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We report the detection of H_2, CI, CI*, CI** and Cl I lines in a nearSolar-metallicity ([Zn/H]=-0.13) damped Lyman-alpha (DLA) system at z_abs=1.962observed 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 becausethey 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 hydrogencolumn density N(H_2)=2.6x10^17 cm^-2 and a mean molecular fractionf=2N(H_2)/(2N(H_2)+N(HI))=1.7x10^-3. The depletion of heavy elements (S, Si,Mg, Mn, Cr, Fe, Ni and Ti) in the central component is similar to that observedin the diffuse neutral gas of the Galactic halo. This depletion isapproximately the same in the six CI-detected components independently of thepresence or absence of H_2. The gas clouds in which H_2 is detected always havelarge densities, n_H>30 cm^-3, and low temperatures, T_01<~100 K. This showsthat presence of dust, high particle density and/or low temperature arerequired for molecules to be present. The photo-dissociation rate derived inthe components where H_2 is detected suggests the existence of a local UVradiation field similar in strength to the one in the Galaxy. Star formationtherefore probably occurs near these H_2-bearing clouds.Comment: 9 pages, 7 figures, to appear in A&A; added reference in Sect.

Detection of molecular hydrogen in a near Solar-metallicity damped Lyman-α system at $z_\mathsf{abs}$ ≈ 2 toward Q 0551-366