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Hydrogen deuteride (HD) is a compound with molecules each consisting of one 1H and one 2H, aka deuterium (D) atom. A bit is produced during molecular cloud formation because hydrogen usually includes a small amount of deuterium. HD emission lines can be used as a tracer for hydrogen, with one advantage over carbon monoxide (CO) in that it has a lower freezing temperature, allowing it to emit within lower temperature clouds. The HD(1-0) rotational spectral line at 112 μm is of interest for determining the amount of gas in protoplanetary disks. The equivalent line from H2 is missing (or much reduced) due to quantum rules, the distinct difference due to the two types of molecules' spins differing by 1/2.