IRAM 30 m CO-line Observation toward PeVatron Candidate G106.3+2.7: Direct Interaction between the Shock and the Molecular Cloud Remains Uncertain

TL;DR

This study investigates the interaction between supernova remnant G106.3+2.7 and surrounding molecular clouds using IRAM 30m CO observations, finding ambiguous evidence of shock disturbance but signs of nearby clouds illuminated by escaped protons.

Contribution

It provides new CO observations and analysis of molecular cloud dynamics around G106.3+2.7, highlighting uncertainties in shock interaction and potential proton illumination.

Findings

Asymmetric CO line profiles suggest complex cloud motions.

Cool molecular clouds are present near the SNR.

Evidence of an expanding molecular structure possibly influenced by the progenitor star's wind.

Abstract

Supernova remnant (SNR) G106.3+2.7 was recently found to be one of the few potential Galactic hadronic PeVatrons. Aiming to test how solid the SNR is associated with the molecular clouds (MCs) that are thought to be responsible for hadronic interaction, we performed a new CO observation with the IRAM 30m telescope toward its "belly" region, which is coincident with the centroid of the $\gamma$-ray emission. There is a filament structure in the local-standard-of-rest velocity interval $-8$ to $-5$ km/s that nicely follows the northern radio boundary of the SNR. We have seen asymmetric broad profiles of $^{12}$CO lines, with widths of a few km/s along the northern boundary and in the "belly" region of G106.3+2.7, but similar $^{12}$CO line profiles are also found outside the SNR boundary. Further, the low $^{12}$CO J=2-1/J=1-0 line ratios suggest the MCs are cool. Therefore, it is still uncertain whether the MCs are directly disturbed by the SNR shocks, but we do find some clues that the MCs are nearby and thus can still be illuminated by the escaped protons from the SNR. Notably, we find an expanding molecular structure with a velocity of $\sim$3.5 km/s and a velocity gradient of the MCs across the SNR from $\sim -3$ to $-7$ km/s, which could be explained as the effect of the wind blown by the SNR's progenitor star.