Submillimeter observations of molecular gas interacting with the supernova remnant W28

Context. Supernovae (SNe) inject large amounts of energy and chemically enriched materials into their surrounding interstellar medium and, in some instances, into molecular clouds (MCs). The interaction of a supernova remnant (SNR) with a MC plays a crucial role in the evolution of the cloud’s physical and chemical properties. Despite their importance, only a handful of studies have been made addressing the molecular richness in molecular clouds impacted by SNRs. (Sub)millimter wavelength observations of MCs affected by SNRs can be used to build a census of their molecular richness, which in turn can motivate various chemical and physical models aimed at explaining the chemical evolution of the clouds. Aims. We carried out multi-molecule and multi-transition observations toward the molecular region F abutting the SNR W28, containing 1720 MHz OH masers, well-established tracers of SNR-MC interactions. We used the detected lines to constrain the physical conditions of this region. Methods. We used the APEX Telescope to observe molecular lines in the frequency range 213–374 GHz. We used non-local thermodynamic equilibrium (non-LTE) RADEX modeling to interpret the observational data. Results. We detected emission from multiple molecular species in the region, namely CH3OH, H2CO, SO, SiO, CN, CCH, NO, CS, HCO+, HCN, HNC, N2H+, CO, and from the isotopologues of some of them. We report the first detection of thermally excited (nonmaser) CH3OH emission toward a SNR. Employing non-LTE RADEX modeling of multiple H2CO and CH3OH lines, we constrained the kinetic temperature and spatial density in the molecular gas. The gas kinetic temperatures range from 60 to 100 K while the spatial density of the gas ranges from 9 × 105 to 5 × 106 cm−3. We obtained an ortho-para ratio ~2 for H2CO, which indicates that formaldehyde is most likely formed on dust grain surfaces and not in the gas phase. Conclusions. Our results show that molecules as complex as H2CO and CH3OH can be detected in SNR-MC interactions. This could motivate chemical modeling to explore their formation pathways