The J = 1 0 and 2 1 transitions
of 36ArH+ were detected in emission with the SPIRE instrument
on the Herschel satellite toward the Crab Nebula supernova remnant, also
known as M 1, by:
M. J. Barlow, B. M. Swinyard, P. J. Owen, J. Cernicharo, H. L. Gomez, R. J. Ivison, O. Krause, T. L. Lim, M. Matsuura, S. Miller, G. Olofsson, and E. T. Polehampton,
Detection of a Noble Gas Molecular Ion, 36ArH+, in the Crab Nebula
Science 342, (2013) 1343.
40Ar is the most abundant isotope on Earth, but it is almost exclusively formed in the radioactive decay of 40K. Interestingly, the only other feature seen clearly in the entire spectrum was the strongest fine structure component of OH+, there is also a hint of the next weaker FS component.
P. Schilke, D. A. Neufeld, H. S. P. Müller, C. Comito, E. A. Bergin, D. C. Lis, M. Gerin, J. H. Black, M. Wolfire, N. Indriolo, J. C. Pearson, K. M. Menten, B. Winkel, Á. Sánchez-Monge, T. Möller, B. Godard, and E. Falgarone,
Ubiquitous argonium (ArH+) in the diffuse interstellar medium: A molecular tracer of almost purely atomic gas
Astron. Astrophys. 566 (2014), Art. No. A29.
The 1 0 transition was observed in absorprtion with HIFI on board of the Herschel satellite toward numerous prominent continuum sources, first, and most notably, toward Sagittarius B2(M), then Sgr B2(N), and later also toward W51e, W49N, W31C, and G34.26+0.15. Interestingly, the molecule displays absorption at velocities associated with diffuse molecular clouds on the line of sight toward the contiunuum sources, but is conspicuously absent (at least in most cases) at the velocity of the corresponding continuum source itself. Model calculations indicate that ArH+ resides mostly in very atomic diffuse molecular clouds with a molecular fraction of around 0.001 to 0.0001 or less. It is thus a very good tracer of such interstellar material. Moreover, it is also a good indicator of the cosmic ray ionization rate in addition to or instead of OH+, H2O+, or H3+.
The authors also detected the 1 0 transition of 38ArH+ toward Sgr B2(M). The 36Ar/38Ar is compatible within error bars with the the local ISM value of about 5.5.