Chemical evolution and depletion pattern in Damped Lyman α systems

¹ Shanghai Astronomical Observatory, CAS, Shanghai 200030, PR China e-mail: hjlyx@center.shao.ac.cn
² Institut d'Astrophysique de Paris, 98bis, Bd. Arago, 75014 Paris, France
³ National Astronomical Observatories, CAS, PR China
⁴ Joint Lab. of Optical Astronomy, CAS, PR China
⁵ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK e-mail: boissier@ast.cam.ac.uk

Corresponding author: N. Prantzos, prantzos@iap.fr

Received: 18 October 2000
Accepted: 7 February 2001

Abstract

Dust depletion plays a key role in understanding the nature of Damped Lyman α systems (DLAs). In this paper we point out a previously unnoticed anticorrelation 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 metal column density ([Zn/H] + )) in DLAs. We suggest that this trend is an unambiguous sign of dust depletion, since metal column density is a measure 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 α and iron-peak elements in DLAs. When observational constraints on the metal column density of DLAs are taken into account (as suggested in Boissé 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 redshift. By considering the aforementioned dependence of abundance ratios on metal 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. At high metal column densities (amount of depletion ~0.5 dex), the solar pattern describes the data quite well, while our models have difficulties 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 the nature of DLAs. The presently uncertain nucleosynthesis of Zn in massive stars (on which a large part of these conclusions is based) should be carefully scrutinised.