A cold metal-poor cloud traced by a weak Mg II absorption at z 0.45

Aims.We present the observations of a weak Mg ii absorption system detected at z ~ 0.452 in the UVES high-resolution spectrum of the QSO HE0001-2340. The weaker of the two Mg ii components forming the system shows associated absorptions due to Si i, Ca i, and Fe i observed for the first time in a QSO spectrum. We investigate the nature of this absorber by comparing its properties with those of different classes of absorbers and reproducing its ionization conditions with photoionization models.