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We report two detections of deuterated molecular hydrogen (HD) in QSOabsorption-line systems at $z > 2$. Toward J2123-0500, we find $N$(HD) $= 13.84\pm 0.2$ for a sub-DLA with metallicity $\simeq 0.5Z_{\odot}$ and $N$(H$_2$) =$17.64 \pm 0.15$ at $z = 2.0594$. Toward FJ0812+32, we find $N$(HD) $= 15.38\pm 0.3$ for a solar-metallicity DLA with $N$(H$_2$) = $19.88 \pm 0.2$ at $z =2.6265$. These systems have ratios of HD to H$_2$ above that observed in denseclouds within the Milky Way disk and apparently consistent with a simpleconversion from the cosmological ratio of D/H. These ratios are not readilyexplained by any available model of HD chemistry and there are no obvioustrends with metallicity or molecular content. Taken together, these two systemsand the two published $z > 2$ HD-bearing DLAs indicate that HD is either lesseffectively dissociated or more efficiently produced in high-redshiftinterstellar gas, even at low molecular fraction and/or solar metallicity. Itis puzzling that such diverse systems should show such consistent HD/H$_2$ratios. Without clear knowledge of all the aspects of HD chemistry that mayhelp determine the ratio HD/H$_2$, we conclude that these systems arepotentially more revealing of gas chemistry than of D/H itself and that it ispremature to use such systems to constrain D/H at high-redshift.

Cosmological concordance or chemical coincidence? Deuterated molecular hydrogen abundances at high redshift