Cold diffuse interstellar medium of Magellanic Clouds: I. HD molecule and cosmic-ray ionization rate

Abstract

HD molecule is one the most abundant molecule in the Universe and due to its sensibility to the conditions in the medium, it can be used to constrain physical parameters in the medium where HD resides. Lately we have shown that HD abundance can be enhanced in the low metallicity medium. Large and Small Magellanic Clouds give us an opportunity to study low metallicity galaxies in details towards different sightlines due to their proximity to our Galaxy. We revisited FUSE space telescope archival spectra towards bright stars in Magellanic Clouds to search for HD molecules, associated with the medium of these galaxies. We reanalysed H$_2$ absorption lines and constrained HD column density at the positions of H$_2$ components. We detected HD towards 24 sightlines (including 19 new detections). We try to measure cosmic ray ionization rate for several systems using measured $N({\rm HD})/N({\rm H_2})$, and in most cases get loose constraints due to insufficient quality of the FUSE spectra.