  Noble gas molecules have not hitherto been detected in space. From spectra
obtained with the Herschel Space Observatory, we report the detection of
emission in the 617.5 GHz and 1234.6 GHz J = 1-0 and 2-1 rotational lines of
{36}ArH^+ at several positions in the Crab Nebula, a supernova remnant known to
contain both H2 molecules and regions of enhanced ionized argon emission.
{36}Ar is believed to have originated from explosive nucleosynthesis in massive
stars during core-collapse supernova events. Its detection in the Crab Nebula,
the product of such a supernova event, confirms this expectation. The likely
excitation mechanism for the observed {36}ArH^+ emission lines is electron
collisions in partially ionized regions with electron densities of a few
hundred per centimeter cubed.
