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A&A 385, 802-815 (2002)
DOI: 10.1051/0004-6361:20020198

Dust depletion and abundance pattern in damped Ly $\alpha$ systems: A sample of Mn and Ti abundances at $z\, < $ 2.2

C. Ledoux1, J. Bergeron1 and P. Petitjean2, 3

1  European Southern Observatory, Karl-Schwarzschild Straße 2, 85748 Garching bei München, Germany
    e-mail: cledoux@eso.org;jbergero@eso.org
2  Institut d'Astrophysique de Paris, 98bis Boulevard Arago, 75014 Paris, France
    e-mail: petitjean@iap.fr
3  DAEC, Observatoire de Paris-Meudon, 92195 Meudon Principal Cedex, France

(Received 3 August 2001 / Accepted 4 February 2002)

Abstract
We analyse a sample of 24 damped Lyman- $\alpha$ (DLA)/moderate DLA systems at intermediate redshifts, $0.3<z_{\rm abs}<2.2$, all with measurement of the weak Mn II absorption lines, to investigate which elemental ratios could possibly be used as tracers of either dust depletion or nucleosynthesis effects. We applied a component-by-component analysis to the five systems of the sample with new observations and, using this procedure, re-analyzed data gathered from the literature whenever possible. We show that the standard method which uses column densities integrated over the whole absorption profiles could substantially underestimate the abundance of rare elements relative to Fe. We find a correlation between the observed [Si/Fe] and [Zn/Fe] ratios, present in our sample at the $2.9\sigma$ significance level. This correlation is fully consistent with a dust depletion sequence only for a Galactic warm disk cloud or halo cloud depletion pattern. The correlation between [Mn/Fe] and [Zn/Fe], detected at the $3.2\sigma$ significance level, cannot be accounted for by any dust depletion sequence: it implies either variations of the intrinsic Mn abundance relative to Fe from -0.3 to +0.1 dex and/or a relation between depletion level and metallicity. The correlation between [Mn/Fe] and metallicity ( $2.6\sigma$ significance level) strengthens the assumption of intrinsic variations of [Mn/Fe] although some marginal correlation between [Zn or Si/Fe] and [Zn/H] is present as well. Extension of the sample toward low metallicity is needed to confirm the correlation between depletion level and metallicity. The variations of [Ti/Fe] vs. [Zn/Fe] cannot be fitted by a single dust depletion sequence either. We then adopt a warm disk cloud or halo cloud depletion pattern and compare the resulting dust-corrected abundance ratios to those observed in Galactic and SMC stars. At high metallicity, [ Fe/H $]_{\rm dc}\ga -0.5$, the intrinsic abundance pattern of Si, Ti, Cr and Mn in DLA absorbers closely follows the trends observed in Galactic stars and these absorbers should thus have a chemical evolution similar to that of our Galaxy. At lower metallicity, some absorbers do follow the trends present in Galactic stars but a substantial fraction of them have elemental ratios (in particular [ Si/Fe $]_{\rm dc}$ and [ Mn/Fe $]_{\rm dc}$) closer to the solar values than Galactic stars. This could be explained by a larger contribution of type Ia supernovae to the chemical enrichment of these DLA absorbers than in Galactic stars of similar metallicity. This metal-poor DLA absorber population could trace H I-rich dwarf galaxies.


Key words: cosmology: observations -- galaxies: halos -- galaxies: ISM -- quasars: absorption lines

Offprint request: C. Ledoux, cledoux@eso.org

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