Detection of a Noble Gas Molecular Ion, 36ArH+, in the Crab Nebula

We Are Stardust Most of the universe's chemical elements were produced in stars, with the heaviest elements being produced when stars explode. Barlow et al. (p. 1343) used the Herschel Space Observatory to obtain submillimeter spectra of the Crab Nebula, the remains of a stellar explosion that was witnessed on Earth in 1054 AD, and detected the first evidence of a noble gas-containing molecular ion in space—36ArH+. Koo et al. (p. 1346) obtained near-infrared spectroscopic observations of the remains of another stellar explosion, Cassiopeia A, with the Palomar 5-m Hale telescope, and found evidence that a substantial amount of phosphorus was formed in the explosion. Among the six elements essential for life (hydrogen, carbon, nitrogen, oxygen, phosphorus, and sulfur), only the origin of phosphorus remained to be confirmed by observation. Spectroscopic observations of the remains of stellar explosions confirm that argon-36 and phosphorus are produced in such energetic events. 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- and 1234.6-gigahertz J = 1-0 and 2-1 rotational lines of 36ArH+ at several positions in the Crab Nebula, a supernova remnant known to contain both molecular hydrogen and regions of enhanced ionized argon emission. Argon-36 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 36ArH+ emission lines is electron collisions in partially ionized regions with electron densities of a few hundred per centimeter cubed.

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