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Item Molecules in the Zabc = 2.8112 damped system toward PKS 0528-250(2015-03-11) Srianand, R.; Petitjean, PatrickItem Suggestions for an interstellar C7H2 Search(2011-07-06) Chandra, Suresh; Musrif, P. G.; Dharmkare, Ram M.Laboratory detection of the ring-chain molecule c-C7H2 has been reported by McCarthy et al. [McCarthy, M.C., Travers, M.J., Gottlieb, C.A., Thaddeus, P., 1997. A&A 483, L139]. Two ring-chain molecules c-C3H2 and c-C5H2 of this series have already been detected in the cosmic objects. We suggest that the c-C7H2 may be identified in cool cosmic objects through its transitions 414–505,515–606,616–707 and 717–808 at 23.241, 21.105, 18.953 and 16.787 GHz, respectively, in absorption against the CMB. Since, in absence of the availability of collisional rates, we have used scaled values for them, we have checked the sensitivity of the results on the collisional rates, by enhancing the rates for the transitions with Dka = 0 by a factor of 10. Though the transitions are not found sensitive to the collisional rates, our results still may be treated as qualitative in nature. These absorption lines may play an important role for identification of c-C H in cool cosmic objects.Item Molecular hydrogen in high-redshift damped lyman-alpha systems: the VLT/UVES database(2008-02) Noterdaeme, P.; Srianand, R.Aims. We present the current status of ongoing searches for molecular hydrogen in high-redshift (1.8 < zabs ≤ 4.2) Damped Lyman-α systems (DLAs) capitalising on observations performed with the ESO Very Large Telescope (VLT) Ultraviolet and Visual Echelle Spectrograph (UVES). Methods. We identify 77 DLAs/strong sub-DLAs, with log N(Hi) ≥ 20 and zabs > 1.8, which have data that include redshifted H2 Lyman and/or Werner-band absorption lines. This sample of Hi, H2 and metal line measurements, performed in an homogeneous manner, is more than twice as large as our previous sample (Ledoux et al. 2003) considering every system in which searches for H2 could be completed so far, including all non-detections. Results. H2 is detected in thirteen of the systems, which have molecular fractions of values between f ≃ 5 × 10−7 and f ≃ 0.1, where f = N(H2)/(2N(H2) + N(Hi)). Upper limits are measured for the remaining 64 systems with detection limits of typically log N(H2) ∼ 14.3, corresponding to log f < −5. We find that about 35% of the DLAs with metallicities relative to solar [X/H] ≥ −1.3 (i.e., 1/20th solar), with X = Zn, S or Si, have molecular fractions log f > −4.5, while H2 is detected – regardless of the molecular fraction – in ∼ 50% of them. In contrast, only about 4% of the [X/H] < −1.3 DLAs have log f > −4.5. We show that the presence of H2 does not strongly depend on the total neutral hydrogen column density, although the probability of finding log f > −4.5 is higher for log N(Hi) ≥ 20.8 than below this limit (19% and 7% respectively). The overall H2 detection rate in log N(Hi) ≥ 20 DLAs is found to be about 16% (10% considering only log f > −4.5 detections) after correction for a slight bias towards large N(Hi). There is a strong preference for H2-bearing DLAs to have significant depletion factors, [X/Fe] > 0.4. In addition, all H2-bearing DLAs have column densities of iron into dust grains larger than log N(Fe)dust ∼ 14.7, and about 40% of the DLAs above this limit have detected H2 lines with log f > −4.5. This demonstrates the importance of dust in governing the detectability of H2 in DLAs. Our extended sample supports neither the redshift evolution of the detection fraction of H2-bearing DLAs nor that of the molecular fraction in systems with H2 detections over the redshift range 1.8 < zabs ≤ 3.Item Anomalous absorption in thioformaldehyde(2009-10-01) Chandra, Suresh; Kumar, Amit; Sharma, M. K.Absorption against the Cosmic Microwave Background (CMB), called the anomalous absorption, is an unusual phenomenon. The transition 111 110 at 4.829 GHz of formaldehyde (H2CO) was the rst one showing the anomalous absorption. The c-C3H2 is the second molecule showing anomalous absorption through its transition 220 211 at 21.590 GHz. Structure of thioformaldehyde (H2CS) is very similar to that of the H2CO. Therefore, we have investigated about the physical conditions under which the transition 111 110 at 1.0465 GHz of H2CS would be found in anomalous absorption in cool cosmic objects. As in case of H2CO, the anomalous absorption of 111 110 of H2CS is found sensitive to the relative collisional rates and it requires that the collisional rate for the transition 111 211 must be smaller than that for the transition 110 212.