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HST/COS Detection of Deuterated Molecular Hydrogen in a DLA at z = 0.18

We report on the detection of deuterated molecular hydrogen, HD, at $z = 0.18$. HD and H$_{\rm 2}$ are detected in HST/COS data of a low metallicity ($Z \sim 0.07Z_\odot$) damped Ly$α$ system at $z = 0.18562$ toward QSO B0120$-$28, with log $N$(H I) = 20.50 $\pm$ 0.10. Four absorption components are clearly resolved in H$_{\rm 2}$ while two components are resolved in HD; the bulk of the molecular hydrogen is associated with the components traced by HD. We find total column densities log $N$(HD) = 14.82 $\pm$ 0.15 and log $N$(H$_{\rm 2}$) = 20.00 $\pm$ 0.10. This system has a high molecular fraction, $f$(H$_{\rm 2}$) = 0.39 $\pm$ 0.10 and a low HD to H$_{\rm 2}$ ratio, log (HD/2H$_{\rm 2}$) $= -5.5 \pm 0.2$ dex. The excitation temperature, $T_{01} = 65 \pm 2$ K, in the component containing the bulk of the molecular gas is lower than in other DLAs. These properties are unlike those in other higher redshift DLA systems known to contain HD, but are consistent with what is observed in dense clouds in the Milky Way.