Temporal Changes in High-velocity Gas Associated with the Monoceros Loop SNR

Abstract We present observations of interstellar Na i and Ca ii absorption lines toward HD 47240, which probes shocked gas in the Monoceros Loop supernova remnant. The observations were obtained at five different epochs spanning the time period from 2017 January to 2024 September. Time variable Na i and Ca ii absorption is observed in two high-velocity components at +63 and +73 km s −1 . These are the same high-velocity features discussed by C. Dirks & D. M. Meyer, who found significant variations in Na i column densities between 2006 January and 2014 January. We discuss our new observations of temporal variability in the context of these and other previous observations of the high-velocity gas toward HD 47240. We speculate that the column density variations are a result of small-scale structure in the post-shock flow passing in front of the background star.