We report the detection of H _ { 2 } , C i , C i ^ { \star } , C i ^ { \star \star } and Cl i lines in a near Solar-metallicity ( [ Zn/H ] = -0.13 ) damped Lyman- \alpha ( DLA ) system at z _ { abs } = 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 ] \sim - 0.8 , probably because they are tied up onto dust grains .
Among the three detected H _ { 2 } -bearing clouds , spanning 55 km s ^ { -1 } in velocity space , we derive a total molecular hydrogen column density N ( H _ { 2 } ) = 2.6 \times 10 ^ { 17 } cm ^ { -2 } and a mean molecular fraction f = 2 N ( H _ { 2 } ) / ( 2 N ( H _ { 2 } ) + N ( H i ) ) = 1.7 \times 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 diffuse 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 H _ { 2 } .
The gas clouds in which H _ { 2 } is detected always have large densities , n _ { H } > 30 cm ^ { -3 } , and low temperatures , T _ { 01 } \la 100 K. This shows that presence of dust , high particle density and/or low temperature are required for molecules to be present .
The photo-dissociation rate derived in the components where H _ { 2 } 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 H _ { 2 } -bearing clouds .