Submillimeter observations of molecular gas interacting with the supernova remnant W28

Abstract: 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 MCs impacted by SNRs. (Sub)millimter wavelength observations of SNR-MC 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/multi-transition observations toward the region F abutting the SNR W28. 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 from $213\rm{-}374\, \textrm{GHz}$. We used non-LTE RADEX modeling to interpret the observational data. Results: We detected emission from multiple molecular species, namely CH${3}$OH, H${2}$CO, SO, SiO, CN, CCH, NO, CS, HCO$^+$, HCN, HNC, N$2$H$^+$, CO, and from isotopologues of some of them. We report the first detection of thermally excited (non-maser) CH${3}$OH emission toward a SNR. Employing non-LTE RADEX modeling of multiple H${2}$CO and CH${3}$OH lines, we constrain the kinetic temperature from 60 to 100$\,$K and the gas density from $9\times 10^{5}$ to $5\times 10^{6}\,\rm cm^{-3}$. We obtained an ortho-para ratio $\sim$2 for H${2}$CO, which indicates that formaldehyde is most likely formed on dust grain surfaces. Conclusions: Our results show that molecules as complex as H${2}$CO and CH$_{3}$OH can be detected in SNR-MC interactions. This could motivate chemical modelling to explore their formation pathways.