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Item Molecular hydrogen at Zabx=1.973 towards Q0013 - 004: Dust depletion pattern in damped Lyman - alpha systems(2002-03-06) Petitjean, Patrick; Srianand, R.; Ledoux, C.We study the dust depletion pattern in different well separated components of the Zabs = 1.973, log N(H I) = 20.83, damped Lyman-a system toward Q 0013-004. The apparent correlation between [Fe/S] and [Si/S] in the components indicates that the abundance pattern is indeed due to dust-depletion. In particular, we find evidence for depletion similar to what is observed in cold gas of the Galactic disk in one of the weakest components ([Fe/Zn] = -1.62, [Fe/S]= -1.82, [Zn/S] = -0.2, [Si/S]= -0.92) in which molecular hydrogen is detected with log N(H2) '" 16.5. This is the first time that such depletion is seen in a DLA system. Extinction due to this component is negligible owing to small total HI column density, log N(HI ) ::;;19.4.This observation supports the possibility that current samples of DLA systems might be biased against the presence of cold and dusty gas along the line of sight. The global metallicities of this peculiar DLA system in which 0 I and C II are spread over ",1050 km S-1 are [P/H] = -0.64, [Zn/H] = -0.75 and [S/H] = -0.76 relative to solar. The overall molecular fraction is in the range -2.7 < log f < -0.6. which is the highest value found for DLA systems. H2 is detected in four components at -625, -475, 0 and 80 km S-1 relative to the strongest component at Zabs = 1.97296.CO is not detected (log N(CO)/ N(H r) < -8) and HD could be present at Zabs = 1.97380. We show that the presence of H2 is closely related to the physical conditions in the gas: high particle density together with low temperature. Excitation of high J levels and molecular fraction vary largely from one component to the ot.her suggesting that the UV radiation field is highly inhomogeneous through the system. Gas pressure, estimated from C I absorptions, is larger than what is observed in the ISM of our Galaxy. This, together with the complex kinematics, suggests that part of the gas is subject to high compression due to either collapse, merging and/or supernovae explosion. This is probably a consequence of star-formation activity in the vicinity of the absorbing gasItem Detection of molecular hydrogen in a near solar-metallicity damped Lyman - alpha system at Zabc ~ 2 toward Q 0551 - 366 Authors: Ledoux, C.(2002-05-01) Ledoux, C.; Srianand, R.; Petitjean, PatrickWe report the detection of H2, C I, C 1*, C I ** and Cl I lines in a near Solar-metallicity ([Zn/H] = -0.13) damped Lyman-a (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 Hs-bearing clouds, spanning 55 km S-l in velocity space, we derive a total molecular hydrogen column density N(H2) = 2.6 X 1017 cm- 2 and a mean molecular fraction f = 2N(H2)/(2N(H2) + N(H I» = 1.7 x 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 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 are 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.Item VLT-UVEs survey for molecular hydrogen in high-redshift damped lyman-alfa system(2011-07-05) Ledoux, C.; Petitjean, Patrick; Srianand, R.We have searched for molecular hydrogen in damped Lyman-α (DLA) and sub- DLA systems at high redshift (zabs > 1.8) using UVES at the VLT down to a detection limit of typically N(H2) = 2×1014 cm−2. Out of the 33 systems in our sample, 8 have firm and 2 have tentative detections of associated H2 absorption lines. Considering that 3 detections were already known from past searches, molecular hydrogen is detected in 13 to 20 percent of the newly-surveyed systems.We report new detections of molecular hydrogen at zabs = 2.087 and 2.595 toward, respectively, Q1444+014 and Q0405−443, and also reanalyse the system at zabs = 3.025 toward Q0347−383. In all of the systems, we measure metallicities relative to Solar, [X/H] (with either X=Zn, or S, or Si), and depletion factors of Fe, [X/Fe], supposedly onto dust grains, and compare the characteristics of our sample with those of the global population of DLA systems (60 systems in total). We find that there is a correlation between metallicity and depletion factor in both our sample and also the global population of DLA systems. Although H2 molecules are detected in systems with [Zn/Fe] as small as 0.3, the DLA and sub-DLA systems where H2 is detected are usually amongst those having the highest metallicities and the largest depletion factors. In particular, H2 is detected in the five systems having the largest depletion factors. Moreover, the individual components where H2 is detected have depletion factors systematically larger than other components in the profiles. In two different systems, one of the H2- detected components even has [Zn/Fe] > 1.4. These are the largest depletion factors ever seen in DLA systems. All this clearly demonstrates the presence of dust in a large fraction of the DLA systems. The mean H2 molecular fraction, f = 2N(H2)/[2N(H2) + N(Hi)], is generally small in DLA systems (typically log f < −1) and similar to what is observed in the Magellanic Clouds. There is no correlation between the observed amount of H2 and the Hi column density. In fact, two systems where H2 is detected have logN(Hi) < 20.3 and, therefore, are sub-DLA systems. From 58 to 75 percent of the DLA systems have log f < −6. This can be explained if the formation rate of H2 onto dust grains is reduced in those systems, probably because the gas is warm (T > 1000 K) and/or the ionizing flux is enhanced relative to what is observed in our Galaxy.Item Probing the time-variation of the fine-structure constant: Results based on Si IV doublets from a UVES sample(2011-07-06) Chand, Hum; Petitjean, Patrick; Srianand, R.; et al.We report a new constraint on the variation of the fine-structure constant based on the analysis of 15 Si iv doublets selected from a ESO-UVES sample. We find ∆α/α = (+0.15 ± 0.43) × 10−5 over a redshift range of 1.59 ≤ z ≤ 2.92 which is consistent with no variation in α. This result represents a factor of three improvement on the constraint on ∆α/α based on Si iv doublets compared to the published results in the literature. The alkali doublet method used here avoids the implicit assumptions used in the many-multiplet method that chemical and ionization inhomogeneities are negligible and isotopic abundances are close to the terrestrial value.