Excitation mechanisms in newly discovered H<SUB>2</SUB>-bearing damped Lyman-&#945; clouds:systems with low molecular fractions

Aims. We probe the physical conditions in high-redshift damped Lyman- &#945;systems (DLAs) using the observed molecular fraction and the rotational excitation of molecular hydrogen. Methods.We search for Lyman- and Werner-band absorption lines of molecular hydrogen in the VLT/UVES spectra of background QSOs at the redshift of known DLAs. Results.We report two new detections of molecular hydrogen in the systems at Zabs = 2.402 and 1.989 toward, respectively, HE 0027-1836 and HE 2318-1107, discovered in the course of the Hamburg-ESO DLA survey. We also present a detailed analysis of our recent H2 detection toward Q 2343+125. All three systems have low molecular fractions, log &#402;&#8804; -4, with f=2N(H2)/(2N(H2)+N(H I)). Only one such H2 system was known previously. Two of them (toward Q 2343+125 and HE 2318-1107) have high-metallicities, [ X/H]&gt; -1, whereas the DLA toward HE 0027-1836 is the system with the lowest metallicity ([ Zn/H]=-1.63) among known H2-bearing DLAs. The depletion patterns for Si, S, Ti, Cr, Mn, Fe and Ni in the three systems are found to be very similar to what is observed in diffuse gas of the Galactic halo. Molecular hydrogen absorption from rotational levels up to J=5 is observed in a single well-defined component toward HE 0027-1836. We show that the width (Doppler parameter) of the H2 lines increases with increasing J and that the kinetic energy derived from the Doppler parameter is linearly dependent on the relative energy of the rotational levels. There is however no velocity shift between lines from different rotational levels. The excitation temperature is found to be 90 K for J=0 to J=2 and &#8764;500 K for higher J levels. Single isothermal PDR models fail to reproduce the observed rotational excitations. A two-component model is needed: one component of low density (&#8764;50 cm-3) with weak illumination (&#935; = 1) to explain the J &#8805; 3 rotational levels and another of high density (&#8764;500 cm-3) with strong illumination (&#935; = 30) for levels. However, the juxtaposition of these two PDR components may be ad-hoc and the multicomponent structure could result either from turbulent dissipation or C-shocks