Temporal Variability of Interstellar Na I Absorption Toward The Monoceros Loop

TL;DR

This study presents the first evidence of temporal variability in interstellar Na I absorption toward a star behind the Monoceros Loop supernova remnant, indicating small-scale fluctuations likely caused by the remnant's expanding gas interacting with the interstellar medium.

Contribution

It provides the first observational evidence of time-variable interstellar Na I absorption associated with a supernova remnant, linking small-scale ISM fluctuations to remnant expansion.

Findings

Significant variation in Na I absorption over eight years.

Variations imply ~10 AU scale fluctuations within the SNR shell.

Similar variations observed in Vela SNR, suggesting a common mechanism.

Abstract

We report the first evidence of temporal variability in the interstellar Na I absorption toward HD 47240, which lies behind the Monoceros Loop supernova remnant (SNR). Analysis of multi-epoch Kitt Peak coud\'{e} feed spectra from this sightline taken over an eight-year period reveals significant variation in both the observed column density and the central velocities of the high-velocity gas components in these spectra. Given the $\sim$1.3 mas yr$^{-1}$ proper motion of HD 47240 and a SNR distance of 1.6 kpc, this variation would imply $\sim$10 AU fluctuations within the SNR shell. Similar variations have been previously reported in the Vela supernova remnant, suggesting a connection between the expanding supernova remnant gas and the observed variations. We speculate on the potential nature of the observed variations toward HD 47240 in the context of the expanding remnant gas interacting with the ambient ISM.