Molecules and their isotopologues in supernova remnants

The Crab Nebula

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.

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 ³⁶ArH⁺ 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. Astrochemical modelling that is underway in our group indicates that the intense flux of high energy charged particles present within the Crab Nebula can produce ionized argon at a rapid enough rate to account for the observed ArH⁺ line fluxes.

Supernova 1987A

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. 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.

he 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). 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.

Publications

• Barlow et al. 2013, Science, 342, 1343.
• Kamenetzky et al. 2013, ApJ, 773, L34.
• Matsuura et al. 2017, MNRAS, in press.