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A new molecular hydrogen cloud is found in the sub-damped Ly-alpha absorber[log N(HI)=19.88+/-0.05] at the redshift z=1.15 toward the bright quasarHE0515-4414 (zem = 1.71). More than 30 absorption features in the Lyman bandsystem of H2 are identified in the UV spectrum of this quasar obtained with theSpace Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope.The H2-bearing cloud shows a total H2 column density N(H2)=(8.7^{+8.7}_-{4.0})10^16 cm^-2, and a fractional molecular abundance f(H2)=(2.3^{+2.3}_{-1.1})10^-3 derived from the H2 lines arising from the J=0-5 rotational levels of theground electronic vibrational state. The estimated rate of photodissociation atthe cloud edge I_0<=1.8 10^{-8} s^-1 is much higher than the mean Galactic diskvalue, I_MW~=5.5 10^{-11} s^-1. This may indicate an enhanced star-formationactivity in the z=1.15 system as compared with molecular clouds at z~=3 whereI~=I_MW. We also find a tentative evidence that the formation rate coefficientof H2 upon grain surfaces at z=1.15 is a factor of 10 larger than a canonicalMilky Way value, R_MW~=3 10^-17 cm^3 s^-1. The relative dust-to-gas ratioestimated from the [Cr/Zn] ratio is equal to k=0.89+/-0.19 (in units of themean Galactic disk value), which is in good agreement with a high molecularfraction in this system. The estimated line-of-sight size of L~=0.25 pc mayimply that the H2 is confined within small and dense filaments embedded in amore rarefied gas giving rise to the z=1.15 sub-damped Ly-alpha absorber.Comment: 10 pages, 7 figures, accepted for publication in A&

Detection of molecular hydrogen atz = 1.15 toward HE 0515–4414