Person:

Dalgarno, Alexander

A new and more comprehensive model of charge-exchange induced X-ray emission, due to ions precipitating into the Jovian atmosphere near the poles, has been used to analyze spectral observations made by the Chandra X-ray Observatory. The model includes for the first time carbon ions, in addition to the oxygen and sulfur ions previously considered, in order to account for possible ion origins from both the solar wind and the Jovian magnetosphere. By comparing the model spectra with newly reprocessed Chandra observations, we conclude that carbon ion emission provides a negligible contribution, suggesting that solar wind ions are not responsible for the observed polar X-rays. In addition, results of the model fits to observations support the previously estimated seeding kinetic energies of the precipitating ions (~0.7-2 MeV u(^{–1})), but infer a different relative sulfur-to-oxygen abundance ratio for these Chandra observations.

The hydrogen negative ion plays a crucial role in the formation of hydrogen molecules in the early universe. Cooling through excitation of (H_2) drives the formation of the first cosmological objects. The (H_2) molecules are produced primarily by a reaction sequence initiated by (H^–). We explore the influence of enhanced photodestruction rates that arise due to absorption by resonance states of (H^–) lying near 11 eV. We examine the feedback effects that occur in radiation fields characteristic of Population III stars, blackbody sources, power-law spectra, and the hydrogen Lyman modulated sawtooth spectra of the high-redshift intergalactic medium.

The cosmological recombination history of lithium, produced during big bang nucleosynthesis, is presented using updated chemistry and cosmological parameters consistent with recent cosmic microwave background (CMB) measurements. For the popular set of cosmological parameters, about a fifth of the lithium ions recombine into neutral atoms by a redshift z ~ 400. The neutral lithium atoms scatter resonantly the CMB at 6708 Å and distort its intensity and polarization anisotropies at observed wavelengths around ~300 μm, as originally suggested by Loeb. The modified anisotropies resulting from the lithium recombination history are calculated for a variety of cosmological models and found to result primarily in a suppression of the power spectrum amplitude. Significant modification of the power spectrum occurs for models that assume a large primordial abundance of lithium. While detection of the lithium signal might prove difficult, it offers the possibility of inferring the lithium primordial abundance and is the only probe proposed to date of the large-scale structure of the universe for z ~ 500-100.