Chemical evolution and depletion pattern in Damped Lyman alpha systems

Dust depletion plays a key role in understanding the nature of Damped Lyman or systems (DLAs). In this paper we point out a previously unnoticed antic orrelation between the observed abundance ratio [X/Zn] (where Zn is assumed to be undepleted and X stands for the refractories Fe, Cr and Ni) and meta l column density ([Zn/H] + log(N-HI)) in DLAs. We suggest that this trend i s an unambiguous sign of dust depletion, since metal column density is a me asure of the amount of dust along the line of sight. Assuming that DLAs are (proto-) galactic disks and using detailed chemical evolution models with metallicity dependent yields we study chemical evolution and dust depletion patterns for ct and iron-peak elements in DLAs. When observational constra ints on the metal column density of DLAs are taken into account las suggest ed in Boisse et al. 1998) we find that our models reproduce fairly well the observed mild redshift evolution of the abundances of 8 elements (Al, Si, S, Cr, Mn, Fe, Zn and Ni) as well as the observed scatter at a given redshi ft. By considering the aforementioned dependence of abundance ratios on met al column density, we further explore the general dust depletion pattern in DLAs, comparing to our model results and to a solar reference pattern. We find that for low metal column densities (no depletion), our models compare fairly well to the data, while a solar pattern has difficulties with Mn. A t high metal column densities (amount of depletion similar to0.5 dex), the solar pattern describes the data quite well, while our models have difficul ties with S. We suggest that further measurements of those key elements, i. e. Zn, S and Mn, will help us to gain more insight into tile nature of DLAs . The presently uncertain nucleosynthesis of Zn in massive stars Ion which a large part of these conclusions is based) should be carefully scrutinised .