Ionization properties and elemental abundances in damped Ly alpha systems

We analyze extant data of Al+2, Al+, and other low ions with the aim of stu dying the ionization properties of damped Ly alpha systems (DLAs) from the analysis of the ratio R(Al+2/Al+) equivalent to N(Al+2)/N(Al+). We find the good correlations log N(Al+)-log N(Si+) and log N(Al+)-log N(Fe+) that we use to indirectly estimate N(Al+) from N(Si+) and/or N(Fe+) measurements. I n this way, we determine the ratio R(Al+2/Al+) for a sample of 20 DLAs. Con trary to common belief, the ratio can attain relatively high values (up to 0.6), suggesting that the gas of the intermediate ionization state plays an important role in DLAs. On the other hand, the lack of any trend between a bundance ratios, such as Si/H and Si/Fe and R(Al+2/Al+) indicates that abun dances are not severely influenced by ionization effects. We find a log R(A l+2/Al+)-log N(H-0) anticorrelation that we use in conjunction with idealiz ed photoionization equilibrium calculations to constrain the ionization pro perties and to predict ionization corrections in DLAs. We consider two poss ible origins for the species of the low- and intermediate-ionization state: (1) neutral regions devoid of Al+2 and/or (2) partially ionized, Al+2-bear ing regions. The log R(Al+2/Al+)-log N(H-0) anticorrelation can be naturall y explained in terms of a two-region model with a soft, stellar-type ionizi ng radiation field. We present abundance ionization corrections for 14 elem ents of astrophysical interest derived with different types of ionizing spe ctra. For most of these elements, the corrections are generally below measu rements errors, which is contrary to the predictions of recent models prese nted in the literature. We briefly discuss the potential effects of inaccur acies in the Al recombination rates used in the photoionization calculation s.