No abstract

We calculate and discuss the light-element freezeout and nonthermal reaction nucleosynthesis in highentropy winds and fireballs for broad ranges of entropy per baryon, dynamic timescales characterizing relativistic expansion, and neutron-to-proton ratios. With conditions characteristic of gamma-ray bursts (GRBs), we find that deuterium production can be prodigious, with final abundance values 2 H=He2%, depending on the fireball isospin, late-time dynamics, and the effects of neutron-decoupling-induced highenergy nonthermal nuclear reactions. This implies that there could potentially be detectable local enhancements in the deuterium abundance associated with GRB events.

We present the mid-infrared spectrum of the planetary nebula IC 2448. In order to determine the chemical composition of the nebula, we use the infrared line fluxes from the Spitzer spectrum along with optical line fluxes from the literature and ultraviolet line fluxes from archival IUE spectra. We determine an extinction of C H ¼ 0:27 from hydrogen recombination lines and the radio to H ratio. Forbidden line ratios give an electron density of 1860 cm À3 and an average electron temperature of 12,700 K. The use of infrared lines allows us to determine more accurate abundances than previously possible because abundances derived from infrared lines do not vary greatly with the adopted electron temperature and extinction, and additional ionization stages are observed. Elements left mostly unchanged by stellar evolution (Ar, Ne, S, and O) all have subsolar values in IC 2448, indicating that the progenitor star formed out of moderately metal deficient material. Evidence from the Spitzer spectrum of IC 2448 supports previous claims that IC 2448 is an old nebula formed from a low-mass progenitor star. Subject headingg s: infrared: general -ISM: abundances -ISM: lines and bandsplanetary nebulae: individual (IC 2448) -stars: AGB and post-AGB