Submillimeter observations of molecular gas interacting with the supernova remnant W28

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: pmazumdar@mpifr-bonn.mpg.de
² Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 150 Science 1-Street, Urumqi, Xinjiang 830011, PR China
³ Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, 150 Science 1-Street, Urumqi, Xinjiang 830011, PR China

Received: 23 January 2020
Accepted: 31 October 2022

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 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.