c © ESO 2002 Astronomy Astrophysics Detection of molecular hydrogen in a near Solar-metallicity damped Lyman- system at zabs 2 toward Q0551−366?

(DLA) system at zabs = 1:962 observed on the line of sight to the quasar Q 0551−366. The iron-peak elements, X = Fe, Cr and Mn are depleted compared to zinc, [X/Zn] −0:8, probably because they are tied up onto dust grains. Among the three detected H2-bearing clouds, spanning 55 km s−1 in velocity space, we derive a total molecular hydrogen column density N(H2) = 2:6 1017 cm−2 and a mean molecular fraction f = 2N(H2)=(2N(H2)+ N(H i)) = 1:7 10−3. The depletion of heavy elements (S, Si, Mg, Mn, Cr, Fe, Ni and Ti) in the central component is similar to that observed in the diuse neutral gas of the Galactic halo. This depletion is approximately the same in the six C i-detected components independently of the presence or absence of H2. The gas clouds in which H2 is detected always have large densities, nH> 30 cm−3, and low temperatures, T01 < 100 K. This shows that presence of dust, high particle density and/or low temperature is required for molecules to be present. The photo-dissociation rate derived in the components where H2 is detected suggests the existence of a local UV radiation field similar in strength to the one in the Galaxy. Star formation therefore probably occurs near these H2-bearing clouds