CNO abundances in the Damped Ly-alpha clouds

Abstract

Damped Ly alpha clouds provide information on the chemistry of distant and metal poor regions of the universe. In these clouds for observational difficulties N and O are among the last elements to be measured, and C has still to be measured with precision. In combining the extant CNO abundances, we avail of a sample which includes 33 N measurements or significant limits, a dozen of O measurements, and only few tentative measurements for C. O is found to track both S and Si rather closely without signatures of any peculiar behaviour and the few [O/Zn] ratios available show a mild alpha/iron-peak enhancement which reinforces earlier evidence. The few tentative C measurements point to roughly solar [C/Zn] and [C/Si] ratios. N abundances are rather complex with N/alpha ratios showing a bimodal distribution. The majority of the values form a first plateau at [N/Si]=-0.82 (+/- 0.13) and about 25 % form a second plateau at [N/Si]= -1.45 (+/- 0.05). The high values are at the level of the Blue Compact Dwarfs but they extend further towards lower metallicities. The lower plateau is a new feature of DLAs with no other counterparts. The two plateaux are found for different values of the N abundance with the transition at [N/H] ~ -3.0. We argue that this results from the delayed release of N between massive and intermediate mass stars with the lower plateau produced by massive stars (M> 8 M_sun) only, which accounts for the small dispersion. The systems on the lower plateau are relatively young and imply a continuous formation of the DLAs. While the interpretation of the N/O in the DLAs needs to be confirmed, these observations could ultimately prove crucial for the interpretation of the early N nucleosynthesis.