Tiny-scale molecular structures in the Magellanic Clouds. I. FUSE, HST and VLT observations

International audienceThe objective of this series of two papers is to investigate small-scale molecular structures in the Magellanic Clouds (hereafter MCs). We report on the FUSE detections of the HD and CO molecules on the lines of sight towards three Large Magellanic stars: Sk -67D05, Sk -68D135, and Sk -69D246. HD is also detected for the first time on the lines of sight towards two Small Magellanic Cloud stars: AV 95 and Sk 159. While the HD and CO abundances are expected to be lower in the Large Magellanic Cloud where molecular fractions are a third of the Galactic value and where the photodissociation flux is up to thousands of times as large, we report an average HD/H2 ratio of 1.4±0.5 ppm and a CO/H2 ratio ranging from 0.8 to 2.7 ppm similar to the Galactic values. We tentatively identify a deuterium reservoir (hereafter D-reservoir) towards the Small Magellanic Cloud, along the light path to AV 95. We derive a D/H ratio ranging from 1.× 10-6 to 1.1× 10-5. Combining FUSE and HST/STIS data we also analyzed the H2, Cl I, Cl II, Fe II, S II, C I, C I*, and C I** content, when available. High resolution VLT observations of Na I, Ca II, and Ca I were obtained in support of the lower resolution FUSE and STIS data for three targets in order to unravel the sightline velocity structures. These observations form the only such set of detections in the Magellanic Clouds to date and allow us to investigate in detail some of the physical properties of the intervening molecular gas. Our detection of the HD and CO molecules in the Magellanic Clouds is an argument for the existence of dense (nH > 100 cm-3) components. Furthermore, we demonstrate that these components are probably extremely small molecular clumps (possibly as small as 10-2 pc) or filaments similar to the tiny-scale atomic structures (TSAS) recently observed in the halo of our Galaxy by Richter et al. (\cite{Richter2003}). For these five sightlines, we also detected molecular hydrogen originating in the Galactic disk. From these observations we conclude that tiny-scale molecular filamentary structures are present in the disk of the Galaxy as well