One of the most surprising discoveries made by the Herschel Space Observatory during its mission was the detection in the Crab Nebula of the noble gas molecule ArH+. This molecular ion, also known as argonium, had previously only been studied in the laboratory. Figure 1 shows a plot of a Herschel SPIRE FTS spectrum of a Crab Nebula knot in which the J=1-0 617.525 GHz and J=2-1 1234.603 GHz rotational transitions of 36ArH+ were both detected (Barlow et al. 2013).
36Ar is the dominant isotope of argon in the Universe and is synthesised by alpha-particle capture chain nuclear reactions in core collapse supernovae such as the Crab Nebula. The next most cosmically abundant isotope of argon is 38Ar, which in the solar wind is five times less abundant than 36Ar. The J=2-1 line of 36ArH+ is displaced by 1.75 GHz from the corresponding transition of 38ArH+ and, using our SPIRE FTS spectra of the Crab Nebula in which we detected 36ArH+, we were able to set a limit to the 36Ar/38Ar ratio of >2. The SPIRE FTS spectral resolving power at the frequency of those ArH+ lines was only 860, so we have scheduled higher spectral resolution ground-based observations of the J=1-0 ArH+ isotopologues in order to directly measure the 36Ar/38Ar ratio. This ratio will help diagnose the nuclear processes that operated in the progenitor star immediately before and during the explosion that produced the Crab Nebula.
ArH+ can be produced by the reaction H2 + Ar+ -> H + ArH+ but it can also be destroyed by the reaction H2 + ArH+ -> Ar + H3+. So although some H2 is necessary for the formation of ArH+, too much H2 can completely remove ArH+, unless more ionized argon is being generated rapidly. Priestley et al. (2017) have performed combined photoionization and photodissociation region (PDR) modelling of a Crab nebula filament subjected to the synchrotron radiation from the central pulsar wind nebula and to a high flux of charged particles. They found that a greatly enhanced cosmic-ray ionization rate over the standard interstellar value is required to account for the lack of detected [C I] emission in the Herschel SPIRE FTS observations of the Crab nebula. They found that the ArH+ and OH+ line strengths and the observed H2 vibration-rotation emission can be reproduced by model filaments with nH = 2 × 104 cm-3, a cosmic-ray ionization rate 107 times larger than the average interstellar value and visual extinctions consistent with the range found for dusty globules in the Crab nebula.
Spectra of SN 1987A obtained with the Herschel SPIRE Fourier Transform Spectrometer recorded weak detections of the J=6-5 and J-7-6 rotational lines of the CO molecule, whilst early ALMA Band 3 and 6 spectra of SN 1987A recorded high signal-to-noise detections of the CO 1-0 and 2-1 rotational transitions of CO, whose FWHM line widths of 2200 km/s indicated an origin within the inner ejecta (Kamenetzky et al. 2013). An excitation analysis of the spectra implied at least 0.01 solar masses of emitting CO molecules. Further ALMA Band 6 and 7 time was allocated during Cycles 2 and 3 for studies of molecules and their isotopic variants in the ejecta of SN 1987A.
The ALMA spectra yielded detections of CO, SiO and HCO+, as well as limits on the SiO isotopologues of 28SiO/29SiO > 128 and 28SiO/30SiO > 189 (compared to ratios of 19.7 and 20.9 for the Sun; see Matsuura et al. 2017). These ratios are consistent with predictions that 28Si should be the dominant silicon isotope, created by alpha-particle capture reactions in the low-metallicity massive progenitor star immediately prior to its explosion as a core collapse supernova.
The above molecular line observations of the Crab Nebula and Supernova 1987A at far-infrared and submillimetre wavelengths have enabled isotope ratios to be measured in core collapse supernova remnants for the first time, providing a new and powerful probe of the nuclear processes that operate in massive stars during and immediately prior to their demise.
last updated 06 July 2022
