of H2 excitation. Spitzer is the only facility at present - and for at least the next decade - which can accomplish these goals.

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