Astronomy and Astrophysics – Astrophysics
Scientific paper
2003-10-30
Astron.Astrophys. 415 (2004) 879-884
Astronomy and Astrophysics
Astrophysics
7 pages, 5 figures accepted by A&A
Scientific paper
10.1051/0004-6361:20034584
The Damped Ly-alpha (DLA) absorber at redshift z_abs=4.383 observed toward QSO BRI 1202-0725 is studied by means of high resolution (FWHM ~ 7 km/s) VLT-UVES spectra. We refine a previously determined Si abundance and derive with confidence abundances for C, N and O which are poorly known in DLAs. The [O/Fe] ratio is ~0.6, but we cannot establish if iron is partially locked into dust grains. The [C/Fe]=0.08 \pm 0.13 and [Si/C]=0.31 \pm 0.07. [N/O] and [N/Si] are about -1, which is consistent with the majority of DLAs. This value is much larger than the one observed for the DLA toward QSO J0307-4945 at z_abs= 4.466. The current interpretation of the bimodal distribution of N abundances in DLAs implies that large [N/alpha] values correspond to relatively old systems. Adopting a scale time of 500 Myrs for full N production by intermediate mass stars, the onset of star formation in our DLA took place already at redshift z> 6.
D'Odorico Valentina
Molaro Paolo P.
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